_______________General Description
The MAX1626/MAX1627 step-down DC-DC switching
controllers provide high efficiency over loads ranging
from 1mA to more than 2A. A unique current-limited,
pulse-frequency-modulated (PFM) control scheme
operates with up to a 100% duty cycle, resulting in very
low dropout voltages. This control scheme eliminates
minimum load requirements and reduces the supply
current under light loads to 90µA (versus 2mA to 10mA
for common pulse-width modulation controllers).
These step-down controllers drive an external P-chan-
nel MOSFET, allowing design flexibility for applications
to 12W or higher. Soft-start reduces current surges dur-
ing start-up. A high switching frequency (up to 300kHz)
and operation in continuous-conduction mode allow the
use of tiny surface-mount inductors. Output capacitor
requirements are also reduced, minimizing PC board
area and system costs.
The output voltage is preset at 5V or 3.3V for the
MAX1626 and adjustable for the MAX1627. Input volt-
ages can be up to 16.5V. The MAX1626/MAX1627 are
functional upgrades for the MAX1649/MAX1651.
________________________Applications
PCMCIA Power Supplies
Personal Digital Assistants
Hand-Held Computers
Portable Terminals
Low-Cost Notebook Computer Supplies
5V to 3.3V Green PC Applications
High-Efficiency Step-Down Regulation
Minimum-Component DC-DC Converters
Battery-Powered Applications
____________________________Features
♦Low Dropout Voltage
♦100% Maximum Duty Cycle
♦Soft-Start Limits Start-Up Current
♦Efficiency >90% (3mA to 2A Loads)
♦Output Power >12.5W
♦90µA Max Quiescent Current
♦1µA Max Shutdown Current
♦Up to 300kHz Switching Frequency
♦16.5V Max Input Voltage
♦Output Voltage: 5V/3.3V (MAX1626)
Adjustable (MAX1627)
♦Current-Limited Control Scheme
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
________________________________________________________________
Maxim Integrated Products
1
MAX1626
V+
CSSHDN
GND
3/5
ON/OFF
P
EXT
REF OUT
OUTPUT
3.3V
INPUT
3.3V to 16.5V
__________________Pin Configuration
1
2
3
4
8
7
6
5
GND
EXT
CS
V+
REF
SHDN
3/5 (FB)
OUT
( ) ARE FOR MAX1627
SO
TOP VIEW
MAX1626
MAX1627
__________Typical Operating Circuit
19-1075; Rev 0; 6/96
PART
MAX1626C/D
MAX1626ESA
MAX1627C/D 0°C to +70°C
-40°C to +85°C
0°C to +70°C
TEMP. RANGE PIN-PACKAGE
Dice*
8 SO
Dice*
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET
______________Ordering Information
* Dice are tested at T
A
= +25°C.
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
MAX1627ESA -40°C to +85°C 8 SO
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA= 0°C to +85°C, unless otherwise noted.)
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.
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
0µA ≤IREF ≤100µA
ILOAD = 0µA
30mA < ILOAD < 2.0A, V+ = 8V
V+ = SHDN = 16.5V (shutdown)
6.0V < V+ < 12.0V, ILOAD = 1A
Operating, no load
Output in regulation
Output forced to 0V
V+ = 5V
3/5 = 0V or V+
MAX1627
Circuit of Figure 1, 3/5 = V+ (Note 1)
MAX1626, 3/5 = V+, output forced to 5V
SHDN = 0V or V+
MAX1627, includes hysteresis
CONDITIONS
mV410REF Load Regulation V1.27 1.30 1.33VREF
Reference Voltage mV/A15Load Regulation mV/V5Line Regulation %100EXT Duty-Cycle Limit
µs
1.5 2.0 2.5
81012
Minimum EXT Off Time
Ω10EXT Resistance µA±1
3/5 Leakage Current V0.5
3/5 Input Voltage Low VV+ - 0.5
3/5 Input Voltage High µA±1SHDN Input Current V0.4SHDN Input Voltage Low
µA
1
I+Supply Current into V+ 70 90 V3.0 16.5V+Input Voltage Range
V1.6SHDN Input Voltage High mV85 100 115VCS
CS Threshold Voltage µA010CS Input Current nA035FB Leakage Current
V2.7 2.8Undervoltage Lockout 4.85 5.00 5.15
µA24 37 50IOUT
OUT Input Current V1.27 1.30 1.33FB Threshold Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
V+ = 3V to 16.5V, ILOAD = 0µA µV/V10 100REF Line Regulation
Circuit of Figure 1, 3/5 = 0V (Note 1) V
3.20 3.30 3.40
VOUT
Output Voltage
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS
(V+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
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.
V+ = SHDN = 16.5V (shutdown)
Operating, no load
ILOAD = 0µA
MAX1627
Circuit of Figure 1, 3/5 = V+
MAX1626, 3/5 = V+, output forced to 5V
MAX1627, includes hysteresis
CONDITIONS
µA
2
IOUT
Supply Current into V+ 100 V3.0 16.5V+Input Voltage
V1.25 1.35Reference mV80 120CS Threshold Voltage nA050FB Leakage Current
V2.9Undervoltage Lockout 4.80 5.20
µA24 50IOUT
OUT Input Current V1.25 1.35FB Threshold Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
Circuit of Figure 1, 3/5 = 0V V
3.16 3.44
VOUT
Output Voltage
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)
100
00.1m 100m 11m 10m 10
EFFICIENCY vs. LOAD CURRENT
(VOUT = +3.3V)
20
10
MAX1626-05
LOAD CURRENT (A)
EFFICIENCY (%)
40
30
60
50
70
80
90
A: V+ = +4.3V
B: V+ = +5V
C: V+ = +8V
D: V+ = +10V
E: V+ = +12V
F: V+ = +15V
CIRCUIT OF FIGURE 1
ABC
D E F
100
00.1m 100m 11m 10m 10
EFFICIENCY vs. LOAD CURRENT
(VOUT = +5V)
20
10
MAX1626-03
LOAD CURRENT (A)
EFFICIENCY (%)
40
30
60
50
70
80
90
A: V+ = +6V
B: V+ = +8V
C: V+ = +10V
D: V+ = +12V
E: V+ = +15V
CIRCUIT OF FIGURE 1
ABC
DE
0.45
00 0.5 1.0 1.5 2.0 2.5
DROPOUT VOLTAGE
vs. LOAD CURRENT
0.05
0.10
0.35
0.25
0.30
0.40
MAX1626-11
LOAD (A)
DROPOUT VOLTAGE (V)
0.20
0.15
3.3V SETTING
VOUT = +3.17V
5V SETTING
VOUT = +4.8V
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
4 _______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)
0-60 -40
MAX1626 SHUTDOWN CURRENT
vs. TEMPERATURE
MAX1626-04
TEMPERATURE (°C)
SHUTDOWN CURRENT (µA)
0.1
0.3
0.2
0.5
0.4
0.6
0.7
0.8
-20 0 20 40 60 80 100 120 140
E
C
B
A
D
APPLICATION CIRCUIT
SHUTDOWN CURRENT:
A: V+ = +15V
B: V+ = +10V
C: V+ = +4V
MAX1626 SHUTDOWN
CURRENT:
D: V+ = +16V
E: V+ = +4V
400
00 2000 4000
EXT RISE AND FALL TIMES
vs. CAPACITANCE
50
100
300
250
350
MAX1626-10
CAPACITANCE (pF)
tRISE AND tFALL (ns)
200
150
tRISE, V+ = +15V
tFALL, V+ = +5V
tRISE, V+ = +5V
tFALL, V+ = +15V
50
0-60 -40 -20 40 60 140
EXT RISE AND FALL TIMES
vs. TEMPERATURE
10
5
15
40
35
45
MAX1626-09
TEMPERATURE (°C)
tRISE AND tFALL (ns)
020 80
25
20
30
100 120
tFALL, V+ = +5V
CEXT = 1nF
3/5 = 0V
OUT = 50kHz, 0.3Vp-p, 3.3VDC
tRISE, V+ = +5V
tRISE, tFALL, V+ = +15V
60-60 -40
MAX1626
V+ QUIESCENT CURRENT
vs. TEMPERATURE
MAX1626-01
TEMPERATURE (°C)
IQ (µA)
62
64
66
68
70
72
-20 0 20 40 60 80 100 120 140
3/5 = 0V
OUT FORCED TO 3.4V
V+ = +16V
V+ = +10V
V+ = +4V
12
0012345
MAX1626 EXT OFF TIME
vs. OUTPUT VOLTAGE
2
8
10
MAX1626-02
OUTPUT VOLTAGE (V)
EXT OFF TIME (µs)
6
4
V+ = +5V
3/5 = GND
3/5 = V+
12
00 0.2 0.4 0.6 0.8 1.0 1.2 1.4
MAX1627 EXT OFF TIME
vs. FB PIN VOLTAGE
2
8
10
MAX1626-03
FB PIN VOLTAGE (V)
EXT OFF TIME (µs)
6
4
V+ = +5V
115
85 -60 -40 -20 0 20 40 60 80 100 120 140
CS TRIP LEVEL vs. TEMPERATURE
90
105
110
MAX1626-12
TEMPERATURE (°C)
CS TRIP LEVEL (mV)
100
95
OUT = 0V
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________________________________________________________________________________
5
1.310
1.280 -60 -40 -20 0 20 40 60 80 100 120 140
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
1.285
1.300
1.305
MAX1626-13
TEMPERATURE (°C)
REFERENCE OUTPUT VOLTAGE (V)
1.295
1.290
IREF = 10µA
IREF = 0µA
IREF = 50µA
IREF = 100µA
100µs/div
MAX1626 LOAD-TRANSIENT RESPONSE
MAX1626-15
V+ = 8V, VOUT = 3.3V, LOAD = 30mA to 2A
A: OUT, 50mV/div, 3.3V DC OFFSET
B: LOAD CURRENT, 1A/div
B
A
5ms/div
MAX1626 LINE-TRANSIENT RESPONSE
MAX1626-16
VOUT = 5V, LOAD = 1A, CIN = 33µF
A: OUT, 100mV/div, 5V DC OFFSET
B: V+ 6V to 12V, 2V/div
B
A
5ms/div
LINE-TRANSIENT RESPONSE
FROM 100% DUTY CYCLE
MAX1626-17
VOUT = 3.3V, LOAD = 1A, CIN = 47µF
A: OUT, 100mV/div, 3.3V DC OFFSET
B: V+ 3.3V to 15V, 5V/div
B
A
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)
500µs/div
MAX1626 SHUTDOWN RESPONSE TIME
AND SUPPLY CURRENT
MAX1626-14
V+ = 8V, VOUT = 5V, LOAD = 1A
A: OUT, 2V/div
B: SUPPLY CURRENT, 1A/div
C: SHDN, 5V/div
B
C
A
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
6 _______________________________________________________________________________________
______________________________________________________________Pin Description
Ground88
1.3V Reference Output. Can source 100µA. Bypass with 0.1µF.44
Positive Supply Input. Bypass with 0.47µF.55
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).
66
Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.77
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.
33
3.3V or 5V Selection. Output voltage is set to 3.3V when this pin is low or 5V when it
is high.
—2
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—
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 uncon-
nected during adjustable-output operation (MAX1627).
11
GND
REF
V+
CS
EXT
SHDN
3/5
FB
OUT
MAX1626
C5
0.47µF
P
D1
RSENSE
0.04Ω
U1
LOGIC-LEVEL MOSFET
C4
0.1µF
3/5
SHDN
REF
V+
EXT
CS
GND OUT
INPUT
L1
22µH, 3A
C1
220µF
LOW-ESR
TANTALUM
C2
68µF LOW-ESR
TANTALUM
C3
68µF LOW-ESR
TANTALUM
L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03
U1: MORTOLA MMSF3PO2HD
OUTPUT
Figure 1. MAX1626 Typical Operating Circuit
MAX1626
MAX1627
MINIMUM ON-TIME
ONE-SHOT
S
OUT
(FB)
3/5
R3
R2
R1
SHDN
V+
CS
R
TRIG
Q
Q
MINIMUM OFF-TIME
ONE-SHOT
CURRENT-SENSE
COMPARATOR
ERROR
COMPARATOR
REF
1.5V
EXT REF
( ) MAX1627 ONLY
MAX1626 ONLY
TRIG
Q
Figure 2. Simplified Functional Diagram
PIN
MAX1626 FUNCTION
MAX1627 NAME
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________________________________________________________________________________ 7
_______________Detailed Description
The MAX1626/MAX1627 are step-down DC-DC con-
trollers designed primarily for use in portable comput-
ers and battery-powered devices. Using an external
MOSFET and current-sense resistor allows design flexi-
bility and the improved efficiencies associated with
high-performance P-channel MOSFETs. A unique, cur-
rent-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 opti-
mizes the MAX1626/MAX1627 for battery-powered
applications, where load currents can vary consider-
ably 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 compo-
nent costs. Figure 1 shows a typical operating circuit
for the MAX1626.
PFM Control Scheme
The MAX1626/MAX1627 use a proprietary, third-genera-
tion, current-limited PFM control scheme. Improvements
include a reduced current-sense threshold and operation
to a 100% duty cycle. These devices pulse only as need-
ed to maintain regulation, resulting in a variable switching
frequency that increases with the load. This eliminates the
current drain associated with constant-frequency pulse-
width-modulation (PWM) controllers, caused by switching
the MOSFET unnecessarily.
When the output voltage is too low, the error compara-
tor sets a flip-flop, which turns on the external P-chan-
nel 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 magnet-
ic 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 induc-
tor 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
turns off. Otherwise the MOSFET remains on, allowing a
duty cycle of up to 100%. This feature ensures the low-
est possible dropout. Once the MOSFET is turned off,
the minimum off-time comparator keeps it off. The mini-
mum 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-conduc-
tion mode. Current through the inductor starts at zero,
rises as high as the peak current limit set by the cur-
rent- sense resistor, then ramps down to zero during
each cycle (Figure 3). Although efficiency is still excel-
lent, output ripple increases and the switch waveform
exhibits ringing. This ringing occurs at the resonant fre-
quency of the inductor and stray capacitance, due to
residual energy trapped in the core when the commuta-
tion 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 cir-
cuit adjusts the switch duty cycle to maintain regulation
without exceeding the peak switching current set by
the current-sense resistor. This provides reduced out-
put 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 out-
put voltages when the input is low enough for the out-
put to drop out of regulation. Dropout depends on the
MOSFET drain-to-source on-resistance, current-sense
resistor, and inductor series resistance, and is propor-
tional to the load current:
Dropout Voltage=
I x R + R + R
OUT DS(ON) SENSE INDUCTOR
[]
MAX1626/MAX1627
EXT Drive Voltage Range
EXT swings from V+ to GND and provides the gate
drive for an external P-channel power MOSFET. A high-
er 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 cir-
cuit 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 opera-
tion, 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 logic-
level 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 guaran-
teed 10mV maximum load regulation while sourcing load
currents up to 100µA. The reference is turned off during
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).
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 nega-
tive 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 out-
put for the MAX1626.
The MAX1627’s output voltage is set using two resis-
tors, R2 and R3 (Figure 5), which form a voltage divider
between the output and GND. R2 is given by:
where VREF = 1.3V. Since the input bias current at FB
has a maximum value of 50nA, large values (10kΩto
200kΩ) can be used for R3 with no significant accuracy
loss. For 1% error, the current through R2 should be at
R2= R3 x V
VOUT
REF −
1
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
8 _______________________________________________________________________________________
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
B
0A
C
A
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
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
B
0A
C
A
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform
least 100 times FB’s input bias current. Capacitor CR2
is used to compensate the MAX1627 for even switch-
ing. Values between 0pF and 330pF work for many
applications. See the
Stability and MAX1627 Feedback
Compensation
section for details.
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:
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:
Standard wire-wound resistors have an inductance
high enough to degrade performance, and are not rec-
ommended. Surface-mount (chip) resistors have very
little inductance and are well suited for use as current-
sense resistors. Power metal-strip resistors feature
1/2W and 1W power dissipation, 1% tolerance, and
inductance below 5nH. Resistance values between
10mΩand 500mΩare 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:
where 2µs is the minimum on-time. Inductor values
between two and six times L(MIN) are recommended.
L = V+ - V
(MIN) (MAX) OUT
()
()
xs
V
R
CS MIN
CS
2µ
R = V
R
POWER RATING (W)
2
CS
CS MAX()
R = V
1.3 x I
CS CS(MIN)
OUT(MAX)
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
_______________________________________________________________________________________ 9
R3
CR2 R2
FROM
OUTPUT
TO FB
Figure 5. Adjustable-Output Operation Using the MAX1627
4.5 5.55.0 6.0 1210 14 16
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
3.5
2.5
2.0
1.5
1.0
0
0.5
VOUT = 5V RSENSE = 0.03Ω
RSENSE = 0.04Ω
RSENSE = 0.05Ω
RSENSE = 0.1Ω
Figure 6a. MAX1626 5V-Operation Current-Sense Resistor
Graph
3.0 4.03.5 4.5 1210 14 16
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
3.5
2.5
2.0
1.5
1.0
0
0.5
VOUT = 3.3V RSENSE = 0.03Ω
RSENSE = 0.04Ω
RSENSE = 0.05Ω
RSENSE = 0.1Ω
Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor
Graph
MAX1626/MAX1627
With high inductor values, the MAX1626/MAX1627 will
begin continuous-conduction operation at a lower frac-
tion 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 current-
sense comparator propagation delay. Peak-current
overshoot reduces efficiency and could cause the exter-
nal components’ current ratings to be exceeded.
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to pre-
vent 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.
1µs is the worst-case current-sense comparator propa-
gation 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-
ings’ resistance should be on the order of the current-
sense 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.
External Switching Transistor
The MAX1626/MAX1627 drive P-channel enhancement-
mode 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 recom-
mended suppliers of switching transistors.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent 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:
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.
Qgspecifies 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.
I
D(MAX LIM MAX CS MAX
SENSE
IV
R
)() ()
≥=
I = V
R VV 1s
L
PEAK CS
CS
OUT
++−
()
×µ
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
10 ______________________________________________________________________________________
KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.
PRODUCTION
METHOD INDUCTORS CAPACITORS DIODES CURRENT-SENSE
RESISTORS MOSFETS
Surface Mount
AVX
TPS series
Sprague
595D series
Motorola
MBRS340T3
Nihon
NSQ series
Dale
WSL series
IRC
LRC series
Miniature
Through-Hole Sumida
RCH875-470M (1.3A)
Sanyo
OS-CON series
low-ESR organic
semiconductor
IRC
OAR series Motorola
Low-Cost
Through-Hole CoilCraft
PCH-45-473 (3.4A) Motorola
1N5817 to
1N5823
Motorola
TMOS power MOSFETs
Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)
CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Siliconix
Little Foot series
Motorola
medium-power
surface-mount products
Nichicon
PL series
low-ESR electrolytics
United Chemi-Con
LXF series
Table 1. Component Selection Guide
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 equiva-
lents, 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 applica-
tions, some Schottky diodes may be inadequate due to
their high leakage currents. In such cases, ultra-high-
speed 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
major contributor to output ripple, so low-ESR capaci-
tors are recommended. Sanyo OS-CON capacitors are
best, and low-ESR tantalum capacitors are second
best. Low-ESR aluminum electrolytic capacitors are tol-
erable, but do not use standard aluminum electrolytic
capacitors.
Voltage ripple is the sum of contributions from ESR and
the capacitor value:
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:
Estimate input and output capacitor values for given
voltage ripple as follows:
where I∆Lis the change in inductor current (around
0.5IPEAK under moderate loads).
These equations are suitable for initial capacitor selec-
tion; 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 compara-
tor’s hysteresis (0.5% of the output voltage) is not prac-
tical, 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 volt-
age ratings higher than the maximum input voltage.
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
CLI
VV
CLI
VV
V
VV
IN L
RIPPLECIN IN
OUT L
RIPPLECOUT OUT IN
IN OUT
=
=−
1
22
1
22
∆
∆
,
,
V
RIPPLE,ESR ≈ ()()R
ESR IPEAK
V
RIPPLE ≈+
,,
VV
RIPPLE ESR RIPPLE C
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
______________________________________________________________________________________ 11
Table 2. Component Suppliers
COMPANY PHONE FAX
(803) 946-0690
AVX USA or (803) 626-3123
(800) 282-4975
Coilcraft USA (847) 639-6400 (847) 639-1469
Coiltronics USA (516) 241-7876 (516) 241-9339
Dale USA (605) 668-4131 (605) 665-1627
International USA (310) 322-3331 (310) 322-3332
Rectifier
IRC USA (512) 992-7900 (512) 992-3377
Motorola USA (602) 303-5454 (602) 994-6430
Nichicon USA (847) 843-7500 (847) 843-2798
Japan 81-7-5231-8461 81-7-5256-4158
Nihon USA (805) 867-2555 (805) 867-2698
Japan 81-3-3494-7411 81-3-3494-7414
Sanyo USA (619) 661-6835 (619) 661-1055
Japan 81-7-2070-6306 81-7-2070-1174
(408) 988-8000
Siliconix USA or (408) 970-3950
(800) 554-5565
Sprague USA (603) 224-1961 (603) 224-1430
Sumida USA (847) 956-0666 (847) 956-0702
Japan 81-3-3607-5111 81-3-3607-5144
United USA (714) 255-9500 (714) 255-9400
Chemi-Con
MAX1626/MAX1627
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 jit-
ter, instability, or degraded performance. The current-
sense 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 lay-
out 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 commuta-
tion 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 exter-
nal components to ensure stable operation. In one-
shot, 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-
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 dur-
ing no-load conditions.
Instability can also be caused by excessive stray capaci-
tance 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-genera-
tion upgrades to the MAX649/MAX651 step-down con-
trollers. 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 cur-
rent-sense 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
the current-sense resistor value by 50% when substitut-
ing for the MAX649 or MAX651.
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
12 ______________________________________________________________________________________
MAX1626
yyyyy
yyyyy
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yyyyy
yyyyy
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yyyy
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C REF
C V+ BYPASS
4x
SCALE
R SENSE
Figure 7. Recommended Placement and Routing of the
Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input
Bypass Capacitors
________________________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.
Micropower Step-Down Converter
When designing a low-power, battery-based applica-
tion, choose an external MOSFET with low gate capaci-
tance (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 tran-
sients, 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 syn-
chronous rectification is recommended.
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
______________________________________________________________________________________ 13
MAX1627
C5
0.47µF
P
D1
RSENSE
0.15Ω
U1
LOGIC-LEVEL MOSFET
C4
0.1µF
OUT
N.C.
SHDN
REF
V+
EXT
CS
GND FB
INPUT
L1
22µH, 3A
R3 R2
CR2
C1
220µF
LOW-ESR
TANTALUM
C2
68µF LOW-ESR
TANTALUM
C3
68µF LOW-ESR
TANTALUM
L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03
U1: MOTOROLA MMSF3P02HD
ADJUSTABLE
OUTPUT
Figure 8. MAX1627 Typical Operating Circuit
MAX1626
C3
0.47µF
P
D1
RSENSE
0.15Ω
U1
LOGIC-LEVEL MOSFET
C4
0.1µF
3/5
SHDN
REF
V+
EXT
CS
GND OUT
INPUT
L1
68µH, 0.7A OUTPUT
C1
100µF
LOW-ESR
TANTALUM
C2
68µF LOW-ESR
TANTALUM
L1: SUMIDA CDR1053-680
D1: MOTOROLA MBRS130T3
U1: MOTOROLA MMSF3P02HD
Figure 9. 0.5A Step-Down Converter
MAX1627
POUTPUT
2.5V, 6A
R3
21.5k, 1% R2
20k, 1%
D1
CR2
220pF
Q1
LOGIC-LEVEL MOSFET
C10
0.1µF
OUT
3V TO 6V
SHDN
N.C.
REF
V+
EXT
CS
GND FB
INPUT
L1
2.7µH >8A
C4
100µFC5
100µF
C9
1.0µF
RCS1, RCS2
0.025Ω
C8
1.0µF
C7
0.1µF
C3
220µF
C2
220µF
C1
220µF
C6
0.1µF
C1–C3: SANYO OS-CON 220 µF, 6.3V
C4, C5: SANYO OS-CON 100 µF, 20V
RCS1, RCS2: 0.025Ω DALE WSL-2512
Q1: MOTOROLA MTB50PO3HDL
D1: NIEC C10T04Q
L1: SUMIDA CDRH127-2R7NC
Figure 10. 6A Step-Down Converter
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
14 ______________________________________________________________________________________
VIA
VIA
VIAS
COMPONENT PLACEMENT GUIDE—COMPONENT SIDE COPPER ROUTING—FRONT SIDE
COPPER ROUTING—BACK SIDE
Figure 11. Recommended PC Board Design for 6A Step-Down Converter
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
______________________________________________________________________________________ 15
___________________Chip Topography
( ) ARE FOR MAX1627
TRANSISTOR COUNT: 375
SUBSTRATE CONNECTED TO V+
REF
0.105"
(2.63mm)
0.081"
(2.06mm)
VCC
CS
EXT
GNDGNDOUT
SHDN
3/5
(FB)
DIM
A
A1
B
C
E
e
H
L
MIN
0.053
0.004
0.014
0.007
0.150
0.228
0.016
MAX
0.069
0.010
0.019
0.010
0.157
0.244
0.050
MIN
1.35
0.10
0.35
0.19
3.80
5.80
0.40
MAX
1.75
0.25
0.49
0.25
4.00
6.20
1.27
INCHES MILLIMETERS
21-0041A
Narrow SO
SMALL-OUTLINE
PACKAGE
(0.150 in.)
DIM
D
D
D
MIN
0.189
0.337
0.386
MAX
0.197
0.344
0.394
MIN
4.80
8.55
9.80
MAX
5.00
8.75
10.00
INCHES MILLIMETERS
PINS
8
14
16
1.270.050
L
0°-8°
HE
D
e
A
A1 C
0.101mm
0.004in.
B
MAX1626/MAX1627
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
________________________________________________________Package Information
16 ______________________________________________________________________________________
