MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers _______________General Description

MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers _______________General Description
19-0305; Rev 3; 3/09
KIT
ATION
EVALU
E
L
B
A
AVAIL
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
The MAX1649/MAX1651 BiCMOS, step-down, DC-DC
switching controllers provide high efficiency over loads
ranging from 1mA to more than 2.5A. A unique, currentlimited pulse-frequency-modulated (PFM) control scheme
gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads),
while using only 100µA of supply current (vs. 2mA to
10mA for PWM converters). Dropout performance down
to 300mV is provided by a high switch duty cycle (96.5%)
and a low current-sense threshold (110mV).
A high switching frequency (up to 300kHz) allows these
devices to use miniature external components.
The MAX1649/MAX1651 have dropout voltages less
than 0.3V at 500mA and accept input voltages up to
16V. Output voltages are preset at 5V (MAX1649), or
3.3V (MAX1651). They can also be adjusted to any
voltage from 1.5V to the input voltage by using two
resistors.
These step-down controllers drive external P-channel
MOSFETs at loads greater than 12.5W. If less power is
required, use the MAX639/MAX640/MAX653 step-down
converters with on-chip FETs, which allow up to a
225mA load current.
________________________Applications
PDAs
High-Efficiency Step-Down Regulation
5V-to-3.3V Green PC Applications
Battery-Powered Applications
____________________________Features
♦ More than 90% Efficiency (10mA to 1.5A Loads)
♦ More than 12.5W Output Power
♦ Less than 0.3V Dropout Voltage at 500mA
♦
♦
♦
♦
100µA Max Quiescent Supply Current
5µA Max Shutdown Supply Current
16V Max Input Voltage
5V (MAX1649), 3.3V (MAX1651), or Adjustable
Output Voltage
♦ Current-Limited Control Scheme
♦ Up to 300kHz Switching Frequency
♦ Up to 96.5% Duty Cycle
______________Ordering Information
PART
TEMP RANGE
MAX1649CPA
0°C to +70°C
8 Plastic DIP
PIN-PACKAGE
MAX1649CSA
MAX1649C/D
MAX1649EPA
MAX1649ESA
MAX1651CPA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
8 SO
Dice*
8 Plastic DIP
8 SO
8 Plastic DIP
MAX1651CSA
MAX1651C/D
MAX1651EPA
MAX1651ESA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 SO
Dice*
8 Plastic DIP
8 SO
*Dice are tested at TA = +25°C.
__________Typical Operating Circuit
INPUT
3.6V TO 16V
__________________Pin Configuration
TOP VIEW
V+
MAX1651
ON/OFF
SHDN
CS
EXT
OUT
REF
FB
GND
P
OUT
1
8
GND
FB
2
7
EXT
6
CS
5
V+
SHDN 3
OUTPUT
3.3V
MAX1649
MAX1651
REF 4
DIP/SO
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
1
MAX1649/MAX1651
_______________General Description
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to GND.......................................-0.3V, +17V
REF, SHDN, FB, CS, EXT, OUT .......................-0.3V, (V+ + 0.3V)
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C) ..........................471mW
Operating Temperature Ranges
MAX1649C_A, MAX1651C_A ..............................0°C to +70°C
MAX1649E_A, MAX1651E_A ............................-40°C to +85°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
V+ Input Voltage Range
V+
Supply Current
I+
CONDITIONS
VOUT < V+
MIN
V+ = 16V, SHDN ≤ 0.4V (operating, switch off)
78
V+ = 16V, SHDN ≥ 1.6V (shutdown)
2
V+ = 10V, SHDN ≥ 1.6V (shutdown)
FB Trip Point
FB Input Current
IFB
Output Voltage
VOUT
Reference Voltage
VREF
TYP
3.0
MAX
UNITS
16
V
100
µA
1
5
MAX1649C, MAX1651C
1.470
1.5
1.530
MAX1649E, MAX1651E
1.4625
1.5
1.5375
MAX1649C, MAX1651C
±50
MAX1649E, MAX1651E
±70
MAX1649, V+ = 5.5V to 16V
4.80
5.0
5.20
MAX1651, V+ = 3.6V to 16V
3.17
3.3
3.43
MAX1649C, MAX1651C, IREF = 0μA
1.470
1.5
1.530
MAX1649E, MAX1651E, IREF = 0μA
1.4625
1.5
1.5375
V
nA
V
V
REF Load Regulation
0µA ≤ IREF ≤ 100µA, sourcing only
4
10
mV
REF Line Regulation
3V ≤ V+ ≤ 16V
40
100
µV/V
Output Voltage
Line Regulation
Circuit of
Figure 1
Output Voltage
Load Regulation
Circuit of
Figure 1
Circuit of
Figure 1
Efficiency
SHDN Input Current
2.6
MAX1651, 3.6V ≤ V+ ≤ 16V,
ILOAD = 1A
1.7
MAX1649, 0A ≤ ILOAD ≤ 1.5A,
VIN = 10V
-47
MAX1651, 0A ≤ ILOAD ≤ 1.5A,
VIN = 5V
-45
MAX1649, V+ = 10V,
ILOAD = 1A
90
mV/V
mV/A
%
MAX1651, V+ = 5V,
ILOAD = 1A
90
V+ = 16V, SHDN = 0V or V+
SHDN Input Voltage High
VIH
3V ≤ V+ ≤ 16V
SHDN Input Voltage Low
VIL
3V ≤ V+ ≤ 16V
2
MAX1649, 5.5V ≤ V+ ≤ 16V,
ILOAD = 1A
1
1.6
_______________________________________________________________________________________
µA
V
0.4
V
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
(V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Current-Limit Trip Level
(V+ to CS)
VCS
CONDITIONS
3V ≤ V+ ≤ 16V
MIN
TYP
MAX
UNITS
80
110
140
mV
CS Input Current
3V ≤ V+ ≤ 16V
±1
µA
Switch Maximum On-Time tON (max)
V+ = 12V
24
32
40
µs
Switch Minimum Off-Time tOFF (min)
V+ = 12V
0.8
1.1
1.8
µs
EXT Rise Time
CEXT = 0.001µF, V+ = 12V
25
ns
EXT Fall Time
CEXT = 0.001µF, V+ = 12V
25
ns
Maximum Duty Cycle
tON
x 100%
tON + tOFF
96.5
%
95
__________________________________________Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
74
I+ (μA)
2.5
I+ (μA)
50
tRISE & tFALL (ns)
V+ = 16V
V+ = 10V
72
2.0
1.5
V+ = 8V
70
1.0
0.5
-60 -40 -20 0
20 40 60 80 100 120 140
EXT RISE AND FALL TIMES
vs. TEMPERATURE (5nF)
180
160
V+ = 5V, tFALL
140
120
V+ = 15V, tRISE
100
80
60
40
90
VOUT = 5V
CIRCUIT OF
FIGURE 1
TEMPERATURE (°C)
-60 -40 -20 0 20 40 60 80 100 120 140
EFFICIENCY
vs. LOAD CURRENT (VOUT = 3.3V)
80
70
TOP TO
BOTTOM:
VIN = 6V
VIN = 8V
VIN = 10V
VIN = 12V
VIN = 15V
60
50
V+ = 15V, tFALL
TEMPERATURE (°C)
V+ = 15V, tFALL
-60 -40 -20 0 20 40 60 80 100 120 140
1
100
1k
10
LOAD CURRENT (mA)
VOUT = 3.3V
CIRCUIT OF
FIGURE 1
90
80
TOP TO
BOTTOM:
70
VIN = 4.3V
VIN = 5V
VIN = 8V
VIN = 10V
VIN = 12V
VIN = 15V
60
50
40
0.1
100
EFFICIENCY (%)
tRISE & tFALL (ns)
200
20 40 60 80 100 120 140
MAX1649/51-A1
V+ = 5V, tRISE
100
EFFICIENCY (%)
CEXT = 5nF
V+ = 15V, tRISE
EFFICIENCY
vs. LOAD CURRENT (VOUT = 5V)
MAX1649/51-02
220
30
TEMPERATURE (°C)
TEMPERATURE (°C)
240
V+ = 5V, tFALL
35
15
0
-60 -40 -20 0
40
20
V+ = 4V
66
V+ = 5V, tRISE
45
25
V+ = 4V
68
CEXT = 1nF
55
3.0
76
MAX1649/51-01
3.5
10k
MAX1649/51-A2
V+ = 16V
60
MAX1649-TOC05
78
4.0
MAX1649-TOC06
80
EXT RISE AND FALL TIMES
vs. TEMPERATURE (1nF)
SHUTDOWN CURRENT
vs. TEMPERATURE
SUPPLY CURRENT vs. TEMPERATURE
40
0.1
1
100
1k
10
LOAD CURRENT (mA)
_______________________________________________________________________________________
10k
3
MAX1649/MAX1651
ELECTRICAL CHARACTERISTICS (continued)
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
SWITCH OFF-TIME
vs. TEMPERATURE
33.5
1.4
99
1.3
33.0
tOFF (μs)
32.5
32.0
DUTY CYCLE (%)
1.2
1.1
1.0
0.9
31.5
0.8
31.0
98
97
96
95
0.7
30.5
94
0.6
30.0
0.5
93
-60 -40 -20 0 20 40 60 80 100 120 140
-60 -40 -20 0 20 40 60 80 100 120 140
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
110
105
100
400
VOUT = 4.80V
300
VOUT = 3.17V
200
100
0
95
0
-60 -40 -20 0 20 40 60 80 100 120 140
0.5
1.0
1.5
LOAD CURRENT (A)
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
150
IREF = 50μA
100
50
IREF = 100μA
2.0
MAX1649-TOC01
1.506
REFERENCE OUTPUT VOLTAGE (V)
IREF = 10μA
200
MAX1649-TOC07
TEMPERATURE (°C)
250
REFERENCE OUTPUT RESISTANCE (Ω)
CIRCUIT OF
FIGURE 1
500
DROPOUT VOLTAGE (mV)
115
CS TRIP LEVEL (mV)
600
MAX1649/51-06
120
MAX1649/51-A3
DROPOUT VOLTAGE
vs. LOAD CURRENT
CS TRIP LEVEL
vs. TEMPERATURE
1.504
1.502
IREF = 10μA
1.500
1.498
1.496
1.494
1.492
0
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
4
100
MAX1649/51-04
1.5
MAX1649/51-03
34.0
MAXIMUM DUTY CYCLE
vs. TEMPERATURE
MAX1649/51-05
SWITCH ON-TIME
vs. TEMPERATURE
tON (μs)
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
MAX1649
LOAD-TRANSIENT RESPONSE
MAX1649
LINE-TRANSIENT RESPONSE
A
A
16V
1.6A
B
B
6V
0A
200μs/div
CIRCUIT OF FIGURE 1, V+ = 10V
A: VOUT = 5V, 100mV/div, AC-COUPLED
B: ILOAD = 30mA TO 1.6A, 1A/div
5ms/div
CIRCUIT OF FIGURE 1, ILOAD = 1A
A: VOUT = 5V, 100mV/div, AC-COUPLED
B: V+ = 6V TO 16V, 5V/div
MAX1649
SHDN RESPONSE TIME
5V
OUTPUT
0V
4V
SHDN
INPUT
0V
1ms/div
CIRCUIT OF FIGURE 1, V+ = 10V, ILOAD = 1A
_______________________________________________________________________________________
5
MAX1649/MAX1651
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
______________________________________________________________Pin Description
PIN NAME
FUNCTION
1
OUT
Sense Input for fixed 5V or 3.3V output operation. OUT is internally connected to the on-chip voltage divider.
Although it is connected to the output of the circuit, OUT does not supply current. Leave OUT unconnected for
adjustable-output operation.
2
FB
Feedback Input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB, and
GND for adjustable-output operation. See Setting the Output Voltage section.
3
SHDN
4
REF
1.5V Reference Output that can source 100µA. Bypass with 0.1µF.
5
V+
Positive Power-Supply Input
6
CS
Current-Sense Input. Connect current-sense resistor between V+ and CS. When the voltage across the resistor
equals the current-limit trip level, the external MOSFET is turned off.
7
EXT
Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.
8
GND
Ground
Active-High Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the reference, output, and external MOSFET are turned off. Connect to GND for normal operation.
VIN
V+
MAX1649
MAX1651
CS
3
4
SHDN
EXT
REF
OUT
FB
C3
0.1μF
2
5
C4
0.1μF
C1
100μF
R1
0.05Ω
6
7
1
P1
Si9430*
OUTPUT
@ 1.5A
L1
47μH**
GND
8
D1
NSQ03A02L
C2
330μF
*SILICONIX SURFACE-MOUNT MOSFET
**SUMIDA CDRH125-470
Figure 1. Typical Application Circuit
_______________Detailed Description
The MAX1649/MAX1651 are BiCMOS, step-down,
switch-mode power-supply controllers that provide
adjustable and fixed outputs of 5V and 3.3V, respectively. Their unique control scheme combines the
advantages of pulse-frequency-modulation (low supply
current) and pulse-width-modulation (high efficiency at
high loads). An external P-channel power MOSFET
allows peak currents in excess of 3A, increasing the
output current capability over previous PFM devices.
Figure 2 is the block diagram.
6
The MAX1649/MAX1651 offer four main improvements
over prior solutions:
1) The converters operate with miniature surface-mount
inductors, due to their 300kHz switching frequency.
2) The current-limited PFM control scheme allows
greater than 90% efficiencies over a wide range of
load currents (10mA to 1.5A).
3) Dropout voltage has been reduced to less than
300mV for many applications.
4) The quiescent supply current is only 100µA.
PFM Control Scheme
The MAX1649/MAX1651 use a proprietary, current-limited PFM control scheme. As with traditional PFM converters, the external power MOSFET is turned on when
the voltage comparator senses that the output is out of
regulation. However, unlike traditional PFM converters,
switching is accomplished through the combination of a
peak current limit and a pair of one-shots that set the
maximum switch on-time (32µs) and minimum switch
off-time (1.1µs). Once off, the off-time one-shot holds
the switch off for 1.1µs. After this minimum time, the
switch either 1) stays off if the output is in regulation, or
2) turns on again if the output is out of regulation.
The MAX1649/MAX1651 also limit the peak inductor current, which allows them to run in continuous-conduction
mode and maintain high efficiency with heavy loads
(Figure 3). This current-limiting feature is a key component of the control circuitry. Once turned on, the switch
stays on until either 1) the maximum on-time one-shot
turns it off (32µs later), or 2) the current limit is reached.
EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET.
_______________________________________________________________________________________
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
MAX1649/MAX1651
V+
FB
DUAL-MODE™
COMPARATOR
MAX1649
MAX1651
50mV
OUT
SHDN
ERROR
COMPARATOR
REF
1.5V
REFERENCE
N
Q
MINIMUM
OFF-TIME TRIG
ONE-SHOT
FROM V+
S
EXT
Q
F/F
MAXIMUM
TRIG ON-TIME Q
ONE-SHOT
R
CURRENT
COMPARATOR
CS
110mV
FROM V+
GND
Figure 2. Block Diagram
Shutdown Mode
Quiescent Current
When SHDN is high, the MAX1649/MAX1651 enter shutdown mode. In this mode, the internal biasing circuitry is
turned off (including the reference) and the supply current drops to less than 5µA. EXT goes high, turning off the
external MOSFET. SHDN is a logic-level input. Connect
SHDN to GND for normal operation.
In normal operation, the device's typical quiescent current is 78µA. In an actual application, even with no load,
additional current is drawn to supply external feedback
resistors (if used) and the diode and capacitor leakage
currents. In the circuit of Figure 1, with V+ at 5V and
VOUT at 3.3V, typical no-load supply current for the
entire circuit is 90µA.
Dual Mode is a trademark of Maxim Integrated Products, Inc.
_______________________________________________________________________________________
7
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
VIN
V+
MAX1649
MAX1651
CS
1.5A
1A
3
0A
4
SHDN
EXT
REF
OUT
GND
(
VOUT
R2 = R3
–1
VREF
)
C1
100μF
R1
0.05Ω
6
P1
Si9430
7
L1
47μH
OUTPUT
@ 1.5A
1
2
R2
8
C3
0.1μF
2μs/div
V+ = 10V, ILOAD = 1.3A
CIRCUIT OF FIGURE 1, R1 = 75mΩ
FB
C4
0.1μF
5
C2
330μF
D1
1N5820
R3
150k
VREF = 1.5V
Figure 3. MAX1649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
Modes of Operation
When delivering high output currents, the MAX1649/
MAX1651 operate in continuous-conduction mode. In
this mode, current always flows in the inductor, and
the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current
capability (Figure 3). This provides excellent load-transient response and high efficiency.
In discontinuous-conduction mode, current through the
inductor starts at zero, rises to a peak value, then
ramps down to zero. Although efficiency is still excellent, the output ripple increases slightly, and the switch
waveform exhibits ringing (at the inductor's self-resonant frequency). This ringing is to be expected and
poses no operational problems.
Dropout
The MAX1649/MAX1651 are in dropout when the input
voltage (V+) is low enough that the output drops below
the minimum output voltage specification (see
Electrical Characteristics). The dropout voltage is the
difference between the input and output voltage when
dropout occurs. See the Typical Operating
Characteristics for the Dropout Voltage vs. Load
Current and Dropout Voltage vs. Temperature graphs.
8
Figure 4. Adjustable-Output Operation
__________________Design Procedure
Setting the Output Voltage
The MAX1649/MAX1651 are preset for 5V and 3.3V output voltages, respectively; tie FB to GND for fixed-output
operation. They may also be adjusted from 1.5V (the
reference voltage) to the input voltage, using external
resistors R2 and R3 configured as shown in Figure 4. For
adjustable-output operation, 150kΩ is recommended for
resistor R3—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
VOUT
R2 = R3 x ——— -1
VREF
(
)
where VREF = 1.5V.
When using external resistors, it does no harm to connect OUT and the output together, or to leave OUT
unconnected.
Current-Sense Resistor Selection
The current-sense resistor limits the peak switch current to 110mV/RSENSE, where RSENSE is the value of
the current-sense resistor, and 110mV is the currentlimit trip level (see Electrical Characteristics).
_______________________________________________________________________________________
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
2.5
rs = 0.030
2.0
rs = 0.040
rs = 0.050
1.5
rs = 0.060
1.0
rs = 0.080
0.5
rs = 0.100
0
where ΔI is the inductor-current overshoot factor,
ILIM = VCS/RSENSE, and 0.3µs is the time it takes the comparator to switch. Set ΔI = 0.1 for an overshoot of 10%.
For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/ILIM works best. To
minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the inductor’s saturation-current rating is greater than ILIM(max).
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where the
inductance is 20% below its nominal value).
3.0
VOUT = 3.3V
2.5
rs = 0.030
2.0
rs = 0.040
1651 Fig05b
VOUT = 5V
1649 Fig05a
MAXIMUM OUTPUT CURRENT (A)
3.0
(V+(max) - VOUT) x 0.3µs
L(min) = ——————————––——
ΔI x ILIM
MAXIMUM OUTPUT CURRENT (A)
Inductor Selection
The MAX1649/MAX1651 operate with a wide range of
inductor values, although for most applications coils
between 10µH and 68µH take best advantage of the con-
trollers’ high switching frequency. With a high inductor
value, the MAX1649/MAX1651 will begin continuous-current operation (see Detailed Description) at a lower fraction of full-load current. In general, smaller values produce higher ripple (see below) while larger values require
larger size for a given current rating.
In both the continuous and discontinuous modes, the
lower limit of the inductor is important. With a too-small
inductor value, the current rises faster and overshoots the
desired peak current limit because the current-limit comparator has a finite response time (300ns). This reduces
efficiency and, more importantly, could cause the current
rating of the external components to be exceeded.
Calculate the minimum inductor value as follows:
rs = 0.050
1.5
rs = 0.060
1.0
rs = 0.080
0.5
rs = 0.100
0
5.0
5.4
5.8
6.2
6.6
16.0
INPUT VOLTAGE (V)
Figure 5a. MAX1649 Current-Sense Resistor Graph
3.0
3.4
3.8
4.2
4.6
16.0
INPUT VOLTAGE (V)
Figure 5b. MAX1651 Current-Sense Resistor Graph
_______________________________________________________________________________________
9
MAX1649/MAX1651
To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a function of the peak current, the peak current must not be
too low.
To choose the proper current-sense resistor for a particular output voltage, determine the minimum input
voltage and the maximum load current. Next, referring to Figures 5a or 5b, using the minimum input voltage, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the sense-resistor (±5%) and inductor
(47µH ±10%) values, the diode drop (0.4), and the
IC’s current-sense trip level (85mV); an external MOSFET on-resistance of 0.07Ω is assumed for VGS = -5V.
Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little inductance
and are well suited for use as current-sense resistors.
A U-shaped wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5mΩ and 0.1Ω
are available (see Table 1).
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
Table 1. Component Selection Guide
PRODUCTION
METHOD
Surface Mount
Miniature
Through-Hole
Low-Cost
Through-Hole
INDUCTORS
CAPACITORS
Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)
Coilcraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
CURRENT-SENSE
RESISTORS
MOSFETS
Siliconix
Little Foot series
AVX
TPS series
Motorola
MBRS340T3
Dale
WSL Series
Sprague
595D series
Nihon
NSQ series
IRC
LRC series
Motorola
medium-power
surface-mount products
IRC
OAR series
Motorola
Sumida
RCH875-470M (1.3A)
Sanyo
OS-CON series
low-ESR organic
semiconductor
Coilcraft
PCH-45-473 (3.4A)
Nichicon
PL series
Motorola
low-ESR electrolytics 1N5817 to
1N5823
United Chemi-Con
LXF series
The peak current of Figure 1 is 2.35A for a 1.5A output.
The inductor used in this circuit is specified to drop by
10% at 2.2A (worst case); a curve provided by the
manufacturer shows that the inductance typically drops
by 20% at 2.7A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency.
Table 1 lists inductor types and suppliers for various
applications. The efficiencies of the listed surfacemount inductors are nearly equivalent to those of the
larger size through-hole versions.
Diode Selection
The MAX1649/MAX1651’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817 through 1N5823 (and their surfacemount equivalents), are recommended. Choose a
diode with an average current rating equal to or greater
than I LIM (max) and a voltage rating higher than
V+(max).
External Switching Transistor
The MAX1649/MAX1651 drive P-channel enhancementmode MOSFET transistors only. The choice of power
transistor is primarily dictated by the input voltage and
the peak current. The transistor’s on-resistance, gatesource threshold, and gate charge must also be appropriately chosen. The drain-to-source and gate-tosource breakdown voltage ratings must be greater than
V+. The total gate-charge specification is normally not
10
DIODES
Motorola
TMOS power MOSFETs
critical, but values should be less than 100nC for best
efficiency. The MOSFET should be capable of handling
the peak current and, for maximum efficiency, have a
very low on-resistance at that current. Also, the onresistance must be low for the minimum available VGS,
which equals V+(min). Select a transistor with an onresistance between 50% and 100% of the currentsense resistor. The Si9430 transistor chosen for the
Typical Operating Circuit has a drain-to-source rating
of -20V and a typical on-resistance of 0.070Ω at 2A with
VGS = -4.5V. Tables 1 and 2 list suppliers of switching
transistors suitable for use with these devices.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter
capacitor is low equivalent series resistance (ESR),
rather than high capacitance. An electrolytic capacitor
with low enough ESR will automatically have high
enough capacitance. The product of the inductor-current variation and the output filter capacitor’s ESR
determines the amplitude of the high-frequency ripple
seen on the output voltage. When a 330µF, 10V
Sprague surface-mount capacitor (595D series) with
ESR = 0.15Ω is used, 40mV of output ripple is typically
observed when stepping down from 10V to 5V at 1A.
The output filter capacitor's ESR also affects efficiency.
Again, low-ESR capacitors perform best. Table 1 lists
some suppliers of low-ESR capacitors.
______________________________________________________________________________________
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
COMPANY
PHONE
FAX
AVX
USA
Coiltronics
Coilcraft
Dale
International
Rectifier
IRC
USA
USA
USA
(207) 282-5111
or
(800) 282-4975
(516) 241-7876
(708) 639-6400
(402) 564-3131
USA
(310) 322-3331
(310) 322-3332
USA
(512) 992-3377
Siliconix
USA
Sprague
USA
USA
Japan
(512) 992-7900
(602) 244-3576
or
(602) 244-5303
(708) 843-7500
81-7-5231-8461
(805) 867-2555
81-3-3494-7411
(619) 661-6835
81-7-2070-6306
(408) 988-8000
or
(800) 554-5565
(603) 224-1961
(708) 956-0666
81-3-3607-5111
USA
(714) 255-9500
Motorola
Nichicon
Nihon
Sanyo
Sumida
United
Chemi-Con
USA
USA
Japan
USA
Japan
USA
Japan
(207) 283-1941
(516) 241-9339
(708) 639-1469
(402) 563-1841
(602) 244-4015
(708) 843-2798
81-7-5256-4158
(805) 867-2556
81-3-3494-7414
(619) 661-1055
81-7-2070-1174
(408) 970-3950
(603) 224-1430
(708) 956-0702
81-3-3607-5144
Proper PC board layout is essential because of high
current levels and fast switching waveforms that radiate noise. Minimize ground noise by connecting the
anode of the rectifier, the input bypass capacitor
ground lead, and the output filter capacitor ground
lead to a single point (“star” ground configuration). A
ground plane is recommended. Also minimize lead
lengths to reduce stray capacitance, trace resistance,
and radiated noise. In particular, the traces connected
to FB (if an external resistor divider is used) and EXT
must be short. Place the 0.1µF ceramic bypass capacitor as close as possible to the V+ and GND pins.
MAX1649/MAX1651 vs. MAX649/MAX651
The MAX1649 and MAX1651 are pin compatible with
the MAX649 and MAX651, but have been optimized for
improved dropout performance and efficiency—particularly with low input voltages. The MAX1649/MAX1651
feature increased maximum switch duty cycle (96.5%)
and reduced current-limit sense voltage (110mV).
Their predecessors, the MAX649/MAX651, use a higher two-step (210mV/110mV) current-limit sense voltage
to provide tighter current-sense accuracy and reduced
inductor peak current at light loads.
___________________Chip Topography
(714) 255-9400
GND
OUT
Input Bypass Capacitor
The input bypass capacitor reduces peak currents
drawn from the voltage source, and also reduces the
amount of noise at the voltage source caused by the
switching action of the MAX1649/MAX1651. The input
voltage source impedance determines the size of the
capacitor required at the V+ input. As with the output filter capacitor, a low-ESR capacitor is recommended.
Bypass the IC separately with a 0.1µF ceramic capacitor placed close to the V+ and GND pins.
EXT
FB
0.106"
(2.692mm)
CS
SHDN
Reference Capacitor
Bypass REF with a 0.1µF or larger capacitor.
REF
V+
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages.
PACKAGE TYPE
PACKAGE CODE
DOCUMENT NO.
8 PDIP
P8-2
21-0041
8 SO
S8-4
21-0043
0.081"
(2.057mm)
TRANSISTOR COUNT: 428
SUBSTRATE CONNECTED TO V+
______________________________________________________________________________________
11
MAX1649/MAX1651
Layout Considerations
Table 2. Component Suppliers
MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
Revision History
REVISION
NUMBER
REVISION
DATE
3
3/09
DESCRIPTION
Corrected Output Voltage conditions and Figure 1 title
PAGES
CHANGED
2, 6
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 2009 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
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