5V/3.3V or Adjustable, MAX1626/MAX1627 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers

5V/3.3V or Adjustable, MAX1626/MAX1627 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers
EVALUATION KIT AVAILABLE
MAX1626/MAX1627
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
General Description
Benefits and Features
The MAX1626/MAX1627 step-down DC-DC controllers
operate over a 2.6V to 16.5V input voltage range. The
controllers deliver load current from 1mA to more than
2A. The MAX1626 has pin-selectable 3.3V and 5V outputs. The MAX1627 supports adjustable outputs from
1.3V to 16V.
A unique current-limited, pulse-frequency-modulation
(PFM) control scheme operates up to 100% duty cycle,
resulting in very low dropout voltage. This control
scheme eliminates minimum load requirements and
reduces supply current under light loads to 90µA
(versus 2mA to 10mA for common pulse-width modulation controllers).
• Reduce External Components and Total Cost
The devices are available in a 8-pin SOIC package
(-40°C to +85°C) and dice (0°C to +70°C).
Applications
• 5V to 3.3V Green PC Applications
• Battery-Powered Applications
• Handheld Computers
• High-Efficiency Step-Down Regulation
• 300KHz PWM Switching Reduces Component Size
• Tiny Surface-Mount Inductor
• Reduce Power Dissipation
• > 90% Efficiency from 3mA to 2A Loads
• Low Dropout Voltage
• 100% Maximum Duty Cycle
• Reduce Number of DC-DC Controllers to Stock
• Wide 2.6V to 16.5V Input Voltage Options
• Selectable 3.3V and 5V or Adjustable 1.3V to 16V
Output Voltage Options
• Reduce System Power Consumption
• 90µA Max Quiescent Current
• 1µA Max Shutdown Current
• Operates Reliably in Adverse Environment
• Soft-Start Limits Startup Current
• Current-Limited Control Scheme
• Increase Design Flexibility
• External P-Channel MOSFET Allows Output Power
of > 12.5W
Ordering Information
• Low-Cost Notebook Computer Supplies
• Minimum Component DC-DC Converters
PART
TEMP RANGE
MAX1626C/D
• PCMCIA Power Supplies
• PDAs and Other Handheld Devices
• Portable Terminals
PIN-PACKAGE
0°C to +70°C
Dice*
MAX1626ESA
-40°C to +85°C
MAX1627C/D
0°C to +70°C
MAX1627ESA
-40°C to +85°C
* Dice are tested at TA = +25°C.
8 SO
Dice*
8 SO
Typical Operating Circuit
Pin Configuration
INPUT
3.3V to 16.5V
TOP VIEW
V+
OUT
1
3/5 (FB)
2
SHDN 3
MAX1626
MAX1627
REF 4
SO
8
GND
7
EXT
6
CS
5
V+
MAX1626
ON/OFF
SHDN
CS
EXT
3/5
OUT
REF
GND
( ) ARE FOR MAX1627
19-1075; Rev 1; 5/15
P
OUTPUT
3.3V
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Supply Voltage, V+ to GND.......................................-0.3V, +17V
OUT, FB, 3/5, SHDN, REF, CS, EXT to GND ...-0.3V, (V+ + 0.3V)
Maximum Current at REF (IREF) ..........................................15mA
Maximum Current at EXT (IEXT) ..........................................50mA
Continuous Power Dissipation (TA = +70°C)
SO (derate 5.88mW/°C above +70°C) ..........................471mW
Operating Temperature Range
MAX1626ESA/MAX1627ESA ............................-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
PARAMETER
Input Voltage Range
Supply Current into V+
SYMBOL
CONDITIONS
V+
I+
MIN
3.0
Operating, no load
70
V+ = SHDN = 16.5V (shutdown)
VOUT
OUT Input Current
IOUT
UNITS
16.5
V
90
2.7
2.8
Circuit of Figure 1, 3/5 = V+ (Note 1)
4.85
5.00
5.15
Circuit of Figure 1, 3/5 = 0V (Note 1)
3.20
3.30
3.40
MAX1626, 3/5 = V+, output forced to 5V
FB Threshold Voltage
MAX1627, includes hysteresis
FB Leakage Current
MAX1627
CS Input Current
CS Threshold Voltage
MAX
1
Undervoltage Lockout
Output Voltage
TYP
VCS
37
50
µA
1.30
1.33
V
0
35
nA
0
10
µA
115
mV
100
1.6
V
SHDN = 0V or V+
3/5 Input Voltage High
0.4
V
±1
µA
V+ - 0.5
V
3/5 Input Voltage Low
3/5 Leakage Current
3/5 = 0V or V+
EXT Resistance
V+ = 5V
Minimum EXT Off Time
0.5
V
±1
µA
10
Ω
Output forced to 0V
8
10
12
Output in regulation
1.5
2.0
2.5
EXT Duty-Cycle Limit
V
24
SHDN Input Voltage Low
SHDN Input Current
V
1.27
85
SHDN Input Voltage High
µA
100
µs
%
Line Regulation
6.0V < V+ < 12.0V, ILOAD = 1A
5
mV/V
Load Regulation
30mA < ILOAD < 2.0A, V+ = 8V
15
mV/A
Reference Voltage
1.30
1.33
V
REF Load Regulation
0µA ≤ IREF ≤ 100µA
4
10
mV
REF Line Regulation
V+ = 3V to 16.5V, ILOAD = 0µA
10
100
µV/V
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VREF
ILOAD = 0µA
1.27
Maxim Integrated | 2
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Electrical Characteristics
PARAMETER
SYMBOL
Input Voltage
CONDITIONS
MIN
V+
Supply Current into V+
TYP
3.0
IOUT
Operating, no load
MAX
UNITS
16.5
V
100
V+ = SHDN = 16.5V (shutdown)
µA
2
Undervoltage Lockout
2.9
Output Voltage
VOUT
OUT Input Current
IOUT
4.80
5.20
Circuit of Figure 1, 3/5 = 0V
3.16
3.44
MAX1626, 3/5 = V+, output forced to 5V
FB Threshold Voltage
MAX1627, includes hysteresis
FB Leakage Current
MAX1627
CS Threshold Voltage
Reference
V
Circuit of Figure 1, 3/5 = V+
ILOAD = 0µA
V
24
50
1.25
1.35
µA
V
0
50
nA
80
120
mV
1.25
1.35
V
Note 1: V+ must exceed VOUT to maintain regulation.
Note 2: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
5V SETTING
VOUT = +4.8V
0.15
D E F
60
50
40
0.10
20
0.05
10
0
0
0.5
1.0
1.5
LOAD (A)
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2.0
2.5
A
B
C
80
70
30
0
90
EFFICIENCY (%)
EFFICIENCY (%)
0.25
0.20
A
80
3.3V SETTING
VOUT = +3.17V
0.30
90
C
B
100
MAX1626-05
0.40
DROPOUT VOLTAGE (V)
100
MAX1626-11
0.45
0.35
EFFICIENCY vs. LOAD CURRENT
(VOUT = +5V)
EFFICIENCY vs. LOAD CURRENT
(VOUT = +3.3V)
MAX1626-03
DROPOUT VOLTAGE
vs. LOAD CURRENT
CIRCUIT OF FIGURE 1
0.1m
1m
10m
A: V+ = +4.3V
B: V+ = +5V
C: V+ = +8V
D: V+ = +10V
E: V+ = +12V
F: V+ = +15V
70
50
40
100m
1
A: V+ = +6V
B: V+ = +8V
C: V+ = +10V
D: V+ = +12V
E: V+ = +15V
30
20
10
0
LOAD CURRENT (A)
D E
60
10
CIRCUIT OF FIGURE 1
0.1m
1m
10m
100m
1
10
LOAD CURRENT (A)
Maxim Integrated | 3
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Typical Operating Characteristics (continued)
0.4
0.3
MAX1626-02
12
V+ = +5V
10
10
B
C
D
E
0.2
8
6
3/5 = V+
4
8
6
4
3/5 = GND
2
2
0.1
0
0
0
-60 -40 -20
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
0
1
2
3
4
25
20
tRISE, tFALL, V+ = +15V
15
1.2
1.4
250
tRISE, V+ = +15V
200
tFALL, V+ = +5V
150
tRISE, V+ = +5V
100
CEXT = 1nF
3/5 = 0V
OUT = 50kHz, 0.3Vp-p, 3.3VDC
50
0
tFALL, V+ = +15V
0
-60 -40 -20 0
20 40 60 80 100 120 140
0
2000
TEMPERATURE (°C)
V+ = +16V
68
V+ = +10V
66
64
V+ = +4V
62
3/5 = 0V
OUT FORCED TO 3.4V
60
OUT = 0V
110
CS TRIP LEVEL (mV)
70
CS TRIP LEVEL vs. TEMPERATURE
115
MAX1626-01
72
4000
CAPACITANCE (pF)
MAX1626
V+ QUIESCENT CURRENT
vs. TEMPERATURE
IQ (μA)
1.0
MAX1626-10
350
tRISE AND tFALL (ns)
tRISE AND tFALL (ns)
tRISE, V+ = +5V
30
5
0.8
300
35
10
0.6
FB PIN VOLTAGE (V)
400
tFALL, V+ = +5V
40
0.4
EXT RISE AND FALL TIMES
vs. CAPACITANCE
MAX1626-09
50
0.2
OUTPUT VOLTAGE (V)
EXT RISE AND FALL TIMES
vs. TEMPERATURE
45
0
5
MAX1626-12
0.5
V+ = +5V
EXT OFF TIME (μs)
0.6
12
EXT OFF TIME (μs)
SHUTDOWN CURRENT (μA)
0.7
A
MAX1626-04
0.8
APPLICATION CIRCUIT
SHUTDOWN CURRENT:
A: V+ = +15V
B: V+ = +10V
C: V+ = +4V
MAX1626 SHUTDOWN
CURRENT:
D: V+ = +16V
E: V+ = +4V
MAX1627 EXT OFF TIME
vs. FB PIN VOLTAGE
MAX1626 EXT OFF TIME
vs. OUTPUT VOLTAGE
MAX1626-03
MAX1626 SHUTDOWN CURRENT
vs. TEMPERATURE
105
100
95
90
85
-60 -40 -20
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0 20 40 60 80 100 120 140
TEMPERATURE (°C)
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (°C)
Maxim Integrated | 4
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Typical Operating Characteristics (continued)
MAX1626 SHUTDOWN RESPONSE TIME
AND SUPPLY CURRENT
REFERENCE OUTPUT VOLTAGE (V)
MAX1626-13
1.310
MAX1626-14
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
1.305
A
IREF = 0μA
IREF = 10μA
1.300
B
1.295
IREF = 50μA
1.290
IREF = 100μA
1.285
C
1.280
0
500μs/div
20 40 60 80 100 120 140
TEMPERATURE (°C)
V+ = 8V, VOUT = 5V, LOAD = 1A
A: OUT, 2V/div
B: SUPPLY CURRENT, 1A/div
C: SHDN, 5V/div
MAX1626 LOAD-TRANSIENT RESPONSE
LINE-TRANSIENT RESPONSE
FROM 100% DUTY CYCLE
MAX1626-16
MAX1626-15
MAX1626 LINE-TRANSIENT RESPONSE
A
MAX1626-17
-60 -40 -20
A
A
B
B
B
100μs/div
V+ = 8V, VOUT = 3.3V, LOAD = 30mA to 2A
A: OUT, 50mV/div, 3.3V DC OFFSET
B: LOAD CURRENT, 1A/div
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5ms/div
VOUT = 5V, LOAD = 1A, CIN = 33μF
A: OUT, 100mV/div, 5V DC OFFSET
B: V+ 6V to 12V, 2V/div
5ms/div
VOUT = 3.3V, LOAD = 1A, CIN = 47μF
A: OUT, 100mV/div, 3.3V DC OFFSET
B: V+ 3.3V to 15V, 5V/div
Maxim Integrated | 5
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Pin Description
PIN
NAME
FUNCTION
1
OUT
Sense input for fixed 5V or 3.3V output operation. OUT is internally connected to an
on-chip voltage divider (MAX1626). It does not supply current. Leave OUT unconnected during adjustable-output operation (MAX1627).
—
2
FB
Feedback Input for adjustable-output operation. Connect to an external voltage
divider between the output and GND (see the Setting the Output Voltage section).
2
—
3/5
3.3V or 5V Selection. Output voltage is set to 3.3V when this pin is low or 5V when it
is high.
3
3
SHDN
Active-High Shutdown Input. Device 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.
4
4
REF
1.3V Reference Output. Can source 100µA. Bypass with 0.1µF.
5
5
V+
Positive Supply Input. Bypass with 0.47µF.
6
6
CS
Current-Sense Input. Connect current-sense resistor between V+ and CS. External
MOSFET is turned off when the voltage across the resistor equals the current-limit
trip level (around 100mV).
7
7
EXT
Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.
8
8
GND
Ground
MAX1626
MAX1627
1
EXT
INPUT
C2
68μF LOW-ESR
TANTALUM
C3
68μF LOW-ESR
TANTALUM
C5
0.47μF
MAX1626
MAX1627
REF
1.5V
REF
ERROR
COMPARATOR
OUT
MINIMUM ON-TIME
ONE-SHOT
TRIG
Q
R2
V+
0.04Ω
3/5
CS
SHDN
REF
GND
R1
P
L1
22μH, 3A
OUT
OUTPUT
C1
220μF
LOW-ESR
TANTALUM
D1
S
Q
R
CURRENT-SENSE
COMPARATOR
L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03
U1: MORTOLA MMSF3PO2HD
Figure 1. MAX1626 Typical Operating Circuit
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R3
3/5
U1
LOGIC-LEVEL MOSFET
EXT
C4
0.1μF
(FB)
Q
TRIG
MINIMUM OFF-TIME
ONE-SHOT
RSENSE
MAX1626
SHDN
V+
CS
( ) MAX1627 ONLY
MAX1626 ONLY
Figure 2. Simplified Functional Diagram
Maxim Integrated | 6
MAX1626/MAX1627
Detailed Description
The MAX1626/MAX1627 are step-down DC-DC controllers designed primarily for use in portable computers and battery-powered devices. Using an external
MOSFET and current-sense resistor allows design flexibility and the improved efficiencies associated with
high-performance P-channel MOSFETs. A unique, current-limited, pulse-frequency-modulated (PFM) control
scheme gives these devices excellent efficiency over
load ranges up to three decades, while drawing around
90µA under no load. This wide dynamic range optimizes the MAX1626/MAX1627 for battery-powered
applications, where load currents can vary considerably as individual circuit blocks are turned on and off to
conserve energy. Operation to a 100% duty cycle
allows the lowest possible dropout voltage, extending
battery life. High switching frequencies and a simple
circuit topology minimize PC board area and component costs. Figure 1 shows a typical operating circuit
for the MAX1626.
PFM Control Scheme
The MAX1626/MAX1627 use a proprietary, third-generation, current-limited PFM control scheme. Improvements
include a reduced current-sense threshold and operation
to a 100% duty cycle. These devices pulse only as needed to maintain regulation, resulting in a variable switching
frequency that increases with the load. This eliminates the
current drain associated with constant-frequency pulsewidth-modulation (PWM) controllers, caused by switching
the MOSFET unnecessarily.
When the output voltage is too low, the error comparator sets a flip-flop, which turns on the external P-channel MOSFET and begins a switching cycle (Figures 1
and 2). As shown in Figure 3, current through the
inductor ramps up linearly, storing energy in a magnetic field while dumping charge into an output capacitor
and servicing the load. When the MOSFET is turned off,
the magnetic field collapses, diode D1 turns on, and
the current through the inductor ramps back down,
transferring the stored energy to the output capacitor
and load. The output capacitor stores energy when the
inductor current is high and releases it when the inductor current is low.
The MAX1626/MAX1627 use a unique feedback and
control system to govern each pulse. When the output
voltage is too low, the error comparator sets a flip-flop,
which turns on the external P-channel MOSFET. The
MOSFET turns off when the current-sense threshold is
exceeded or when the output voltage is in regulation. A
one-shot enforces a 2µs minimum on-time, except in
current limit. The flip-flop resets when the MOSFET
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5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
turns off. Otherwise the MOSFET remains on, allowing a
duty cycle of up to 100%. This feature ensures the lowest possible dropout. Once the MOSFET is turned off,
the minimum off-time comparator keeps it off. The minimum off-time is normally 2µs, except when the output is
significantly out of regulation. If the output is low by
30% or more, the minimum off-time increases, allowing
soft-start. The error comparator has 0.5% hysteresis for
improved noise immunity.
In the MAX1626, the 3/5 pin selects the output voltage
(Figure 2). In the MAX1627, external feedback resistors
at FB adjust the output.
Operating Modes
When delivering low and medium output currents, the
MAX1626/MAX1627 operate in discontinuous-conduction mode. Current through the inductor starts at zero,
rises as high as the peak current limit set by the current- sense resistor, then ramps down to zero during
each cycle (Figure 3). Although efficiency is still excellent, output ripple increases and the switch waveform
exhibits ringing. This ringing occurs at the resonant frequency of the inductor and stray capacitance, due to
residual energy trapped in the core when the commutation diode (D1 in Figure 1) turns off. It is normal and
poses no operational problems.
When delivering high output currents, the MAX1626/
MAX1627 operate in continuous-conduction mode
(Figure 4). In this mode, current always flows through
the inductor and never ramps to zero. The control circuit adjusts the switch duty cycle to maintain regulation
without exceeding the peak switching current set by
the current-sense resistor. This provides reduced output ripple and high efficiency.
100% Duty Cycle and Dropout
The MAX1626/MAX1627 operate with a duty cycle up
to 100%. This feature extends usable battery life by
turning the MOSFET on continuously when the supply
voltage approaches the output voltage. This services
the load when conventional switching regulators with
less than 100% duty cycle would fail. Dropout voltage
is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the
MOSFET drain-to-source on-resistance, current-sense
resistor, and inductor series resistance, and is proportional to the load current:
Dropout Voltage =
[
IOUT x RDS(ON) + RSENSE + RINDUCTOR
]
Maxim Integrated | 7
MAX1626/MAX1627
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
A
A
B
B
C
0A
C
0A
10μs/div
CIRCUIT OF FIGURE 1, V+ = 8V, VOUT = 5V, LOAD = 100mA
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
10μs/div
CIRCUIT OF FIGURE 1, V+ = 8V, VOUT = 5V, LOAD = 1.5A
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform
EXT Drive Voltage Range
shutdown. Bypass the reference with 0.1µF for normal
operation. Place the bypass capacitor within 0.2 inches
(5mm) of REF, with a direct trace to GND (Figure 7).
EXT swings from V+ to GND and provides the gate
drive for an external P-channel power MOSFET. A higher supply voltage increases the gate drive to the
MOSFET and reduces on-resistance (RDS(ON)). See
External Switching Transistor section.
Quiescent Current
The device’s typical quiescent current is 70µA.
However, actual applications draw additional current to
supply MOSFET switching currents, OUT pin current, or
external feedback resistors (if used), and both the diode
and capacitor leakage currents. For example, in the circuit of Figure 1, with V+ at 7V and VOUT at 5V, typical
no-load supply current for the entire circuit is 84µA.
When designing a circuit for high-temperature operation, select a Schottky diode with low reverse leakage.
Shutdown Mode
When SHDN is high, the device enters shutdown mode.
In this mode, the feedback and control circuit, reference,
and internal biasing circuitry are turned off. EXT goes
high, turning off the external MOSFET. The shutdown
supply current drops to less than 1µA. SHDN is a logiclevel input. Connect SHDN to GND for normal operation.
Reference
The 1.3V reference is suitable for driving external loads,
such as an analog-to-digital converter. It has a guaranteed 10mV maximum load regulation while sourcing load
currents up to 100µA. The reference is turned off during
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Soft-Start
Soft-start reduces stress and transient voltage slumps
on the power source. When the output voltage is near
ground, the minimum off-time is lengthened to limit peak
switching current. This compensates for reduced negative inductor current slope due to low output voltages.
Design Information
Setting the Output Voltage
The MAX1626’s output voltage can be selected to 3.3V
or 5V under logic control by using the 3/5 pin. The 3/5
pin requires less than 0.5V to ensure a 3.3V output, or
more than (V+ - 0.5)V to guarantee a 5V output. The
voltage sense pin (OUT) must be connected to the output for the MAX1626.
The MAX1627’s output voltage is set using two resistors, R2 and R3 (Figure 5), which form a voltage divider
between the output and GND. R2 is given by:
⎛V
⎞
R2 = R3 x ⎜ OUT − 1⎟
⎝ VREF
⎠
where VREF = 1.3V. Since the input bias current at FB
has a maximum value of 50nA, large values (10kΩ to
200kΩ) can be used for R3 with no significant accuracy
Maxim Integrated | 8
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current. Capacitor CR2
is used to compensate the MAX1627 for even switching. Values between 0pF and 330pF work for many
applications. See the Stability and MAX1627 Feedback
Compensation section for details.
FROM
OUTPUT
R2
CR2
TO FB
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to VCS/RSENSE, where RSENSE is the value of
the current-sense resistor and VCS is the current-sense
threshold. VCS is typically 100mV, but can range from
85mV to 115mV. Minimizing the peak switching current
will increase efficiency and reduce the size and cost of
external components. However, since available output
current is a function of the peak switching current, the
peak current limit must not be set too low.
Set the peak current limit above 1.3 times the maximum
load current by setting the current-sense resistor to:
RCS =
VCS(MIN)
1.3 x IOUT(MAX)
Alternatively, select the current-sense resistor for 5V
and 3.3V output applications using the current-sense
resistor graphs in Figures 6a and 6b. The current-sense
resistor’s power rating should be 20% higher than:
RPOWER RATING (W) =
V2CS(MAX)
RCS
R3
Figure 5. Adjustable-Output Operation Using the MAX1627
ommended. Surface-mount (chip) resistors have very
little inductance and are well suited for use as currentsense resistors. Power metal-strip resistors feature
1/2W and 1W power dissipation, 1% tolerance, and
inductance below 5nH. Resistance values between
10mΩ and 500mΩ are available.
Inductor Selection
The essential parameters for inductor selection are
inductance and current rating. The MAX1626/MAX1627
operate with a wide range of inductance values. In many
applications, values between 10µH and 68µH take best
advantage of the controller’s high switching frequency.
Calculate the minimum inductance value as follows:
Standard wire-wound resistors have an inductance
high enough to degrade performance, and are not rec-
3.5
3.5
VOUT = 3.3V
RSENSE = 0.03Ω
3.0
2.5
RSENSE = 0.04Ω
2.0
RSENSE = 0.05Ω
MAXIMUM OUTPUT CURRENT (A)
MAXIMUM OUTPUT CURRENT (A)
VOUT = 5V
1.5
1.0
RSENSE = 0.1Ω
0.5
RSENSE = 0.03Ω
3.0
RSENSE = 0.04Ω
2.5
RSENSE = 0.05Ω
2.0
1.5
RSENSE = 0.1Ω
1.0
0.5
0
0
4.5
5.0
5.5
6.0
10
12
14
16
INPUT VOLTAGE (V)
Figure 6a. MAX1626 5V-Operation Current-Sense Resistor
Graph
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3.0
3.5
4.0
4.5
10
12
14
16
INPUT VOLTAGE (V)
Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor
Graph
Maxim Integrated | 9
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
where 2µs is the minimum on-time. Inductor values
between two and six times L(MIN) are recommended.
With high inductor values, the MAX1626/MAX1627 will
begin continuous-conduction operation at a lower fraction of the full load (see Detailed Description). Low-value
inductors may be smaller and less expensive, but they
result in greater peak current overshoot due to currentsense comparator propagation delay. Peak-current
overshoot reduces efficiency and could cause the external components’ current ratings to be exceeded.
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to prevent overheating and core saturation. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and inductance
starts to fall. The heating current rating is the maximum
DC current the inductor can sustain without overheating.
The peak switching current is the sum of the current limit
set by the current-sense resistor and overshoot during
current-sense comparator propagation delay.
IPEAK =
KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.
Table 1. Component Selection Guide
PRODUCTION
METHOD
INDUCTORS
Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)
Miniature
Through-Hole
Low-Cost
Through-Hole
External Switching Transistor
The MAX1626/MAX1627 drive P-channel enhancementmode MOSFETs. The EXT output swings from GND to
the voltage at V+. To ensure the MOSFET is fully on,
use logic-level or low-threshold MOSFETs when the
input voltage is less than 8V. Tables 1 and 2 list recommended suppliers of switching transistors.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, current rating, total gate charge (Qg), and RDS(ON). The
drain-to-source breakdown voltage rating should be at
least a few volts higher than V+. Choose a MOSFET
with a maximum continuous drain current rating higher
than the peak current limit:
(V + − VOUT ) × 1μs
VCS
+
RCS
L
1µs is the worst-case current-sense comparator propagation delay.
Inductors with a core of ferrite, Kool Mu™, METGLAS™,
or equivalent, are recommended. Powder iron cores
are not recommended for use with high switching
frequencies. For optimum efficiency, the inductor wind-
Surface Mount
ings’ resistance should be on the order of the currentsense resistance. If necessary, use a toroid, pot-core,
or shielded-core inductor to minimize radiated noise.
Table 1 lists inductor types and suppliers for various
applications.
CAPACITORS
ID(MAX) ≥ ILIM (MAX) =
VCS(MAX)
RSENSE
The Qg specification should be less than 100nC to
ensure fast drain voltage rise and fall times, and reduce
power losses during transition through the linear region.
Qg specifies all of the capacitances associated with
charging the MOSFET gate. EXT pin rise and fall times
vary with different capacitive loads, as shown in the
Typical Operating Characteristics. RDS(ON) should be
as low as practical to reduce power losses while the
MOSFET is on. It should be equal to or less than the
current-sense resistor.
DIODES
CURRENT-SENSE
RESISTORS
MOSFETS
Siliconix
Little Foot series
AVX
TPS series
Motorola
MBRS340T3
Dale
WSL series
Coilcraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Sprague
595D series
Nihon
NSQ series
IRC
LRC series
Motorola
medium-power
surface-mount products
Sumida
RCH875-470M (1.3A)
Sanyo
OS-CON series
low-ESR organic
semiconductor
IRC
OAR series
Motorola
Coilcraft
PCH-45-473 (3.4A)
Nichicon
PL series
Motorola
low-ESR electrolytics 1N5817 to
1N5823
United Chemi-Con
Motorola
TMOS power MOSFETs
LXF series
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Maxim Integrated | 10
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Table 2. Component Suppliers
COMPANY
PHONE
FAX
AVX
USA
Coilcraft
Coiltronics
Dale
International
Rectifier
IRC
Motorola
Nichicon
USA
USA
USA
(803) 946-0690
or
(800) 282-4975
(847) 639-6400
(516) 241-7876
(605) 668-4131
USA
(310) 322-3331
(310) 322-3332
USA
USA
USA
Japan
USA
Japan
USA
Japan
(512) 992-3377
(602) 994-6430
(847) 843-2798
81-7-5256-4158
(805) 867-2698
81-3-3494-7414
(619) 661-1055
81-7-2070-1174
Siliconix
USA
Sprague
Sumida
USA
USA
Japan
(512) 992-7900
(602) 303-5454
(847) 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
(847) 956-0666
81-3-3607-5111
USA
(714) 255-9500
Nihon
Sanyo
United
Chemi-Con
(803) 626-3123
(847) 639-1469
(516) 241-9339
(605) 665-1627
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
VRIPPLE ≈ VRIPPLE,ESR + VRIPPLE,C
To simplify selection, assume initially that two-thirds of
the ripple results from ESR and one-third results from
capacitor value. Voltage ripple as a consequence of
ESR is approximated by:
VRIPPLE,ESR ≈ (RESR )(IPEAK )
Estimate input and output capacitor values for given
voltage ripple as follows:
1 LI2
2 ΔL
(408) 970-3950
CIN =
(603) 224-1430
(847) 956-0702
81-3-3607-5144
COUT =
(714) 255-9400
Diode Selection
The MAX1626/MAX1627’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817–1N5822 family or surface-mount equivalents, are recommended. Ultra-high-speed rectifiers
with reverse recovery times around 50ns or faster, such
as the MUR series, are acceptable. Make sure that the
diode’s peak current rating exceeds the peak current
limit set by RSENSE, and that its breakdown voltage
exceeds V+. Schottky diodes are preferred for heavy
loads due to their low forward voltage, especially in
low-voltage applications. For high-temperature applications, some Schottky diodes may be inadequate due to
their high leakage currents. In such cases, ultra-highspeed rectifiers are recommended, although a Schottky
diode with a higher reverse voltage rating can often
provide acceptable performance.
Capacitor Selection
Choose filter capacitors to service input and output
peak currents with acceptable voltage ripple.
Equivalent series resistance (ESR) in the capacitor is a
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major contributor to output ripple, so low-ESR capacitors are recommended. Sanyo OS-CON capacitors are
best, and low-ESR tantalum capacitors are second
best. Low-ESR aluminum electrolytic capacitors are tolerable, but do not use standard aluminum electrolytic
capacitors.
VRIPPLE,CINVIN
1 LI2
2 ΔL
⎛
⎞
VIN
⎜
VRIPPLE,COUT VOUT ⎝ VIN − VOUT ⎟⎠
where IΔL is the change in inductor current (around
0.5IPEAK under moderate loads).
These equations are suitable for initial capacitor selection; final values should be set by testing a prototype or
evaluation kit. When using tantalum capacitors, use
good soldering practices to prevent excessive heat
from damaging the devices and increasing their ESR.
Also, ensure that the tantalum capacitors’ surge-current
ratings exceed the start-up inrush and peak switching
currents.
Pursuing output ripple lower than the error comparator’s hysteresis (0.5% of the output voltage) is not practical, since the MAX1626/MAX1627 will switch as
needed, until the output voltage crosses the hysteresis
threshold. Choose an output capacitor with a working
voltage rating higher than the output voltage.
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on V+ and CS, caused by the circuit’s switching
action. Use a low-ESR capacitor. Two smaller-value
low-ESR capacitors can be connected in parallel for
lower cost. Choose input capacitors with working voltage ratings higher than the maximum input voltage.
Maxim Integrated | 11
MAX1626/MAX1627
Place a surface-mount ceramic capacitor very close to
V+ and GND, as shown in Figure 7. This capacitor
bypasses the MAX1626/MAX1627, and prevents spikes
and ringing on the power source from obscuring the
current feedback signal and causing jitter. 0.47µF is
recommended. Increase the value as necessary in
high-power applications.
Bypass REF with 0.1µF. This capacitor should be
placed within 0.2 inches (5mm) of the IC, next to REF,
with a direct trace to GND (Figure 7).
Layout Considerations
High-frequency switching regulators are sensitive to PC
board layout. Poor layout introduces switching noise into
the current and voltage feedback signals, resulting in jitter, instability, or degraded performance. The currentsense resistor must be placed within 0.2 inches (5mm)
of the controller IC, directly between V+ and CS. Place
voltage feedback resistors (MAX1627) next to the FB pin
(no more than 0.2") rather than near the output. Place
the 0.47µF input and 0.1µF reference bypass capacitors
within 0.2 inches (5mm) of V+ and REF, and route
directly to GND. Figure 7 shows the recommended layout and routing for these components.
High-power traces, highlighted in the Typical Operating
Circuit (Figure 1), should be as short and as wide as
possible. The supply-current loop (formed by C2, C3,
RSENSE, U1, L1, and C1) and commutation-current loop
(D1, L1, and C1) should be as tight as possible to
reduce radiated noise. Place the anode of the commutation diode (D1) and the ground pins of the input and
output filter capacitors close together, and route them to
a common “star-ground” point. Place components and
route ground paths so as to prevent high currents from
causing large voltage gradients between the ground pin
of the output filter capacitor, the controller IC, and the
reference bypass capacitor. Keep the extra copper on
the component and solder sides of the PC board, rather
than etching it away, and connect it to ground for use as
a pseudo-ground plane. Refer to the MAX1626
Evaluation Kit manual for a two-layer PC board example.
Stability and MAX1627 Feedback
Compensation
Use proper PC board layout and recommended external components to ensure stable operation. In oneshot, sequenced PFM DC-DC converters, instability is
manifested as “Motorboat Instability.” It is usually
caused by excessive noise on the current or voltage
feedback signals, ground, or reference, due to poor PC
board design or external component selection.
Motorboat instability is characterized by grouped
switching pulses with large gaps and excessive low-
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5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
R SENSE
MAX1626
4x
SCALE
C REF
C V+ BYPASS
Figure 7. Recommended Placement and Routing of the
Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input
Bypass Capacitors
frequency output ripple. It is normal to see some
grouped switching pulses during the transition from
discontinuous to continuous current mode. This effect
is associated with small gaps between pulse groups
and output ripple similar to or less than that seen during no-load conditions.
Instability can also be caused by excessive stray capacitance on FB when using the MAX1627. Compensate for
this by adding a 0pF to 330pF feed-forward capacitor
across the upper feedback resistor (R2 in Figure 5).
MAX1626/MAX1627 vs.
MAX1649/MAX1651 vs.
MAX649/MAX651
The MAX1626/MAX1627 are specialized, third-generation upgrades to the MAX649/MAX651 step-down controllers. They feature improved efficiency, a reduced
current-sense threshold (100mV), soft-start, and a 100%
duty cycle for lowest dropout. The MAX649/ MAX651
have a two-step (210mV/110mV) current-sense threshold. The MAX1649/MAX1651 are second-generation
upgrades with a 96.5% maximum duty cycle for
improved dropout performance and a reduced currentsense threshold (110mV) for higher efficiency, especially
at low input voltages. The MAX1649/ MAX1651 are
preferable for special applications where a 100% duty
cycle is undesirable, such as flyback and SEPIC circuits.
Since the MAX1626’s pinout is similar to those of the
MAX649 and MAX1649 family parts, the MAX1626 can
be substituted (with minor external component value
changes) into fixed-output mode applications, provided
the PC board layout is adequate. The MAX1627 can
also be substituted when MAX649 or MAX1649 family
parts are used in adjustable mode, but the feedback
resistor values must be changed, since the MAX1627
has a lower reference voltage (1.3V vs. 1.5V). Reduce
Maxim Integrated | 12
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
INPUT
INPUT
C2
68μF LOW-ESR
TANTALUM
C3
68μF LOW-ESR
TANTALUM
C2
68μF LOW-ESR
TANTALUM
C5
0.47μF
C3
0.47μF
V+
V+
RSENSE
0.15Ω
MAX1627
N.C.
OUT
CS
SHDN
U1
LOGIC-LEVEL MOSFET
P
EXT
REF
C4
0.1μF
GND
L1
22μH, 3A
R2
C1
220μF
LOW-ESR
TANTALUM
REF
U1
LOGIC-LEVEL MOSFET
P
EXT
ADJUSTABLE
OUTPUT
D1
CR2
CS
SHDN
FB
R3
RSENSE
0.15Ω
MAX1626
3/5
GND
OUTPUT
OUT
C4
0.1μF
C1
100μF
LOW-ESR
TANTALUM
D1
L1: SUMIDA CDR1053-680
D1: MOTOROLA MBRS130T3
U1: MOTOROLA MMSF3P02HD
L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03
U1: MOTOROLA MMSF3P02HD
Figure 8. MAX1627 Typical Operating Circuit
L1
68μH, 0.7A
Figure 9. 0.5A Step-Down Converter
the current-sense resistor value by 50% when substituting for the MAX649 or MAX651.
Applications
The MAX1626/MAX1627 typical operating circuits
(Figures 1 and 8) are designed to output 2A at a 5V
output voltage. The following circuits provide examples
and guidance for other applications.
INPUT 3V TO 6V
C5
100μF
C4
100μF
C6
0.1μF
Micropower Step-Down Converter
When designing a low-power, battery-based application, choose an external MOSFET with low gate capacitance (to minimize switching losses), and use a low
peak current limit to reduce I2R losses. The circuit in
Figure 9 is optimized for 0.5A.
High-Current Step-Down Converter
The circuit in Figure 10 outputs 6A at 2.5V from a 5V or
3.3V input. High-current design is difficult, and board
layout is critical due to radiated noise, switching transients, and voltage gradients on the PC board traces.
Figure 11 is a recommended PC board design. Choose
the external MOSFET to minimize RDS(ON). Keep the
gate-charge factor below the MAX1626/MAX1627’s
drive capability (see Ext Rise and Fall Times vs.
Capacitance graph in the Typical Operating
Characteristics). Otherwise, increased MOSFET rise
and fall times will contribute to efficiency losses. For
higher efficiencies, especially at low output voltages,
the MAX796 family of step-down controllers with synchronous rectification is recommended.
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C7
0.1μF
C8
1.0μF
C9
1.0μF
V+
RCS1, RCS2
0.025Ω
MAX1627
N.C.
OUT
SHDN
REF
C10
0.1μF
GND
CS
P
EXT
FB
Q1
LOGIC-LEVEL MOSFET
L1
2.7μH >8A
OUTPUT
2.5V, 6A
D1
R3
21.5k, 1%
R2
20k, 1%
CR2
220pF
C1
C2
C3
220μF 220μF 220μF
C1–C3: SANYO OS-CON 220μF, 6.3V
C4, C5: SANYO OS-CON 100μF, 20V
RCS1, RCS2: 0.025Ω DALE WSL-2512
Q1: MOTOROLA MTB50PO3HDL
D1: NIEC C10T04Q
L1: SUMIDA CDRH127-2R7NC
Figure 10. 6A Step-Down Converter
Maxim Integrated | 13
MAX1626/MAX1627
VIA
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
VIAS
VIA
COMPONENT PLACEMENT GUIDE—COMPONENT SIDE
COPPER ROUTING—FRONT SIDE
COPPER ROUTING—BACK SIDE
Figure 11. Recommended PC Board Design for 6A Step-Down Converter
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Maxim Integrated | 14
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
MAX1626/MAX1627
Package Information
Chip Topography
OUT
GND
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a
“+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but the
drawing pertains to the package regardless of RoHS status.
GND
EXT
3/5
(FB)
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
8 SO
S8-4
21-0041
90-0096
0.105"
(2.63mm)
CS
SHDN
VCC
REF
0.081"
(2.06mm)
( ) ARE FOR MAX1627
TRANSISTOR COUNT: 375
SUBSTRATE CONNECTED TO V+
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Maxim Integrated | 15
MAX1626/MAX1627
5V/3.3V or Adjustable,
100% Duty-Cycle, High-Efficiency,
Step-Down DC-DC Controllers
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
CHANGED
DESCRIPTION
0
6/96
Initial release
—
1
5/15
Updated General Description, Applications, and Benefits and Features sections
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and
max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
© 2015 Maxim Integrated Products, Inc. | 16
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