NEWS BRIEFS IN-DEPTH ARTICLE DESIGN SHOWCASE

NEWS BRIEFS IN-DEPTH ARTICLE DESIGN SHOWCASE
Volume Eighteen
NEWS BRIEFS
Forbes rates Maxim among America's best small companies
2
IN-DEPTH ARTICLE
Energy management for small portable systems
3
DESIGN SHOWCASE
Switching regulator/transformer steps down from high voltage
13
RS-485 data interface gives isolated full-duplex operation
14
Autotransformer boosts maximum VOUT
16
±15V regulator accepts inputs from 2V to 12V
17
NEW PRODUCTS
Data Converters
• 14-bit, 85ksps serial ADC has 20µA shutdown
(MAX194)
• 10-bit, VOUT DACs operate from 5V
(MAX503/504/515)
19
19
High Speed: Video, Comparators
• 50V, 200MHz amplifier drives high-resolution monitors
(MAX445)
• 950MHz JFET video buffer includes 75Ω trimmed termination resistor (MAX4005)
19
20
Analog Switches and Multiplexers
20
20
• CMOS analog multiplexers offer extremely low leakage
• Active, in-line circuit protectors guard signal lines
(MAX338/339)
(MAX366/367)
• Analog switches and multiplexers guarantee matched,
low on-resistances
(MAX381/383/385/398/399) 21
Power Management
(MAX688/689)
21
21
(MAX793/794/795)
(MAX809/810)
22
22
• 3.3V linear regulator’s automatic shutdown suits portable applications (MAX687/688/689)
• 3V and 3.3V linear regulators offer accuracy and low dropout
µP Supervisors
• Multifunction µP supervisors monitor 3V and 3.3V systems
• Smallest available µP supervisors come in SOT-23 packages
Interface
• AutoShutdown™ lowers RS-232 transceivers’ supply current to 1µA
• 3.3V RS-485/RS-422 transceivers transmit to 10Mbps
(MAX3212/23/43)
23
(MAX3483/85/86/88/90/91) 22
Hybrid/MCM Packaging
• Custom packaging accommodates high-frequency, high-power circuits
23
News Briefs
FORBES RATES MAXIM AMONG AMERICA’S BEST SMALL COMPANIES
For the fifth consecutive year, Maxim Integrated Products is among America’s
most prosperous small companies. We are one of only 14 “top tier” companies on Forbes
magazine’s list of America’s Best Small Companies to have achieved top 200 status at least
four times in the past five years.
We announced 36 new products during the first quarter and 31 products during the second
quarter, for a total of 795 new products introduced since the company was formed—the most of any
company in the industry.
MAXIM REPORTS 35TH CONSECUTIVE QUARTER OF INCREASED EARNINGS
Maxim Integrated Products, Inc., reported record net revenues of $56,184,000 for the second quarter of
fiscal 1995, compared to $36,143,000 for the same period a year ago. This represents a 55.4% gain in net
revenues from the same quarter a year ago. Net income of $8,930,000 (or $0.27 per share) for the quarter marked
the 35th consecutive increasingly profitable quarter for Maxim and compared to net income of $5,686,000 (or
$0.18 per share) for the same quarter in fiscal 1994.
Operating income for the quarter was 23.5% of net revenues, again one of the industry’s highest. Gross
margins increased to $32,868,000 (58.5% of net revenues) from $21,166,000 (58.6% of net revenues) for the
same quarter in fiscal 1994.
During the quarter, cash and short-term investments increased $8,707,000 ($0.26 per share). Accounts
receivable levels were 31 days outstanding, and inventory days declined to 63 days from 66 days in the prior
quarter. The Company continued its stock repurchase program, repurchasing stock for $2,126,000 during the
quarter. The Company also purchased for cash $6,635,000 of capital equipment.
Maxim effected a two-for-one stock split during the second quarter in the form of a stock dividend. At the
annual meeting in November, shareholders approved a substantial increase in Maxim’s stock option plan,
confirming the Company’s philosophy that people make the difference in great organizations.
New products developed and announced during the quarter were consistent with our model for 3-year
sales and profit growth. New product announcements increased 13% per year from 1990 to 1993, and they are
expected to increase 30% per year from 1993 to 1996. Based on past performance, we foresee that products
developed during the 1993–1996 time frame will strongly influence sales and profits growth during 1996–1999.
We have now substantially completed integration of the operation acquired from Tektronix last year, and
our emerging high-frequency businesses are growing on plan. To date, we have announced eight new standard
products based on the acquired high-frequency bipolar technology. The existing high-frequency business has not
declined as our worst-case plan allowed. Ramp-up of the manufacturing capability in Oregon is on track. In the
second quarter, 30% of our wafers were manufactured in the acquired 0.8 micron facility. Wafer output from this
facility has increased 25% quarter-to-quarter. By the end of Q495, we anticipate that 50% of our wafers will be
produced in Beaverton. Over the next several quarters, if required, production can be increased to three times our
current total wafer consumption with additional capital expenditures of less than $20 million.
Also during the quarter, Maxim began a three-quarter program to modernize its manufacturing facilities,
including upgrading wafer fabrication from 4" to 6" wafers and replacing outdated test equipment and handlers in
all of our facilities. Second quarter results included a $5.6 million charge related to this program.
Energy management
for small portable
systems
INPUT
3V TO 8V
C1
47µF
C6
2.2µF
L1
D2
1N4148
C2
47µF
2
V+
C5 5
REF
0.1µF
Numerous diverse and conflicting constraints burden the
designer of small hand-held products. Aside from the
customary restrictions on size and weight, these
constraints include cost limitations, strict time schedules,
battery-life goals measured in weeks instead of hours,
and host computers that are (sometimes) overtaxed with
the demands of power management.
EXT
1
D1
1N5817
OUTPUT
5V/0.3A
Q1
L2
C3
47µF
MAX1771
HI = OFF
LO = ON
4
SHDN
6
AGND
7 GND
CS
R1
0.082Ω
FB
3
R3
470k
R2
200k
C4
47pF
Because power requirements for hand-held applications
vary widely with product use, no single “best” power
source exists for these applications. A device used intermittently is more concerned with no-load quiescent
current than with full-load efficiency, and so may operate
satisfactorily with alkaline batteries. Cell phones, on the
other hand, must contend with high peak loads and
frequent use. This mode of operation emphasizes conversion efficiency over quiescent current, so cell phones are
better served with a rechargeable battery.
8
L1 & L2 = CTX20-4
COILTRONICS (407) 241-7876
Q1 = 1/2 IRF7101
INTERNATIONAL RECTIFIER (310) 322-3331
EFFICIENCY AT I LOAD
INPUT
VOLTAGE
3.5V
4.0V
5.0V
6.0V
Figure 1.
In hand-held product design, size limitations often dictate
the number of battery cells early in the process. This is
frustrating to the electrical engineer, and a substantial
constraint, since the number (and type) of cells allowed
determines the operating-voltage range, which in turn
strongly affects the cost and complexity of the power
supply. High cell counts enable the use of linear regulators and simple circuitry at the cost of extra weight and
limited efficiency. Low cell counts compel the use of a
more costly switching regulator, but the low cost of the
battery may justify this expense.
10mA
100mA
200mA
300mA
81%
82%
82%
82%
83%
84%
84%
84%
84%
84%
85%
86%
84%
85%
86%
86%
This regulator topology supplies 5V for inputs ranging
from 3V to 8V. The operation shifts smoothly between stepup and step-down conversion without steps or mode
changes. During shutdown, the output turns off completely
and sources no current.
BATTERY
VIN
CURRENT PATH FROM
VIN TO VOUT BROKEN BY Q2
TYPICAL
BOOST DC-DC
CONVERTER
LOW = OFF
SHUTDOWN
VOUT
TO LOAD
Q1
Q2
Four-cell designs
Four-cell batteries often provide an attractive compromise between weight and operating life. That number is
particularly popular for alkaline batteries because they
are commonly sold in multiples of four. Four-cell power
for 5V circuitry presents a design challenge, however. As
the battery discharges, the regulator must first step down,
and then step up. This requirement precludes use of the
simpler, one-function regulator topologies that can only
step down, step up, or invert.
Figure 2.
Typical dc-dc boost converters provide a current path from
input to output, even when powered down. To interrupt this
path, you must add a disconnect switch (Q2).
VOUT is capacitively coupled to the switching circuitry
(Figure 1). The absence of a transformer is one of
several advantages this configuration has over flybacktransformer regulators and combination step-up/linear
regulators.
One effective solution to this problem is the SEPIC
(single-ended primary inductance converter), in which
3
INPUT
5V FROM
2C ADAPTOR
C1
2.2µF
D1
1N4001
D2
1N4148
C3
47µF
16V
8
10k
10k
V+
B2
LITHIUM
CR2032
D4
1N4148
L1*
22µH
LX 7
L2*
22µH
D3
1N5817
OUTPUT
3.3V/200mA
LOGIC
150k
B1
(3 AA CELLS)
Q1
Si9433
SILICONIX
(408) 988-8000
FB 3
150k
2
LBI
125k
150k
REF 5
IC1
MAX761
ON/OFF
4
LOW WHEN
B1 < 3V
(OR WHEN ADAPTER
VOLTAGE < 3V)
LBO 1
SHDN
GND
6
Figure 3.
C2
0.1µF
1.5V
* L1 AND L2 ARE SUMIDA CD54 SERIES.
SUMIDA (408) 956-0666
This low-current step-up/step-down regulator supplies 3.3V at 200mA. Q1 automatically disconnects the B1 battery when you connect an
ac adapter, and a diode-OR circuit allows B2 to back-up the 3.3V output.
capability under full load and improves the low-VIN efficiency by boosting gate drive to the external MOSFET. If
VIN does not fall below 4V, you can substitute a 3Vthreshold FET for Q1 and omit D2. In that case, pin 2
connects directly to VIN, which assumes an upper limit of
16.5V.
As another improvement over boost designs (in which
current drains from the battery during shutdown unless
you add a cut-off switch—see Figure 2), the SEPIC
output fully turns off in response to a shutdown
command. As VIN falls during normal operation, the
SEPIC circuit smoothly regulates VOUT without shifting
its mode of operation as V OUT approaches V IN . Its
power-conversion efficiency peaks at 86%, near 200mA
(Figure 1).
Three cells to 3.3V
The circuit of Figure 3 employs the same principles as that
of Figure 2, but adds battery-backup capability. It also
foregoes the external FET for a lower-current internal one.
Separate coils for L1 and L2 (vs. a single transformer)
allow the use of a 22µH coil for each of multiple versions
of the circuit—such as you would need in a product that
required power supplies of 3.3V, 5V, 12V, and 30V, for
example. The input-voltage range is 3V to 13V.
Coils L1 and L2 (Figure 1) should be the same type and
have the same value, but coupling between them is not
required. They can be wound on the same core for convenience, but the circuit works equally well if they are
completely separate. Each coil passes only one half of the
peak switching current (IPEAK = 100mV/R1 = 1.22A), so
each can be rated accordingly.
Capacitor C2 couples energy to the output and requires
low ESR to handle high ripple currents. A low-ESR
Sanyo OS-CON capacitor, for instance, offers 3% more
efficiency than does a less expensive 1µF ceramic
capacitor. Tantalum capacitors are not recommended
because high ESR causes them to self-heat at high ripple
currents.
During normal operation, the ac adapter’s 5V output
powers the circuit and turns off Q1. Disconnecting the
adapter removes 5V, turns on Q1, and allows the three
AA cells to provide power. If the input voltage drops
below 3.0V, a low-battery comparator in IC1 alerts the
system by driving LBO low. And for backup, a diode-OR
connection allows the optional lithium battery (coin cell
B2) to maintain the 3.3V output. To simplify the
switchover circuit from adapter to main battery, this
design requires the ac adapter’s 5V output to be
somewhat regulated—to between 4V and 5.5V.
Diode D2 provides a supply voltage for the IC (pin 2) by
capturing switching pulses at the drain of Q1. Although
this voltage (approximately the sum of VIN and VOUT)
limits the maximum VIN to 8V, it improves the start-up
4
INPUT
3.8V TO 16V
0.1µF
INPUT
3.8V TO 11.5V
100µF
1 IN
V+
0.05Ω
4
SHDN
EXT
REF
OUT
FB
Q1
Si9430
(SILICONIX)
3
OUTPUT
3.3V/1.5A
L1
22µH
1
GND
2
GND
6
7
NSQ03A02L
NIHON
(805) 867-2555
8
0.1µF
LO = OFF
OUTPUT
3.3V/400mA
10µF
2
MAX1651
3
8
10µF
5
CS
OUT
GND
MAX604
GND
GND
4 OFF
SET
7
6
5
330µF
SAFE OPERATING REGION AT 70°C
4.0
100
700
DROPOUT LINE
EFFICIENCY
600
70
3.6
OUTPUT CURRENT (mA)
3.8
80
DROPOUT (V)
EFFICIENCY (%)
90
500
400
300
SAFE REGION
200
60
100
DROPOUT
VOLTAGE
3.4
50
0
0.2
0.4
0.6
0.8
1.0
0
2
1.2
LOAD CURRENT (A)
Figure 4.
A low-dropout switch-mode controller and p-channel
MOSFET supply 3.3V at 1.5A with inputs as low as 3.8V.
Efficiency exceeds 90% for most of the operating range.
Figure 5.
3
4
5 6 7 8 9 10 11 12
INPUT VOLTAGE (V)
This combination of internal MOSFET pass transistor and
high-power SO-8 package provides a linear regulator with
low dropout, an operating current of 15µA, and an output
capability of over 400mA.
Low-dropout, step-down converter
Linear regulators
Low-voltage logic, such as that powered from 3.3V, now
enables the use of 4-cell inputs for simple step-down
configurations that optimize efficiency and cost. For 3.3V
outputs, the key specification is dropout voltage—the
minimum allowable difference between VIN and VOUT.
“End-of-life” voltage for the battery varies according to
cell type and the product’s pattern of use, but (for all but
lithium batteries) it falls in the range of 0.8V to 1V per
cell. As a result, it’s not uncommon for 3.3V regulators to
operate with input voltages as low as 3.6V.
Still the lowest-cost approach for many step-down applications (short of no regulator at all) is linear regulation,
provided its efficiency and battery-life limitations are
acceptable, and its power dissipation at higher VIN is
manageable.
For portable designs, even a simple linear regulator can
provide some twists. As an example, dropout voltage (the
low-VIN level at which output regulation is lost) should
often be regarded as a part of normal operation rather
than a fault. That is, to extend operating time it may be
advisable to allow the regulator to fall out of regulation
without shutting down. The regulator’s behavior during
dropout (especially its quiescent current) is important in
these designs.
The design of Figure 4 offers an uncomplicated means
for delivering intermediate current loads at 3.3V from
four cells. The IC drives a low-threshold p-channel
MOSFET, and minimizes current-sense losses with a low
current-sense voltage of 110mV. For best performance,
the MOSFET on-resistance should be specified in
conjunction with the circuit’s lowest operating voltage—
about 3.6V in this case.
The simple linear regulator of Figure 5 offers exceptional dropout behavior with little effect on operating
current. Essentially an 8-pin surface-mount package, it
delivers more than 400mA. Because the internal pass
element is a MOSFET instead of a bipolar transistor, the
circuit’s dropout voltage is nearly zero at light loads.
5
68µF
INPUT
0.9V* TO 3.3V
L1
47µH
MAX856
Q1 MMDFZP02E
MOTOROLA
(602) 244-3576
1N5817
LX 8
CONTROL
68µF
OUTPUT
3.3V/200mA*
1M
MAXIMUM START-UP LOAD CURRENT vs.
START-UP VOLTAGE
GND 7
0.1µF
REF
1000
OUT 6
1 SHDN
WITH LOAD SWITCH
3/5 2
1M
LBI 5
1.5V
LB0
1M
Q2
START-UP LOAD CURRENT (mA)
3
100
10
WITHOUT LOAD SWITCH
1
0.1
L0 = OFF
1M
2N3904
1M
0.01
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
START-UP INPUT VOLTAGE (V)
* OPTIONAL CIRCUITRY FOR FULL-LOAD
LOW-VOLTAGE START UP. SEE FIGURE 6b.
(b)
(a)
Figure 6.
This low-power, CMOS step-up converter (a) generates 3.3V from 1-cell and 2-cell inputs. The optional load-disconnect circuitry (dashed
lines) enables the circuit to start with inputs as low as 0.8V (b).
And, its quiescent current does not rise as V IN approaches VOUT.
in two AA cells exceeds the 3Whrs in a 6-cell, 9V
alkaline battery by 50%, even though the two batteries
are comparable in size and weight.
This last characteristic is especially important for small
portables whose steady-state load is no greater than
100µA. In such designs, the milliamp or more of
quiescent-current rise (typical of a low-dropout regulator
with bipolar pass transistor) accelerates the battery
discharge at a time when the battery can least afford it:
near the end. Typically, the IC in Figure 5 draws 15µA of
operating current whether in or out of dropout.
The step-up regulator of Figure 6a provides high, 88%
efficiency for 2-cell and 1-cell inputs, and its high,
500kHz switching frequency enables the use of very
small inductors. The IC’s quiescent current is only 60µA
at light or zero loads—an attractive feature for portable
products whose supply voltage must remain active when
the product is turned “off.” As the product enters such an
idle or suspend mode, load current falls to microamps
and must not be dominated by current into the regulator
IC. For equipment that truly shuts down, the IC provides
a very low-current shutdown mode in which it draws less
than 1µA.
Boosting from low-cell-count batteries
The cell count for batteries in earlier-generation designs
was high—not to provide more energy, but rather to
allow generation of the system voltages with low-cost
linear regulators (or even with no regulator at all). The
latest generation of voltage-conversion ICs, on the other
hand, lets you reduce the cell count while adding a
minimum number of external parts. Usually, this extra
cost is more than offset by the benefits of lower cell
count: smaller size, less weight, and (sometimes) longer
battery life. To illustrate, the 4.5Whrs of available energy
One-cell regulators
It makes sense to operate from a 1-cell battery when size
is of prime importance. Reasonable efficiency and cost is
now possible when operating with inputs below one volt,
so many hand-held applications have become new candidates for 1-cell operation. The switching frequency for
6
INPUT
1V TO 6V
Inductorless conversion suits tight spaces
10µH
1
ILIM
2
IN
5
LX
OUT 6
ACTIVE
RECTIFIER
7 SHDN
LO = OFF
Despite the advances made in inductor-based switching
regulators, most designers would regard the ideal
converter circuit as one that has no inductor. The
capacitor-based alternatives (charge-pump converters)
were hampered in the past by their lack of regulation and
limited output current. Though still low compared to that
of switching regulators, their output current is now
adequate for many designs. And in some cases, the
charge-pump advantages are compelling—low cost,
small size, and reduced EMI. Charge pumps are particularly useful in PCMCIA systems and other “credit-card”
products in which the component height is limited.
RLIM 1kΩ
47µF
OUTPUT
3.3V/
100mA
47µF
CONTROL
1
N
SEL 8
REF
MAX778
Figure 7.
PGND
4
Figures 8, 9, and 10 illustrate three inductorless voltage
converters. In Figure 8, the output of a 2-cell battery or
other low-voltage source is converted to a regulated 5V
±4%. The IC changes its operational mode with input
voltage, producing a tripler at low VIN, a doubler at high
VIN, and a tripler-doubler at mid-range that changes
modes every switching cycle. Efficiency ranges from
85% to 65%. Low supply current—typically 75µA for
no-load operating conditions and 1µA in shutdown—
makes the circuit useful as a coin-cell-powered backup
supply for DRAM or PSRAM.
AGND
3
This single-IC boost converter has an internal synchronous
rectifier. It maintains a regulated 3.3V output for inputs
ranging from 1V to 6V.
low-cost ICs now approaches 1MHz, which permits the
use of small magnetic components available from
multiple sources. It’s not unusual, therefore, for the dc-dc
circuitry to occupy less space than the battery it replaced.
The optional diode-capacitor network in Figure 8
generates an unregulated negative voltage between -1.4V
and -3V. Acting as a negative supply, this output simplifies analog designs by allowing the use of inexpensive op
In Figure 6a, the addition of Q1 and Q2 within the
dashed lines allows the regulator to start with lower input
voltages and higher load currents. Q1 also disconnects
the load and battery from each other during shutdown,
and the on-chip comparator does not allow Q1 to turn on
again until VOUT has risen to at least 3V. Figure 6b illustrates this circuit’s loaded-start capability and its remarkably low typical start-up voltage (0.8V).
INPUT
2V TO 3.6V
10µF
Figure 7 shows a low-parts-count step-up regulator that
also starts under load and operates with inputs down to
0.8V. Its 500kHz switching frequency and adjustable
peak coil current (set by RLIM) allows use of a tiny, lowcost surface-mount coil. The on-board active
(synchronous) rectifier not only eliminates the external
diode, it also enables the shutdown input to turn off the
output completely—a useful feature not common in
boost designs, and one that requires an external FET in
Figure 6.
LO = ON
2
IN
OUT
MAX619
7
1
GND
3
6
10µF
OUTPUT
5V/20mA FOR VIN > 2V
5V/50mA FOR VIN > 3V
SHDN
C1+
C2+
4
C1
0.22µF
C2
0.22µF
8
C1-
C2-
5
CMPSH-35 DUAL SCHOTTKY
CENTRAL SEMICONDUCTOR
(516) 435-1110
0.1µF
-1.4V TO -3V/5mA
2.2µF
CIRCUITRY FOR OPTIONAL NEGATIVE OUTPUT
The active rectifier and control circuitry in the IC of
Figure 7 maintain regulation for inputs to 6.2V—an
achievement which, if not of benefit in single-cell
designs, may be useful elsewhere. The price for these
improvements is higher quiescent current: 190µA for
Figure 7 vs. 30µA for Figure 6.
Figure 8.
7
With a few external capacitors, one IC boosts a 2-cell or 3cell input to 5V, and delivers 50mA (for 3V inputs) with
only 75µA of quiescent current. With an additional SOT-23
dual diode and two capacitors, it also produces a small
negative output.
INPUT
4.5V TO 10V
1
IN 8
C1+
1µF
INPUT
4.75V TO 5V
2
CHARGE
PUMP
OUT 6
3
5
1µF
1µF
MAX850
C1C4
4.7µF
NEGOUT
VCC
4 C2+ S1
C3
0.1µF
S2
0.22µF
VOUT 6
R2
3 C2- S1
ERROR
AMP
LO = OFF
OUTPUT
12V/30mA
FLASH VPP
OUTPUT
OUTPUT
-4.1V/-5mA
10µF
4 SHDN
FB 5
ERROR
AMP
-1.28V
REF
C5
4.7µF
R1
S2
7 C1+
S1
0.22µF
GND
7
VREF
SHDN 8
HI = ON
OUTPUT NOISE AND RIPPLE
S2
1 C1-
MAX662A
S1
OSCILLATOR
GND
500µV/div
SWITCH CLOSURES SHOWN FOR CHARGE PUMP IN THE TRANSFER MODE
10µs/div
VIN = 6.0V, VOUT = -4.1V, IOUT = 5mA, AC COUPLED
Figure 9.
Figure 10. Intended for biasing efficient GaAsFET RF power amplifiers, this charge-pump voltage inverter includes a superquiet linear regulator that limits output ripple and noise
below 1mVp-p.
For programming flash memory, this circuit generates a
regulated 12V/30mA programming voltage without inductors. It’s small enough to fit into “smart cards” the size of
a credit card.
amps. The negative rail assures that such op amps can
swing completely to ground.
Though more efficient, a GaAsFET costs more and
requires a small negative bias voltage. Typical charge
pumps generate too much noise for this application, but
an output voltage regulator in the chip of Figure 10 holds
the output noise and ripple to 1mVp-p. Tying the FB
terminal to ground sets the regulated output to -4.1V (you
can set other output levels with two external resistors).
Regulation and low noise are achieved with an output
linear regulator—unlike the circuits of Figures 8 and 9,
which regulate by gating the charge pump’s switching
action.
Another charge-pump circuit, built in less than 0.1in.2 of
board area, converts 5V to the 12V level required for
programming “flash” memory chips (Figure 9).
Common in PCMCIA cards, flash memory is popular for
compact portable applications because it provides large
amounts of nonvolatile storage in a small space, and
because it needs power only for read and write operations. Some flash ICs operate on 5V, but those with the
highest memory densities require 12V for programming.
A third application that benefits from the use of charge
pumps is the optimization of RF-transmitter efficiency in
cellular and other voice/data wireless transceivers. “Talk
time” in these transceivers is extended by the use of
power amplifiers based on gallium-arsenide FETs
(GaAsFETs), which are more efficient than those based
on bipolar transistors.
Intermittent high-current loads
A second requirement in many hand-held wireless
designs is a quick response to abrupt load changes. The
power supply may idle at milliamp levels for most of the
time, but to handle short RF transmissions or bursts of
CPU activity it must also deliver high-amplitude currents
for short intervals. Especially demanding is the RF trans-
8
INPUT
3 CELLS
(3V...5V)
HALT RUN
1
4
3
L1
10µH
33µF
LX
SHDN
OUT
LBO
C1
22µF
6
MAX757
REF
FB
2
GBI
GND
7
0.1µF
5
D1
8
R1
1Ω
OUTPUT
5.8V
C2
C3
C4
R2
54.9k
C5
IOUT
1.5A
R3
15k
577µs
4.6ms
5.8V
5.35V
L1 = SUMIDA CD75 (708) 956-0666
D1 = NIHON EC15Q502L (805) 867-2555
C1 = 22µF TANTALUM
C2–C5 = 470µF TANTALUM
VOUT
TIME
Figure 11. This circuit includes a large capacitive reservoir that supplies 1.5A transient loads in a GSM cellular telephone. The average load is only
200mA, so the 8-pin, surface-mount, boost-regulator IC requires no external MOSFET.
produces an output range of 20V to 30V, adjusted either
by digital control or by an external potentiometer. This
circuit’s high switching frequency and adjustable
inductor-current limit enable the use of small surfacemount inductors and output-filter capacitors. For loads
below 10mA, for instance, the Murata-Erie LQH4 coil
shown is only 2.6mm high.
mitter in a GSM cellular telephone or other digital
wireless system employing TDMA (time-division
multiple access) techniques.
For cellular handsets, a desirable battery combination for
minimal size and weight is three NiCd cells. The lowestcost RF transmitters for this application operate at or near
6V. You might expect the expense of a switching
regulator capable of delivering 2W at 6V to force the use
of a five-cell battery. But, the high current is drawn only
for 600µs or so at a 10% duty cycle, so a small step-up IC
can supply the load.
Note that the potentiometer’s configuration is not
arbitrary (see the optional circuit in Figure 12).
Connecting the pot between FB and ground (rather than
FB and VOUT) ensures that an open or noisy pot wiper
will produce a low output voltage rather than a maximum
(and possibly destructive) output. Moreover, connecting
the pot and its wiper to ground minimizes the trace area
at FB; if you swap R8 and R9 the VOUT noise will likely
increase.
In Figure 11, a reservoir capacitor powers both the
TDMA logic and the RF circuitry. The capacitor supplies
an average 200mA, but at 1.5A its output drop is less
than 500mV after 577µs. A 1Ω resistor (R1) isolates the
RF load from the dc-dc converter IC. While 4 x 470µF is
certainly a lot of buffer capacitance in a hand-held
device, the four surface-mount capacitors are far smaller
and cheaper than two additional battery cells. The
circuit’s average power-conversion efficiency is 80%,
and its quiescent supply current is only 60µA.
In 2- or 3-cell applications you can optimize efficiency
by biasing the IC from 5V (if available) instead of the
battery voltage. The inductor still draws current from the
battery, but higher voltage at the chip’s V+ pin improves
efficiency by providing more gate drive to Q1, which
lowers its on-resistance. On the other hand, if battery
voltage exceeds 5V then V+ should connect directly to
the battery. VOUT can be adjusted by a 4-bit, 3.3V CMOS
digital code or by the optional potentiometer, as shown.
LCD bias supplies
The bias requirements for LCD panels in portable gear
cover broad ranges of voltage and current, depending on
the display’s technology, screen size, and cost. Bias
voltages may be positive or negative and as high as
±30V. The boost converter in Figure 12, for example,
9
idle until the main battery dies or is removed, then
supports the 3.3V rail by boosting the output of a lithium
coin cell. The 5V and 3.3V main outputs are also overridden by pnp linear regulators (Q2 and Q4), which
become active when you plug in an external unregulated
dc supply. This action also unloads the main battery. The
two ICs include several control and supervisory lines in
addition to the four output voltages.
3V TO 16V
BATTERY
INPUT
D1
1N5819
2
V+
EXT
HI = OFF
4
5
0.1µF
6
MAX1771
CS
SHDN
1
FB
3
R3
25k
OUTPUT
+20V TO + 30V/10mA
Q1
22µF
35V
8
Simple battery charging
R1
0.4Ω
REF
AGND
22µF
L1
47µH
5V
0.1µF
GND 7
For small hand-held products, a lack of space and a
limited budget often preclude sophisticated schemes for
battery monitoring and charging. The goal in these cases
is to squeeze the maximum performance from “bare
bones” hardware. If available, though, CPU resources
(combined with low-cost analog circuitry) offer a convenient means for charge control.
L1 = MURATA ERIE LQH4
OR
SUMIDA CD54
R2
392k
R4
300k
BIT 3
R5
600k
R6
1.2M
BIT 2
BIT 1
R7
2.4M
16-STEP OUTPUT
ADJUSTS FROM
+20V TO +30V.
ASSUMES 0V
AND 3.3V CMOSLOGIC LEVELS.
BIT 0
The 8-pin, step-down, switching regulator IC of Figure
14 is configured as a high-efficiency 1A current source,
activated via a logic-level signal. The op amp (IC2)
monitors the charging current with a sense resistor (R10)
and applies feedback to the regulator chip. This “high
side” current sensing lets the negative battery terminal
connect directly to ground.
TO FB
TO OUTPUT
R8
10k
R9
25k
R10
470k
POTENTIOMETER
CIRCUIT ADJUSTS
FROM +22V TO +30V
WITH VALUES SHOWN
Switch-mode battery charging offers advantages, even for
low-cost applications; it dissipates less power and makes
full use of an ac adapter as a power source. Linearregulator designs typically require wall cubes with twice
the power rating, after you consider high- and lowamplitude extremes for the ac-line voltage. Linear designs
also require heatsinks to implement fast charging.
Figure 12. This circuit produces a bias (contrast) voltage for LCD
panels that can be adjusted either with a potentiometer or
digitally with a 4-bit homemade D/A converter.
Multiple supply voltages
Many portable designs require more than one supply
voltage. Even as IC manufacturers add to the list of
functions that can be powered from standard 3.3V and
5V levels, the need to optimize performance, weight,
battery life, and cost continues to justify additional
voltages. Fortunately, the use of multi-output ICs
minimizes the number of components needed to create
these voltages. These ICs minimize the board area and
the number of “glue” components required, while
improving the system’s low-load efficiency and other
performance parameters.
The circuit shown generates a regulated current for
charging a 3-cell battery. A 5.1V zener diode (D3)
clamps the output at approximately 6.3V when the
battery is removed. You can adjust for other battery
voltages and currents by changing R5, R10, and D3. The
operating-voltage range is 5V to 16V, but surges to 24V
are allowed (with some output error).
With two ICs you can design a four-output power supply
for hand-held organizers, computers, or data terminals
(Figure 13). The output voltages are 5V for PCMCIA
slots and analog circuitry, 3.3V for CPU and RAM, 12V
for flash memory, and -17V for LCD backplane bias.
If a fast charge is desired but no CPU resources are
available, an “all-in-one” controller may solve the
problem (Figure 15). IC1 is a low-cost NiCd charge
controller operating in a low-loss, switch-mode charging
configuration. The DRV pin drives a p-channel MOSFET
(Q1) via the bipolar-transistor buffer Q1–Q2. The cell
count (2 to 16), charge rate, and trickle-charge current are
pin programmed via the IC’s PGM0–PGM3 inputs.
A fifth regulator—a micropower boost circuit—is
included for backup during battery replacement. It sits
The circuit terminates a fast charge automatically by
detecting a negative slope in the curve of battery voltage
10
B2 LITHIUM
CR2032
C5
100µF
V1
+3.3V
250mA
L1
D1 22µH
1N5817
C1
100µF
L2
C4
22µH
0.1µF
16
15
V+
LX3
D12
CS12
L3
220µH
7
D4
1N914
D3
1N4001
8
13
Q2
2N2955
R2
330Ω
C2
47µF
R1
0.22Ω
LXB
FB12
FB3
PFO
LIN
BKUP
VREF
C3
0.22µF
B1
2x
AA CELLS
Q1
V2
+12V/120mA
Flash VPP
11
MAX718
DCIN
5
12
D2
1N5817
12ON
AGND 12/5
GND
14
10
9
3.3V LOW
3
WALL CUBE ON
1
ACTIVATE BACKUP
2
12V ON
4
TO 3.3V
6
C6 100µF
C7
0.1µF
L4
D5 22µH
1N5817
V3
+5V
200mA
16
15
V+
CS+
LX3
C8
100µF
D7
1N4001
EXTERNAL
≈ 9V
DC SOURCE
MAX722
DHI
FB3
DLO
13
R6
330Ω
R3
1Ω
CS- 10
7
Q4
2N2955
9
3
2
1
11
D6
Q3 1N5818
L5
47µH
LIN
FBN
C9
2.2µF
SHDN
GND
14
R4
1.5M
8
3/5
NEGON
V4
-17V LCD
POWER
12
R5
110k
VREF 5
PFO 4
AGND
6
5V LOW
C10
0.22µF
5V AND LCD OFF
LCD ON
Figure 13. These two ICs perform a multitude of power-related tasks in a system powered by two AA cells. They generate four supply voltages,
supervise the system power, control a lithium backup battery, and provide a switchover between battery and wall-adapter outputs.
vs. time. For safety, it also provides an adjustable timeout
as backup for terminating the charge. Note that NiMH
batteries require termination at zero slope rather than
negative slope. For NiMH batteries, replace the MAX713
with the pin-compatible MAX712.
Figure 15 accommodates nominal 12V inputs such as a
car battery, and is therefore limited to charging batteries
of six cells or less. As shown, the PGM0–PGM3 connections set the fast-charge rate at one ampere and the
trickle-charge rate at 1/16 of that. The backup timer is set
for 90 minutes.
(Circle 1)
Reference:
1. High Frequency Power Converters, Sevens and
Wittlinger, Harris.
11
Si9405
Q1
0.15Ω
R5
0.15Ω
DALE IHSM-5832
L1
22µH
R10
0.15Ω
IOUT
1A ±5%
VIN
16µF
16µF
D2
6
CS
7
EXT
5
V+
C4
10µF
16V
D1
1N4740
10V
N.C.
1
MAX649
OUT
REF
SHDN
3
R1
200Ω
1W
R9
1k, 1%
D3
1N4733
5.1V
IC1
FB
C6
220µF
2
R11
1k, 1%
IC2
R6
2.7k
4
GND
8
MC34071
(MOTOROLA)
R7
10k, 1%
0.1µF
R8
10k, 1%
U2 = MOTOROLA MC34071
R4
100k
R2
510k
OFF
R3
100k
ON
Q1
2N3904
Figure 14. This 1A switch-mode current source supplies charging current to a grounded battery by sensing current on the “high side.” An op amp
senses the output current and supplies feedback to the dc-dc converter IC.
Q3
IRF9024
INPUT
8V TO 16V
LED
D3
R1
1k
C5
10µF
R2
4.7k
C6
10µF
D2
L1
COILCRAFT
(708) 639-6400
D03340
220µH
Q1
CMPTA06
FAST
CHARGE
Q4
CMPTA06
Q2
2N2907
CENTRAL
SEMICONDUCTOR
(516) 435-1110
D1
D1, D2 ARE
MBRS340T3
MOTOROLA
(602) 244-3576
470Ω
5
THI
8
15
V+
14
C1
1µF
DRV
FASTCHG
BATT+
BATT+
3
PGM SETTINGS
SHOWN ARE
FOR 3 CELLS,
1C CHARGE
RATE, AND 90
MIN TIMEOUT.
4
9
10
1 AMP (FAST)
2
PGM0
MAX713
BATT-
C3
10µF
12
BATT-
PGM1
TLO
6
R3
0.25Ω
PGM2
GND
13
PGM3
REF VLIMIT TEMP CC
16
1
7
11
C2
220pF
C4
0.1µF
Figure 15. A low-cost battery-charge controller is the heart of a low-dissipation, fast-charge switch-mode circuit. When the battery is fully charged,
the circuit shifts automatically to a C/16 trickle charge.
12
DESIGN SHOWCASE
Switching regulator/transformer steps down
from high voltage
Adding a transformer to a step-up dc-dc regulator
enables the regulator to accept inputs of 20V and
higher while operating in a flyback step-down mode
(Figure 1). The circuit of Figure 1 handles inputs up
to 30V (as shown), but is easily modified for higher
specific voltages. It was developed for use in a small
industrial controller whose non-ventilated case
required close attention to power dissipation.
A snubber network across T1 reduces this breakdownvoltage requirement, at the cost of efficiency. The
simplest snubber is the RC network shown in Figure
1. (A zener-diode type allows higher efficiency.) The
amount of “snubbing” required depends on the output
load, the circuit layout, and the parasitic elements
present. For IOUT = 250mA and VIN between 20V and
30V, this circuit’s efficiency is 66% (rising to 72%
when you remove the snubber). R2 limits the peak
current through Q1 and L1 to 0.33A.
IC1 is well suited to the application because it drives
an external switching transistor and derives its power
from an internal shunt regulator—both of which can
be made to accommodate a wide range of input
voltages. IC1 includes internal feedback resistors for
5V, 12V, or 15V outputs (the connection shown is for
5V). To set output voltages other than these, connect
a feedback divider between the circuit’s regulated
output and IC1’s feedback terminal (pin 6).
The internal shunt regulator is a zener diode, biased by
R1 at approximately 2mA (the allowed range is 1mA
to 20mA). Replacing R1 with a constant-current source
lets the circuit cope with a wider input voltage range.
The shunt regulator’s output is about 6V, which limits
the gate drive to Q1; therefore, Q1 should have a logiclevel gate threshold. (The VN88 MOSFET also works
well, though its 2.5V maximum gate threshold is
slightly high for this criterion.)
Though not optimal for efficiency, the transformer’s
1:1 turns ratio simplifies procurement by allowing the
use of a standard product such as the Coiltronix CTX
transformer shown. Its 1:1 ratio also enhances
stability by producing a duty cycle well below 50%.
An ideal 1:1 transformer would generate VIN + VOUT
at the bottom of the primary, but leakage inductance
causes real transformers to produce a somewhat
higher voltage. That voltage appears across Q1, so
Q1’s minimum breakdown voltage should be approximately 2VIN + VOUT.
INPUT
20V TO 30V
The SHDN input (pin 7, shown grounded) is a digital
on/off switch for the load and the switching circuit.
The shunt regulator remains active during shutdown
and provides a useful supply voltage for backup
memory, a real-time clock, or any general-purpose
logic (including a latching on/off switch). The shunt
regulator can supply 4000-series logic directly, or can
supply 5V-specified devices via two “dropper” diodes.
(Circle 2)
1
R1
10k
C1
47µF
2
V5
3
EXTL
V+
C2
0.1µF
9
EXTH
MAX773
SGND
CS
C3
0.1µF
Figure 1.
OUTPUT
5V/250mA
D1
C5
47nF
R3
1k
REF
8
SHDN
7
LBI
5
FB
6
GND
10
1N5401
T1
CTX 100-4P
12
13
Q1
TN25A
C4
100µF
11
R2
0.68Ω
This step-up switching regulator and 1:1 transformer steps down to 5V from input voltages as high as 30V.
13
DESIGN SHOWCASE
RS-485 data interface gives isolated,
full-duplex operation
10M bits/second (10Mbps) and line lengths to 1200
meters. Differential transmission provides noise
immunity. The circuit shown features controlledslew-rate drivers that minimize EMI and the reflections caused by improperly terminated cables. It also
enables error-free transmissions to 250kbps. To
achieve data rates to 2.5Mbps, substitute a full-slewrate MAX1480A for IC2, a MAX485 for IC3, and
R2–R5 values per Table 1.
The simple RS-485 circuit of Figure 1 provides fullduplex communications (simultaneous transmission
and reception) with only two essential packages (IC2
and IC3). Its balanced and differential data lines are
necessary for high-noise environments or for longdistance transmission between a computer and its
peripherals. Such transmissions are difficult, if not
impossible, with the single-ended circuitry of an RS232 transceiver.
The RS-485 standard allows for bidirectional, multipoint, party-line communications, with data rates to
IC3
8 V
CC
MAX483
4 DI
A 7
D
B 6
3 DE
1 R0
2 RE
R
GND
INPUT
5V
5
IC2
C1
22µF
C2
0.1µF
MAX1480B
VCC1
VCC1
D1
SYSTEM
GROUND
D2
IC1
74HC04
OR
EQUIVALENT
GND
PS
SD
DRIVER
INPUT
DI
RL RECEIVER
I/O
120Ω
VCC2
R1
DI′
DRIVER
R2
ENABLE
DE
DE′
RECEIVER
OUTPUT
DI
VCC2
DE
GND
R3
RO
VCC2
RO
1
28
2
27
3
26
4
25
5
24
6
7
MAX
253
23
MAX
483
22
8
21
9
20
10
19
11
18
12
17
13
16
14
15
R6
AC1
AC2
ISO VCC
B (Z)
ISO RO DRV
RL
120Ω
A (Y)
DRIVER
I/O
ISO DI IN
R4
ISO DE IN
ISO GND
R5
ISO DI DRV
ISO VCC
ISO DE DRV
ISO GND
ISO RO LED
ISO
GND
SYSTEM
GROUND
TABLE 1. PULL-UP AND LED DRIVE RESISTORS
Figure 1.
IC2
IC3
R1 (Ω)
R2 (Ω)
R3 (Ω)
MAX1480A
MAX485
200
200
MAX1480B
MAX483
200
510
R4 (Ω)
R5 (Ω)
R6 (Ω)
MAX DATA RATE
360
3k
360
200
2.5Mbps
3k
2.2k
3k
200
250kbps
IC2 and IC3 provide full-duplex data communications for cable lengths as long as 1200 meters.
14
IC2 is a complete half-duplex interface that includes
transceivers, optocouplers, a power driver, and a
transformer. The optocouplers transmit digital signals
across the internal isolation barrier, and the centertapped transformer transmits power across the barrier
from its logic (non-isolated) side to its isolated side.
The isolation barrier in IC2 typically withstands
1600Vrms for one minute or 2000Vrms for one second.
Any TTL/CMOS-logic family can drive the IC2
digital inputs through a series resistor. With resistive
pull-ups, the receiver outputs can drive any such logic
as well. IC2’s isolated outputs meet all RS-485
specifications.
IC3, powered by the isolated VCC, upgrades the halfduplex operation of IC2 to full duplex using IC2’s
own dedicated optocouplers. Pin 3 must be tied low
to disable IC3’s driver, and pin 4 should be left
floating. The driver outputs for IC2 and IC3 exhibit
high impedance when DE is low; bringing DE high
enables the outputs to function as line drivers.
(Circle 3)
15
DESIGN SHOWCASE
Autotransformer boosts maximum VOUT
Step-up dc-dc converters that operate from small
input voltages often have correspondingly low
maximum breakdown voltages of 5V to 6V, which
limits the maximum output voltage available from
such devices. Adding an autotransformer lets you
double VOUT without exceeding the IC’s breakdown
voltage.
6V, it produces a regulated 9V output with no more
than 4.5V across the IC (Figure 1). The circuit is
suitable for use in smoke alarms and other batterypowered equipment. It delivers 30mA at 9V from a
1.1V input, and as much as 90mA at 9V from a 1.5V
input.
A similar circuit for 2-cell inputs (Figure 2) delivers
30mA at 9V from 1.6V, and 80mA at 9V from 3.6V.
A properly wound center-tapped inductor acts like a
transformer with a 1:1 turns ratio. Combined with an
IC that normally boosts single-cell inputs as high as
(Circle 4)
T1
CTX20-1
(COILTRONICS)
T1
CTX33-1
(COILTRONICS)
1N5817
IC1
100µF
100µF
OUTPUT
9V
B1
(TWO
CELL)
R1
43k
MAX779
1
5
ILIM
LX
2
IN
8
FB
3
AGND
7
4
SHDN
PGND
3
R2
1k
LX
SHDN
OUT
REF
GND
7
0.1µF
VOUT
R1 = R2 ( 0.2025 -1)
B1
(ONE
CELL)
68µF
MAX857
1
FB
OUTPUT VOLTAGE vs. LOAD CURRENT
8
6
1µF
2
R2
10k
OUTPUT VOLTAGE vs. LOAD CURRENT
10
9
9
VIN = 3.6V
VIN = 1.5V
VIN = 1.1V
8
VOUT (V)
VOUT (V)
R1
68k
1N914
VOUT
R1 = R2 ( 1.25 -1)
10
8
7
7
6
6
5
0
20
40
60
80
VIN = 1.6V
VIN = 2.0V
5
100
0
LOAD CURRENT (mA)
Figure 1.
OUTPUT
9V
1N5817
68µF
IC1
20
40
60
80
100
LOAD CURRENT (mA)
An autotransformer allows a low-voltage step-up converter to boost single-cell inputs as high as 10V.
Figure 2.
16
Similar to Figure 1, this circuit accepts 2-cell inputs
and generates regulated outputs as high as 10V.
DESIGN SHOWCASE
±5V regulator accepts inputs from 2V to 12V
1:1 winding ratio, which causes the -5V output
magnitude to track that of the 5V output. This
negative-output generation isn’t possible with the
standard step-up topology (Figure 2) because neither
winding would see a VOUT-proportional voltage.
Configured as in Figure 1, the step-up dc-dc
converter IC1 and associated components produce
±5V from input voltages ranging from 2V to 12V.
Input voltages are negative with respect to the output
ground terminal. Transistor Q1 shifts the feedback
voltage to a level compatible with the IC, which is
about 1.5V relative to the chip’s GND pin.
Figure 1 offers two other advantages over the Figure
2 configuration. First, it remains in regulation when
VIN rises above the nominal output level. In Figure 2,
the inductor-diode leakage path forces VOUT to track
VIN for this condition. In Figure 1, VIN is limited by
IC1’s absolute-maximum voltage rating: V+ to
BATT- must not exceed 17V, so for VOUT = 5V the
input range is 2V to 12V. Second, the Figure 1 circuit
has no leakage path from input to output during
shutdown. With 50kΩ output loads and R1 = 100kΩ,
the total shutdown current is only 26µA.
By taking V+ from the highest voltage in the circuit
(VIN + VOUT), the chip minimizes internal loss by
maximizing the gate drive to its internal switching
MOSFET. When this MOSFET (between LX and
GND) turns off, the energy stored in T1’s primary
flows to the V+ output, generating a voltage across
the primary equal to V+ plus a diode drop.
The -5V output is generated similarly by the additional winding plus D2 and C6. Regulation is via T1’s
V+
R2
43k
Q1
BC2142
SHUTDOWN
T1
COILTRONICS
CTX20-4
C5
47µF
V+ LOAD
VOUT
GND
C1
47µF
C2
0.1µF
D1
1N5817
C3
0.1µF
8
V+
C6
47µF
VL
IC1
MAX761
VIN
7
100µA
LX
N1
V- LOAD
V4
R1
100k
SHDN
LBI GND
FB
REF
2
5
6
3
D2
1N5817
R3
15k
C4
0.1µF
VL = VIN WHEN N1 IS ON
VL = VOUT + VDIODE WHEN N1 IS OFF
Figure 1.
This regulator circuit produces ±5V from just two battery cells, whose terminal voltage may range above and below the positive
output level.
17
The efficiency in Figure 1 is about 70%—a little
lower than that of a standard step-up circuit (Figure
3). This efficiency data is based on VIN = 2.5V, representing two AA cells at 50% discharge. The circuit
can start with 50Ω loads and a 2.0V input, but it can’t
quite regulate with that combination of input and
load—the V+/V- outputs will sag to 3.88V/-3.68V.
modulation) control causes a variation in the
frequency of output ripple and noise. If this is undesirable, IC1 can be replaced with the MAX752 dc-dc
converter, whose current-mode PWM (pulse-width
modulation) control produces a constant switching
frequency (and somewhat lower efficiency due to
higher quiescent current).
Output noise (mostly fast spikes) is nominally
200mVp-p for a wide range of output loads. In
addition, IC1’s current-limited PFM (pulse-frequency
(Circle 5)
Table 1. 43k and 15k FEEDBACK RESISTORS
V+
V+
LOAD (Ω)
50
50
50
50
50
50
100
100
100
100
100
100
1000
1000
1000
1000
1000
∞
∞
Shutdown
V+
VL
VIN
LX
SHDN
VOUT
N1
GND
GND
SHUTDOWN LEAKAGE PATH
VL = VIN WHEN N1 IS ON
VL = VOUT - VIN WHEN N1 IS OFF
VL IS NEVER EQUAL TO VOUT
VLOAD (Ω) VIN (V)
2.5
Note 1
2.5
∞
2.5
10,000
2.5
1000
2.5
100
2.5
50
2.5
Note 1
2.5
∞
2.5
10,000
2.5
1000
2.5
100
2.5
50
2.5
Note 1
2.5
∞
2.5
10,000
2.5
1000
2.5
100
2.5
Note 1
2.5
∞
2.5
IIN (mA)
280
280
283
301
494
605
138
138
139
151
273
469
14.6
14.3
15.6
27.7
137
0.803
0.802
26µA
V+ (V)
4.93
4.93
4.93
4.93
4.90
4.78
5.00
5.00
5.00
5.00
4.96
4.93
5.00
5.00
5.00
5.00
5.00
5.00
5.00
V- (V)
7.17
5.38
5.08
4.87
4.63
6.84
5.28
5.06
4.86
4.73
6.27
5.08
4.92
4.66
Eff (%)
69.4
69.4
69.1
68.0
58.1
58.6
72.5
72.5
72.7
73.0
70.7
58.9
68.5
69.9
70.7
71.1
70.7
5.07
Measurements from veroboard prototype.
Efficiency would improve with ground plane PCB.
Note 1: V- components disconnected.
Figure 2.
The leakage path (dotted line) in this conventional
step-up switching regulator prevents regulation when
VIN exceeds VOUT.
Figure 3.
18
Conversion efficiency for the Figure 1 circuit is about
70%, depending on the input voltage and the output
loads.
NEW PRODUCTS
14-bit, 85ksps
serial ADC has
20µA shutdown
The 14-bit MAX194 analog-to-digital
converter (ADC) has ultra-low power
consumption, high accuracy, and high
speed, making it ideal for medical, instrumentation, and industrial applications. As
the low-power leader in 14-bit applications, it consumes only 80mW at the
maximum operating speed of 85ksps.
Because the MAX194 is the only device in
its class with shutdown capability (to
10µA), its power consumption drops even
further at lower sampling rates. At 1ksps,
for instance, the consumption is only
1mW.
10-bit, VOUT DACs
operate from 5V
• Draws only 140µA from 5V (MAX515)
• Buffered voltage outputs swing railto-rail
• Internal voltage reference (MAX503/
MAX504)
• Small 8-pin SO footprint (MAX515)
The MAX503/MAX504/MAX515
voltage-output digital-to-analog converters
(DACs) combine ultra-low power
consumption and small size with operation
from a single 5V supply. These features
make the devices ideal for a wide range of
applications—especially portable and
battery-powered systems. The serial-input
MAX515 draws only 140µA of operating
current. The parallel-input MAX503 and
serial-input MAX504 include internal
references, and draw only 260µA. Both
include a shutdown mode that lowers the
supply current to 40µA.
Besides power savings, the DACs save
real estate on the pc board. They come in
small packages, and their rail-to-rail output
buffers eliminate the op amp and associated components required with a currentoutput DAC. The MAX503 and MAX504
are capable of 4-quadrant multiplication,
and include true 10-bit accuracy, power-on
reset, and configurable gains of 1 or 2. To
The MAX194’s capacitive-DAC architecture provides an inherent track/hold
function with a fast, 2.4µs acquisition time.
Its internal calibration circuitry maintains
true 14-bit accuracy over temperature by
745mW
250mW
95mW
AD67
9
CUT
POWER
100x
CS50
14
AD78
72
MAX1
(Circle 7)
5V
5V
10-BIT
10-BIT
DAC
DAC
AMP
AMP
REF
REF
MAX503
MAX504
(Circle 6)
94
The MAX515 comes in an 8-pin
DIP/SO package and the MAX504 comes
in a 14-pin DIP/SO. The parallel-input
MAX503 comes in a 24-pin DIP/SO or a
shrink small-outline package (SSOP),
which requires less board area than an 8pin DIP. All are tested for operation over
the commercial (0°C to +70°C) and
extended-industrial (-40°C to +85°C)
temperature ranges. Prices for the
MAX515 start at $2.50 (1000 up, FOB
USA).
515
The MAX194 comes in 16-pin DIP
and SO packages, in versions tested for the
commercial (0°C to +70°C), extendedindustrial (-40°C to +85°C), and military
(-55°C to +125°C) temperature ranges.
Prices start at $14.00 (1000 up, FOB
USA).
<1mW
simplify equipment upgrades, the
MAX503/MAX504/MAX515 devices are
both software and hardware (plug-in)
compatible with Maxim’s 12-bit MAX530/
MAX531/MAX539 converters.
DATA IN
IN
DATA
correcting for linearity and offset errors,
and its separate analog and digital powersupply terminals minimize the effects of
digital noise. The device has a serial data
interface
and
pin-selectable
unipolar/bipolar input ranges.
VOLTAGE
VOLTAGE
OUT
OUT
50V, 200MHz
amplifier drives
high-resolution
monitors
The monolithic MAX445 is a lowcost, variable-gain transconductance
amplifier that drives high-resolution CRT
monitors directly. Combining a variablegain preamp with a high-voltage (50Vp-p)
open-collector output stage, it is suitable
for workstation and medical-imaging
displays with video resolutions as high as
1280 x 1024 and 1530 x 1280.
An internal bandgap reference enables
external adjustments at the differentialinput preamp for gain (contrast) and
output offset. A control input (TTL/
BLANK) turns off the output current
regardless of input signal. With an
external peaking network, the MAX445
delivers 2.5ns rise times at 45Vp-p into an
external load of 200Ω and 8pF (including
the CRT and parasitics).
The MAX445 comes in a 24-pin
power-tab DIP, which requires additional
heatsinking to maintain its internal
junctions within the recommended range
for operating temperature. The device is
characterized for a case-temperature
operating range of 0°C to +90°C.
(Circle 8)
19
NEW PRODUCTS
950MHz JFET video
buffer includes
75Ω trimmed
resistor
The MAX4005 is the first ultra-highspeed video buffer with a trimmed, 75Ω
output resistor to minimize reflections
produced by mismatched impedances on a
transmission cable. The buffer’s JFET
input stage has an extremely low input
current (10pA), making the MAX4005
ideal for high-speed applications that
require isolation between a highimpedance signal source and a lowimpedance 75Ω cable.
CMOS analog
multiplexers offer
extremely low
leakage
The MAX338/MAX339, 8-channel/
dual 4-channel multiplexers exhibit
extremely low leakage currents: INO(OFF)
is less than 20pA at +25°C, and ICOM(ON)
is less than 50pA at +25°C. The maximum
channel on-resistance is 400Ω, and the onresistances in a device match to within
10Ω. Because the channels conduct
equally well in both directions, either
device is suitable for use as a multiplexer
or demultiplexer. Switching-transition
times are less than 500ns.
Active, in-line
circuit protectors
guard signal lines
The MAX366 and MAX367 each
contain multiple 2-terminal circuit protectors. Placed in series with signal lines, the
protectors guard sensitive circuitry against
fault conditions that produce line voltages
near or beyond the supply voltages. During
a fault, the line voltage can differ from the
opposite-polarity supply voltage by as
much as 40V. The protectors are active
during power-up, during power-down, and
when the supplies are off.
The MAX366 contains three protectors, and the MAX367 contains eight. Each
High-speed performance parameters
include a 950MHz, -3dB bandwidth; gain
flatness within ±0.1dB to 60MHz; a
1000V/µs slew rate; and 350ps rise/fall
times. The MAX4005 also offers
precision: 3mV maximum offset voltage,
±1nA maximum input current, a -28dB
3rd-order intercept at 100MHz, better than
-60dB 3rd-harmonic distortion at 50MHz,
and low differential gain and phase errors
of 0.11% and 0.03°.
pliers for 75Ω signal distribution. The
MAX4005 comes in an 8-pin SO package,
tested for the commercial (0°C to +70°C)
temperature range. Prices start at $2.75
(FOB USA).
(Circle 9)
MAX4005 applications—for video,
medical, test and measurement, diagnostic,
and ATE systems—include video buffers
and line drivers, impedance transformers,
remote-sense amplifiers, and fanout multi-
These new ICs feature extremely low
charge injection—only 1.5pC at +25°C
(5pC maximum). Fabricated with Maxim’s
44V silicon-gate process, they guarantee
protection per MIL-STD-883, Method
3015.7 against electrostatic discharge
(ESD) greater than 2000V. Each operates
from a single supply of +4.5V to +30V or
dual supplies of ±4.5V to ±20V. All
control inputs remain TTL compatible
over the specified ranges of temperature
and supply voltage.
tance, consider the MAX328 and MAX329
multiplexers.) MAX338/MAX339 devices
come in 16-pin DIP and narrow-SO
packages, in versions tested for the
commercial (0°C to +70°C), extendedindustrial (-40°C to +85°C), and military
(-55°C to +125°C) temperature ranges.
Prices start at $2.39 (1000 up, FOB USA).
(Circle 10)
The MAX338 and MAX339 are
improved, pin-compatible electrical
upgrades that replace the industry-standard
DG508A and DG509A at no additional
cost. (For even lower leakage and charge
injection at the expense of higher on-resis-
protector is a series connection of two nchannel FETs and one p-channel FET,
configured so the overall on-resistance is
very high with power off and about 100Ω
with power on. Leakage currents are less
than 1nA at +25°C. The devices are
suitable for analog or digital lines, and
operate with unipolar supplies of +4.5V to
+36V or bipolar supplies of ±2.25V to
±18V. Each protector is fully symmetrical,
which allows the input and output
terminals to be freely interchanged.
As the signal voltage approaches
within 1.5V (approximately) of either
supply voltage, the on-resistance increases
dramatically and limits the output voltage
and fault current. On the protected side,
20
the signal voltage clamps approximately
1.5V below the supply rail, maintaining its
polarity without polarity reversals or
“glitches.”
The MAX366 comes in 8-pin DIP and
SO packages; the MAX367 comes in 18pin DIP and SO packages. Both are
available in versions tested for the
commercial (0°C to +70°C), extendedindustrial (-40°C to +85°C), and military
(-55°C to +125°C) temperature ranges.
Prices for the MAX366 start at $1.42
(1000 up, FOB USA).
(Circle 11)
NEW PRODUCTS
Design improvements have guaranteed
extremely low charge injection (<5pC) and
low power consumption per package
(<10µW for switches, <300µW for muxes).
Switch leakage is low: <250pA at +25°C
and <2.5nA at +85°C. For muxes at 5V and
+85°C, the NO-off leakage is <1nA and the
COM-off leakage is <2.5nA. Mux transition
times are <100ns at 5V. The switches turn
on in <175ns, turn off in <100ns, and
guarantee ESD protection in excess of 2kV.
Analog switches
and multiplexers
guarantee matched,
low on-resistances
The MAX381/MAX383/MAX385
dual analog switches and the MAX398/
MAX399 (8-channel and dual 4-channel
analog multiplexers) offer precision, high
speed, and low-voltage operation. Ideal for
5V systems, these devices feature low onresistances (<35Ω for switches, <100Ω for
muxes) flat to within 4Ω (maximum) over
the analog signal range. On-resistances are
also matched between channels to within
2Ω for switches and 10Ω for muxes.
Switch configurations are SPST, NO
(MAX381); SPDT, NO (MAX385); and
SPDT, NO/NC (MAX383).
MAX398 and MAX399 multiplexers are
pin compatible with the industry-standard
DG408/DG409 and DG508A/DG509A, and
they come in 16-pin DIP and SO packages.
So do the MAX381/MAX383/MAX385
switches, which are pin-compatible with the
industry-standard DG401/DG403/DG405.
All are available in versions tested for the
commercial (0°C to +70°C), extended-industrial (-40°C to +85°C), and military (-55°C to
+125°C) temperature ranges. Prices start at
$1.47 for the MAX381, $2.57 for the
MAX383/MAX385, and $2.50 for the
MAX398/MAX399 (1000 up, FOB USA).
These CMOS devices are fabricated
with Maxim’s low-voltage silicon-gate
process. They maintain fast switching and
CMOS-logic compatibility while operating
with a single positive supply (+2.7V to
+16.5V) or dual supplies (±2.7V to ±8V).
(Circle 12)
matically when VOUT drops below 2.96V.
Preceding the shutdown, an internal powerfail comparator issues an early warning of
low output voltage. While in shutdown, the
output is latched off and remains off until
the ON input is pulsed. This procedure
prevents the further discharge that can
damage depleted battery cells in a portable
telephone or other battery-powered
equipment.
3V and 3.3V linear
regulators have
automatic
shutdown
• MAX687/MAX688/MAX689 for
portable applications
The MAX687/MAX688/MAX689 are
low-dropout linear regulators whose inputto-output voltage is limited only by an
external pnp pass transistor. Base-drive
capability exceeds 10mA, enabling a highgain pass transistor to supply more than
1A of load current.
MAX687/MAX688/MAX689 devices
are available in 8-pin DIP, SO, and µMAX
packages, in versions tested for the commercial (0°C to +70°C) and extended-industrial
(-40°C to +85°C) temperature ranges. Prices
start at $1.60 (1000 up, FOB USA).
The MAX687’s output voltage is fixed
at 3.3V, and the device shuts down auto-
(Circle 13)
LOW-COST LINEAR-REGULATOR CONTROLLER
VIN
2.2µF
10nF
C1
C1
SHDN
MAX688
PFO
µMAX
ZMM718
or
FZT749
VOUT
(or DIP/SO Package)
22µF
C1
21
3V and 3.3V linear
regulators offer
accuracy and low
dropout
• MAX688/MAX689 for 4A applications
• 0.8V dropout with 4A IOUT
The MAX688 (3.3V) and MAX689
(3.0V) ICs form linear regulators in which
an external pnp transistor determines the
dropout voltage. The ICs can sink
minimum-guaranteed base currents of
10mA, allowing high gain transistors
(ß>100) to deliver load currents greater
than 1A. Or, two external transistors in a
quasi-Darlington configuration can boost
the output current to 4A or more.
The MAX688 and MAX689 differ
only in output voltage, and offer an activelow SHDN input in place of the automaticshutdown feature. As SHDN falls, the chip
first enters a <25µA standby mode in
which the internal comparators and
reference remain active, enabling the
normal standby transition to occur at a
well-defined level (specified to within
±2%). Thus, a declining battery voltage
can be used to trigger the shutdown.
Seventy millivolts of hysteresis prevents
chatter between the normal and standby
modes, and full shutdown (<1µA) occurs
when SHDN falls below 200mV.
MAX688/MAX689 devices are
available in 8-pin DIP, SO, and µMAX
packages, in versions tested for the
commercial (0°C to +70°C) and extendedindustrial (-40°C to +85°C) temperature
ranges. Prices start at $1.60 (1000 up,
FOB USA).
(Circle 14)
NEW PRODUCTS
Smallest available
µP supervisors
come in SOT-23
packages
MAX809 and MAX810 microprocessor supervisors are the smallest such
devices available. Fully specified over
temperature, they assert a reset signal
whenever VCC falls below a preset threshold. When used in 3V or 5V systems, they
provide excellent reliability and low cost
by eliminating all external components and
adjustments. Typical supply currents are
only 24µA (L and M versions) and 17µA
(R, S, and T versions).
Multifunction µP
supervisors
monitor 3V and
3.3V systems
The MAX793/MAX794/MAX795*
microprocessor supervisors monitor and
control the activities of 3V and 3.3V µPbased applications, such as battery-powered
computers and controllers, automotive
systems, and portable equipment.
Supervisory features include active-low and
active-high reset outputs, low-line early
warning, internal switch for the backup
battery, internal switch for main power,
driver for external FET or pnp switch,
The MAX809 and MAX810 differ
only in the polarity of their reset outputs.
The MAX809 issues an active-low RESET
(valid for V CC down to 1V), and the
MAX810 issues an active-high RESET.
Both ignore fast transients on the VCC rail,
but once a reset is asserted, it remains
active for at least 140ms after VCC returns
above the trip threshold. The available
thresholds are designated by letter suffix:
4.63V (L), 4.38V (M), 3.08V (T), 2.93V
(S), and 2.63V (R).
Applications include computers,
controllers, intelligent instruments, and
portable/battery-powered equipment. The
MAX809 and MAX810 come in 3-pin
power-fail comparator, battery-OK output,
software watchdog, isolation to guarantee
battery freshness, manual-reset input, and
chip-enable gating.
The MAX793 offers all the above
features with four choices of resetthreshold range, as indicated by suffix
letter: U (3.00V to 3.13V), T (3.00V to
3.15V), S (2.85V to 3.00V), and R (2.55V
to 2.70V). The MAX794 is similar, but
substitutes a user-programmable threshold
for the battery-OK function. The 4function MAX795 device offers the
U/T/S/R ranges in an 8-pin package.
All three devices guarantee reliable
resets for V CC as low as 1V, on-board
SOT-23 packages, with specifications
guaranteed over the extended industrial
temperature range (-40°C to +85°C).
Prices start at $0.80 (3000 up, FOB USA).
MAX809
SMALLEST SIZE,
NO EXTERNAL COMPONENTS
LOWEST-COST SOLUTION
VCC
RESET
SOT-23
(Circle 15)
chip-enable gates with a maximum propagation delay of 10ns, and the capability to
withstand backup-battery voltages higher
than V CC during normal operation. The
MAX793 and MAX794 offer independent
watchdog timers with 1.6 second timeouts,
and an uncommitted voltage monitor for
power-fail or low-battery warnings.
The MAX793 and MAX794 come in
16-pin DIP and narrow-SO packages; the
MAX795 comes in 8-pin DIP and SO
packages. All are available in versions
tested for the commercial (0°C to +70°C)
and extended-industrial (-40°C to +85°C)
temperature ranges.
* Contact factory for availability.
(Circle 16)
3.3V RS-485/RS422 transceivers
transmit to 10Mbps
Maxim’s low-power 3.3V transceivers
provide true RS-485 and RS-422 communications without the extra die size and
extra pins associated with internal charge
pumps. Instead, the devices include a
proprietary output stage with low forward
drop (patent pending) that delivers an
industry first—2V minimum into 100Ω or
1.5V into 54Ω, while operating from
supply voltages as low as 3.0V. Each IC
(MAX3483, MAX3485, MAX3486,
MAX3488, MAX3490, and MAX3491)
contains one driver and one receiver. As
many as 32 of these transceivers may
connect to one bus.
Slew-rate-limited drivers in the
MAX3483 and MAX3488 reduce EMI and
reflections 100 times, compared with other
RS-485 devices. These transceivers meet
RS-485 and RS-422 specifications down to
3V, and guarantee error-free transmission
at data rates to 250kbps. Partial slew-rate
limiting in the MAX3486 allows transmission to 2.5Mbps, and the nonlimited
MAX3485/MAX3490/MAX3491
transceivers run effortlessly at 10Mbps.
The full-duplex MAX3488 and
MAX3490 are pin-compatible with the 75179
transceiver, and the full-duplex MAX3491
(with separate driver/receiver enables) is pincompatible with the 75180. The half-duplex
MAX3483/MAX3485/MAX3486 are pincompatible with the 75176. All six Maxim
transceivers operate with 1mA supply currents
and dissipate only 3.3mW—100-times less
than their 5V counterparts. All but the
MAX3490 and MAX3491 have low-current
2nA shutdown modes.
22
Driver-overload protection includes
foldback current limiting, which guards
each output against short circuits and other
fault conditions over the whole range of
input common-mode voltage (-7V to 12V).
Thermal-shutdown circuitry prevents
excessive power dissipation by disabling
the driver outputs. As a fail-safe measure,
each receiver output guarantees a logichigh level when both inputs are open.
The MAX3483, MAX3485, MAX3486,
MAX3488, and MAX3490 come in 8-pin
DIP and SO packages; the MAX3491 comes
in 14-pin DIP and SO packages. All are
available in versions tested for the commercial (0°C to +70°C) and extended-industrial
(-40°C to +85°C) temperature ranges. Prices
start at $1.75 (1000 up, FOB USA).
(Circle 17)
NEW PRODUCTS
AutoShutdown™
lowers RS-232
transceivers’
supply current
to 1µA
Maxim’s new RS-232 transceivers
include a proprietary AutoShutdown
function (patent pending): except when
actively in use, they automatically enter a
low-power mode. As a result, the supply
currents fall to 1µA when the input signals
have non-valid RS-232 levels. The patentpending internal circuitry saves power and
extends battery life: between data transmissions, when the cable is disconnected,
and when the transceiver at the far end of
the cable is turned off. These power
savings require no modification of the
existing BIOS or operating system.
The MAX3212 and MAX3243* each
contain three drivers and five receivers,
providing complete serial ports ideal for
notebook and subnotebook computers. The
MAX3212 operates with a supply voltage
of 2.7V to 3.6V (yet remains compatible
Custom packaging
accommodates
high-frequency,
high-power
circuits
Maxtek’s custom multichip modules
(MCMs) contain circuits that comprise 20
to 200 separate components running at
frequencies from 50MHz to 15GHz.
Sampling heads and other specialized
MCMs can operate at up to 50GHz.
Maxtek’s in-house laser-trimming capability enables adjustment of resistors to
within 0.1%, capacitors to within 0.5pF,
and time events to within picoseconds.
The MCM optimizes or tunes the
performance of other circuitry in a typical
application. At 50MHz and above, for
example, the attenuator/preamp/ADC
portion of a data-acquisition circuit may
lack the gain necessary to flatten a step
response. Substituting an MCM for the
attenuator cures this problem by compensating the preamp’s roll-off. The MCM
undergoes final adjustment in an active
with 5V logic), and the MAX3243 operates
(with four small external capacitors) from
3.0V to 5.5V. Over their operating ranges
of temperature and supply voltage, both
transceivers meet all EIA/TIA-232E,
EIA/TIA-562, and V.28/V.24 specifications. The guaranteed-minimum data rates
(235kbps for the MAX3212 and 120kbps
for the MAX3243) assure compatibility
with popular data-communications
software for personal computers.
To produce the ±5V-minimum transmitter outputs specified by RS-232, the
MAX3212 employs an internal switchmode controller that generates ±6.5V from
a single, low-cost, external inductor. The
MAX3243 generates the same levels with
a capacitive doubler-inverter circuit
followed by a proprietary low-dropout
transmitter. Both devices drive serial mice,
and both have convenient flow-through
pinouts.
One receiver in each device maintains a
second, complementary output active
regardless of shutdown status. When VCC is
turned off, that output can monitor an
laser-trimming jig, in which a test system
flattens the step response by adjusting the
operating circuit in 0.01% increments.
MCMs let you combine ICs and other
components representing many different
technologies. Prescalers and mixers, for
example, may require a mixture of silicon
and gallium-arsenide chips. Data-acquisition modules may require a mixture of
high- and low-power components. An
MCM can combine all of these in one
package, along with crystals and other
types of optical and electromechanical
devices. The available MCM options
include standard or custom surface-mount
types, socketed daughter boards, flex
circuits, hermetic packages, custom
packages with integrated heatsinks, and
JEDEC packages with more than 100 pins.
Maxtek is a new company formed by
Maxim and Tektronix to perform design,
testing, and manufacturing of complex,
custom multichip modules and hybrids. As
the descendent of Tektronix’ internal MCM
facility, Maxtek has produced more than
8,000,000 HF MCMs in the past 20 years.
23
external modem or other circuit without
forward-biasing the circuit’s protection
diode. The MAX3212’s receiver-enable
input (EN) can three-state the receiver
outputs or activate all five, with no effect on
the shutdown current. (The MAX3223—a
smaller, dual-transmitter/dual-receiver
version of the MAX3243—offers an EN
input in place of the always-active receiver
output.) MAX3212/MAX3243 devices
include FORCEON/FORCEOFF controls
for overriding the AutoShutdown™ circuit
if desired.
The MAX3212 and MAX3243 come
in 28-pin wide-SO and SSOP packages,
and the MAX3223 comes in a 20-pin DIP
and SSOP. All are available in versions
tested for the commercial (0°C to +70°C)
and extended-industrial (-40°C to +85°C)
temperature ranges. Prices start at $1.85
for the MAX3223 and $3.29 for the
MAX3212/MAX3243 (1000 up, FOB
USA).
(Circle 18)
* Contact factory for availability.
™AutoShutdown is a trademark of Maxim
Integrated Products.
Maxtek MCMs can operate from
-15°C to +70°C. Prices range from $2,000
to $4 each on quantities of 50 to 500,000
per year, and prototype charges begin at
$10,000 with deliveries as short as four
weeks. For a limited time, Maxtek is
offering free engineering consultations by
telephone on the design of actual highfrequency MCMs. Please contact Maxtek
for more information (1-800-4-MAXTEK).
ORIGINAL PREAMP OUTPUT
100mV
80mV
60mV
40mV
20mV
10ns/div
ATTENUATOR
PREAMP
ADC
SOLUTION
COMPENSATED
ATTENUATOR INPUT
IMPROVED
PREAMP OUTPUT
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