MAXIM MAX1626, MAX1627 Technical data

查询MAX1626供应商

19-1075; Rev 0; 6/96

EVALUATION KIT MANUAL

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High-Efficiency, Step-Down DC-DC Controllers

_______________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-chan­nel MOSFET, allowing design flexibility for applications
to 12W or higher. Soft-start reduces current surges dur­ing 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 volt­ages 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

__________________Pin Configuration

TOP VIEW

3/5 (FB)

SHDN

( ) ARE FOR MAX1627

5V/3.3V or Adjustable, 100% Duty-Cycle,

____________________________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

______________Ordering Information

TEMP. RANGE PIN-PACKAGE

0°C to +70°C

-40°C to +85°C
0°C to +70°C

V+

MAX1626

CSSHDN

P

3/5

REF

GND

EXT

OUT

Dice*
8 SO
Dice*

OUTPUT

3.3V

OUT

REF

1
2
3
4

MAX1626
MAX1627

SO



PART

MAX1626C/D

MAX1626ESA
MAX1627C/D
MAX1627ESA -40°C to +85°C 8 SO

* Dice are tested at TA= +25°C.

__________Typical Operating Circuit

INPUT

3.3V to 16.5V

8

GND
EXT

7

CS

6
5

V+

ON/OFF

MAX1626/MAX1627

________________________________________________________________

Maxim Integrated Products

1

For free samples & the latest literature: http://www.maxim-ic.com, or phone [phone]

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

ABSOLUTE MAXIMUM RATINGS

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 (I
Maximum Current at EXT (I
Continuous Power Dissipation (T

SO (derate 5.88mW/°C above +70°C)..........................471mW

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.

)..........................................15mA

REF

) ..........................................50mA

EXT

= +70°C)

A

ELECTRICAL CHARACTERISTICS

(V+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA= 0°C to +85°C, unless otherwise noted.)

Operating, no load

V

I+Supply Current into V+

V+ = SHDN = 16.5V (shutdown)

Circuit of Figure 1, 3/5 = V+ (Note 1)

OUT

Circuit of Figure 1, 3/5 = 0V (Note 1)
MAX1626, 3/5 = V+, output forced to 5V

OUT

MAX1627, includes hysteresis
MAX1627

CS

SHDN = 0V or V+

3/5 = 0V or V+
V+ = 5V
Output forced to 0V
Output in regulation

6.0V < V+ < 12.0V, I
30mA < I

LOAD

I

= 0µA

LOAD

REF

REF

≤ 100µA

0µA ≤ I
V+ = 3V to 16.5V, I

MAX1626/MAX1627

Output Voltage
OUT Input Current

CS Threshold Voltage

3/5 Input Voltage High
3/5 Input Voltage Low
3/5 Leakage Current

Minimum EXT Off Time

Reference Voltage

Operating Temperature Range

MAX1626ESA/MAX1627ESA............................-40°C to +85°C

Storage Temperature Range.............................-65°C to +160°C

Lead Temperature (soldering, 10sec).............................+300°C

CONDITIONS

70 90

4.85 5.00 5.15

3.20 3.30 3.40

81012

1.5 2.0 2.5

= 1A

LOAD

< 2.0A, V+ = 8V

= 0µA µV/V10 100REF Line Regulation

LOAD

UNITSMIN TYP MAXSYMBOLPARAMETER

1

V3.0 16.5V+Input Voltage Range

µA

V2.7 2.8Undervoltage Lockout
V

µA24 37 50I

V1.27 1.30 1.33FB Threshold Voltage
nA035FB Leakage Current
µA010CS Input Current

mV85 100 115V

V1.6SHDN Input Voltage High

V0.4SHDN Input Voltage Low
µA±1SHDN Input Current

VV+ - 0.5

V0.5
µA±1

Ω10EXT Resistance
µs

%100EXT Duty-Cycle Limit
mV/V5Line Regulation
mV/A15Load Regulation

V1.27 1.30 1.33V

mV410REF Load Regulation

2 _______________________________________________________________________________________

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

ELECTRICAL CHARACTERISTICS

(V+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

CONDITIONS

Operating, no load

Supply Current into V+

Output Voltage
OUT Input Current

I

V

OUT

V+ = SHDN = 16.5V (shutdown)

Circuit of Figure 1, 3/5 = V+

OUT

Circuit of Figure 1, 3/5 = 0V
MAX1626, 3/5 = V+, output forced to 5V

OUT

MAX1627, includes hysteresis
MAX1627

I

= 0µA

LOAD

Note 1: V+ must exceed V
Note 2: Specifications from 0°C to -40°C are guaranteed by design, not production tested.

to maintain regulation.

OUT

4.80 5.20

3.16 3.44

100

2

MAX1626/MAX1627

UNITSMIN TYP MAXSYMBOLPARAMETER

V3.0 16.5V+Input Voltage

µA

V2.9Undervoltage Lockout
V

µA24 50I

V1.25 1.35FB Threshold Voltage

nA050FB Leakage Current

mV80 120CS Threshold Voltage

V1.25 1.35Reference

__________________________________________Typical Operating Characteristics

(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)

DROPOUT VOLTAGE

0.45

0.40

0.35

0.30

0.25

0.20

0.15

DROPOUT VOLTAGE (V)

0.10

0.05
0

vs. LOAD CURRENT

3.3V SETTING
= +3.17V

V

OUT

0 0.5 1.0 1.5 2.0 2.5

LOAD (A)

_______________________________________________________________________________________ 3

5V SETTING

= +4.8V

V

OUT

MAX1626-11

EFFICIENCY vs. LOAD CURRENT

(V

= +3.3V)

100

90

A

80
70
60
50
40

EFFICIENCY (%)

30
20
10

CIRCUIT OF FIGURE 1

0

0.1m 100m 11m 10m 10

C

B

LOAD CURRENT (A)

OUT

D E F

A: V+ = +4.3V
B: V+ = +5V
C: V+ = +8V
D: V+ = +10V
E: V+ = +12V
F: V+ = +15V


MAX1626-05

EFFICIENCY (%)

EFFICIENCY vs. LOAD CURRENT

(V

= +5V)

100

90

A
80
70

60
50
40
30
20
10

CIRCUIT OF FIGURE 1

0

0.1m 100m 11m 10m 10

OUT

C

B

LOAD CURRENT (A)

D

E

A: V+ = +6V
B: V+ = +8V
C: V+ = +10V
D: V+ = +12V
E: V+ = +15V


MAX1626-03

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

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)

MAX1626 SHUTDOWN CURRENT

0.8

0.7

0.6

0.5

0.4

0.3

0.2

SHUTDOWN CURRENT (µA)

0.1
0

-60

MAX1626/MAX1627

vs. TEMPERATURE

APPLICATION CIRCUIT
SHUTDOWN CURRENT:
A: V+ = +15V
B: V+ = +10V
C: V+ = +4V

MAX1626 SHUTDOWN
CURRENT:
D: V+ = +16V
E: V+ = +4V


-20 0 204060 80 100120

-40
TEMPERATURE (°C)

A

MAX1626-04

B

C

D

EXT OFF TIME (µs)

E

140

MAX1626 EXT OFF TIME

vs. OUTPUT VOLTAGE

12

10

8

6

4

2

0

012345

3/5 = V+

3/5 = GND

OUTPUT VOLTAGE (V)

V+ = +5V

MAX1626-02

12

10

8

6

4

EXT OFF TIME (µs)

2

MAX1627 EXT OFF TIME

vs. FB PIN VOLTAGE

0

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

FB PIN VOLTAGE (V)

V+ = +5V

MAX1626-03

EXT RISE AND FALL TIMES

50
45
40
35

(ns)

30

FALL

25

AND t

20

RISE

t

15
10

5
0

-60 -40 -20 40 60 140

vs. TEMPERATURE

t

, V+ = +5V

FALL

t

RISE

C

= 1nF

EXT

3/5 = 0V
OUT = 50kHz, 0.3Vp-p

0

20 80

TEMPERATURE (°C)

t

RISE

, t

FALL

3.3V

,

, V+ = +5V

, V+ = +15V

DC

100 120

MAX1626-09

400
350
300

(ns)

250

FALL

200

AND t

150

RISE

t

100

50

EXT RISE AND FALL TIMES

vs. CAPACITANCE

t

, V+ = +15V

RISE

, V+ = +5V

t

FALL

t

, V+ = +5V

RISE

, V+ = +15V

t

0

0 2000 4000

CAPACITANCE (pF)

FALL

MAX1626

V+ QUIESCENT CURRENT

72

70

68

66

(µA)

Q

I

64

62

60

-60

vs. TEMPERATURE

V+ = +16V

V+ = +10V

V+ = +4V

3/5 = 0V
OUT FORCED TO 3.4V

-20 0 204060 80 100120

-40
TEMPERATURE (°C)

MAX1626-01

140

CS TRIP LEVEL vs. TEMPERATURE

115

OUT = 0V

110

105

100

95

CS TRIP LEVEL (mV)

90

85

-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

4 _______________________________________________________________________________________

MAX1626-10

MAX1626-12

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)

MAX1626/MAX1627

REFERENCE OUTPUT VOLTAGE

vs. TEMPERATURE

1.310

1.305

1.300

1.295

1.290

1.285

REFERENCE OUTPUT VOLTAGE (V)

1.280

I

= 0µA

REF

= 50µA

I

REF

-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

MAX1626 LOAD-TRANSIENT RESPONSE

A

I

REF

= 10µA

I

REF

= 100µA

MAX1626-15

MAX1626-13

MAX1626 LINE-TRANSIENT RESPONSE

A

MAX1626 SHUTDOWN RESPONSE TIME

AND SUPPLY CURRENT

A

B

C

V+ = 8V, V
A: OUT, 2V/div
B: SUPPLY CURRENT, 1A/div
C: SHDN, 5V/div

500µs/div

= 5V, LOAD = 1A

OUT

LINE-TRANSIENT RESPONSE

FROM 100% DUTY CYCLE

MAX1626-16

A

MAX1626-14

MAX1626-17

B

V+ = 8V, V
A: OUT, 50mV/div, 3.3V DC OFFSET
B: LOAD CURRENT, 1A/div


100µs/div

= 3.3V, LOAD = 30mA to 2A

OUT

_______________________________________________________________________________________

B

V

= 5V, LOAD = 1A, CIN = 33µF

OUT

A: OUT, 100mV/div, 5V DC OFFSET
B: V+ 6V to 12V, 2V/div

5ms/div

B

V

= 3.3V, LOAD = 1A, CIN = 47µF

OUT

A: OUT, 100mV/div, 3.3V DC OFFSET
B: V+ 3.3V to 15V, 5V/div

5ms/div

5

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

______________________________________________________________Pin Description

MAX1626

PIN

MAX1627

NAME

Sense input for fixed 5V or 3.3V output operation. OUT is internally connected to an

11

OUT

on-chip voltage divider (MAX1626). It does not supply current. Leave OUT uncon­nected during adjustable-output operation (MAX1627).

2—

—2

FB

3/5

Feedback Input for adjustable-output operation. Connect to an external voltage
divider between the output and GND (see the

3.3V or 5V Selection. Output voltage is set to 3.3V when this pin is low or 5V when it
is high.

Active-High Shutdown Input. Device is placed in shutdown when SHDN is driven

33

SHDN

high. In shutdown mode, the reference, output, and external MOSFET are turned off.
Connect to GND for normal operation.

MAX1626/MAX1627

V+

REF

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

66

CS

MOSFET is turned off when the voltage across the resistor equals the current-limit
trip level (around 100mV).

Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND.77
Ground88

C5

0.47µF

R



SENSE

0.04Ω

U1 
LOGIC-LEVEL MOSFET

L1 

22µH, 3A

L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03
U1: MORTOLA MMSF3PO2HD

INPUT

68µF LOW-ESR

TANTALUM

C4

0.1µF

C2

3/5

SHDN

REF

MAX1626

GND

C3
68µF LOW-ESR
TANTALUM

V+

CS

EXT

OUT

D1

EXT

GND

P

Figure 1. MAX1626 Typical Operating Circuit

OUTPUT


C1
220µF
LOW-ESR
TANTALUM

FUNCTION

Setting the Output Voltage

EXT REF

1.5V

MAX1626
MAX1627

MINIMUM ON-TIME

ONE-SHOT

TRIG

Q

Q

TRIG

MINIMUM OFF-TIME

ONE-SHOT



S

CURRENT-SENSE
COMPARATOR

REF

Q
R

Figure 2. Simplified Functional Diagram

ERROR
COMPARATOR

R2

R1

( ) MAX1627 ONLY
MAX1626 ONLY

section).

OUT

(FB)

R3

3/5

SHDN

V+
CS

6 _______________________________________________________________________________________

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

_______________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

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=
I x R + R + R

OUT DS(ON) SENSE INDUCTOR

[]

MAX1626/MAX1627

_______________________________________________________________________________________ 7

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

A
B

C

0A

CIRCUIT OF FIGURE 1, V+ = 8V, V
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET

MAX1626/MAX1627

Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform

C: INDUCTOR CURRENT, 1A/div

10µs/div

= 5V, LOAD = 100mA

OUT

EXT Drive Voltage Range

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 (R

External Switching Transistor

section.

DS(ON)

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 V

at 5V, typical

OUT

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

). See

A
B

C

0A

CIRCUIT OF FIGURE 1, V+ = 8V, V
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div

Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform

10µs/div

= 5V, LOAD = 1.5A

OUT

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 nega­tive inductor current slope due to low output voltages.

________________Design Information

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:

where V

REF

has a maximum value of 50nA, large values (10kΩ to
200kΩ) can be used for R3 with no significant accuracy
loss. For 1% error, the current through R2 should be at

Setting the Output Voltage

R2= R3 x



V



V



OUT

REF



−

1





= 1.3V. Since the input bias current at FB

8 _______________________________________________________________________________________

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

least 100 times FB’s input bias current. Capacitor C
is used to compensate the MAX1627 for even switch­ing. Values between 0pF and 330pF work for many
applications. See the

Compensation

Stability and MAX1627 Feedback

section for details.

Current-Sense-Resistor Selection

The current-sense comparator limits the peak switching
current to VCS/R

SENSE

, where R

SENSE

is the value of
the current-sense resistor and VCSis the current-sense
threshold. VCSis 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:

V

R =

CS

1.3 x I

CS(MIN)

OUT(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:

2

V

R =

POWER RATING (W)

CS MAX()

R

CS

Standard wire-wound resistors have an inductance
high enough to degrade performance, and are not rec­ommended. Surface-mount (chip) resistors have very
little inductance and are well suited for use as current-

3.5
V

= 5V

OUT

3.0

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

4.5 5.55.0 6.0 1210 14 16



R

SENSE

R

SENSE

R

SENSE

R

SENSE



INPUT VOLTAGE (V)

= 0.03Ω

= 0.04Ω

= 0.05Ω

= 0.1Ω

R2

C

R2

FROM
OUTPUT

R2

TO FB

R3

Figure 5. Adjustable-Output Operation Using the MAX1627

sense 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:

V+ - V

()

L =

(MIN)

(MAX) OUT

V

()

CS MIN

R

CS

where 2µs is the minimum on-time. Inductor values
between two and six times L

3.5
V

= 3.3V

OUT

3.0

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

0

3.0 4.03.5 4.5 1210 14 16
INPUT VOLTAGE (V)

are recommended.

(MIN)



R

= 0.03Ω

SENSE

R

= 0.04Ω

SENSE

R

= 0.05Ω

SENSE

R

= 0.1Ω

SENSE

xs

2µ

MAX1626/MAX1627

Figure 6a. MAX1626 5V-Operation Current-Sense Resistor
Graph

_______________________________________________________________________________________ 9

Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor
Graph

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

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.

MAX1626/MAX1627

I =

PEAK

V

CS

R

CS

+−

VV 1s

()

+

OUT

L

×µ

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 wind­ings’ resistance should be on the order of the current­sense resistance. If necessary, use a toroid, pot-core,

KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.

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 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 R
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

CS MAX

I

D(MAX LIM MAX

I

≥=

)()

()

R

SENSE

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

. R

DS(ON)

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.

DS(ON)

should be

. The

Table 1. Component Selection Guide

PRODUCTION

METHOD

Surface Mount

Miniature
Through-Hole

Low-Cost
Through-Hole

10 ______________________________________________________________________________________

INDUCTORS CAPACITORS DIODES

Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)

CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)

Sumida
RCH875-470M (1.3A)

CoilCraft
PCH-45-473 (3.4A)

AVX
TPS series

Sprague
595D series

Sanyo
OS-CON series
low-ESR organic
semiconductor

Nichicon
PL series
low-ESR electrolytics

United Chemi-Con
LXF series

Motorola
MBRS340T3

Nihon
NSQ series

Motorola
1N5817 to
1N5823

CURRENT-SENSE

RESISTORS

Dale
WSL series

IRC
LRC series

IRC
OAR series

Siliconix
Little Foot series

Motorola
medium-power
surface-mount products

Motorola

Motorola
TMOS power MOSFETs

MOSFETS

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

Table 2. Component Suppliers

COMPANY PHONE FAX

AVX USA or [phone]

Coilcraft USA [phone] [phone]
Coiltronics USA [phone] [phone]
Dale USA [phone] [phone]
International

Rectifier
IRC USA [phone] [phone]
Motorola USA [phone] [phone]
Nichicon USA [phone] [phone]

Nihon USA [phone] [phone]

Sanyo USA [phone] [phone]

Siliconix USA or [phone]

Sprague USA [phone] [phone]

Sumida USA [phone] [phone]

United
Chemi-Con

USA [phone] [phone]

Japan 81-7-5231-8461 81-7-5256-4158

Japan 81-3-3494-7411 81-3-3494-7414

Japan 81-7-2070-6306 81-7-2070-1174

Japan 81-3-3607-5111 81-3-3607-5144
USA [phone] [phone]

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 R

SENSE

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.

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 capaci­tors are recommended. Sanyo OS-CON capacitors are

[phone]
[phone]

[phone]
[phone]

Diode Selection

, and that its breakdown voltage

Capacitor Selection

best, and low-ESR tantalum capacitors are second
best. Low-ESR aluminum electrolytic capacitors are tol­erable, but do not use standard aluminum electrolytic
capacitors.

Voltage ripple is the sum of contributions from ESR and
the capacitor value:

V

≈+

RIPPLE

VV

,,

RIPPLE ESR RIPPLE 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:

V

RIPPLE,ESR

≈ ()()R

ESRIPEAK

Estimate input and output capacitor values for given
voltage ripple as follows:

2

1

LI

∆

L

2

=

VV

,

RIPPLE CIN IN

=

VV

RIPPLE COUT OUTININ OUT

2

1

LI

∆

L

2

,



V



−

VV



is the change in inductor current (around

under moderate loads).

where I

0.5I

PEAK

C

IN

C

OUT

∆L

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.

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

MAX1626/MAX1627






______________________________________________________________________________________ 11

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

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 lay-

MAX1626/MAX1627

out and routing for these components.
High-power traces, highlighted in the

Circuit

(Figure 1), should be as short and as wide as
possible. The supply-current loop (formed by C2, C3,
R

, U1, L1, and C1) and commutation-current loop

SENSE

(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.

Typical Operating

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 low­frequency 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

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

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) current­sense threshold. The MAX1649/MAX1651 are second­generation upgrades with a 96.5% maximum duty cycle
for improved dropout performance and a reduced cur­rent-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 substitut­ing for the MAX649 or MAX651.

R

SENSE

12 ______________________________________________________________________________________

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

INPUT

68µF LOW-ESR

TANTALUM

C2

68µF LOW-ESR

TANTALUM

C3

C5

0.47µF

INPUT

68µF LOW-ESR

TANTALUM

C2

C3

0.47µF

MAX1626/MAX1627

V+

R



SENSE

0.15Ω

U1 
LOGIC-LEVEL MOSFET

P

22µH, 3A

D1

L1: SUMIDA CDRH125-220
D1: NIHON NSQ03A03

C

R2

U1: MOTOROLA MMSF3P02HD

L1 

ADJUSTABLE

OUTPUT

C1
220µF
LOW-ESR
TANTALUM

0.1µF

C4

N.C.

MAX1627

OUT
SHDN
REF

GND FB

CS

EXT

R3 R2

Figure 8. MAX1627 Typical Operating Circuit

________________________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.

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.

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 R
gate-charge factor below the MAX1626/MAX1627’s
drive capability (see Ext Rise and Fall Times vs.
Capacitance graph in the

Characteristics

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.

Micropower Step-Down Converter

High-Current Step-Down Converter

. Keep the

DS(ON)

Typical Operating

). Otherwise, increased MOSFET rise

V+

MAX1626

0.1µF

3/5

SHDN

REF

C4

GND

CS

EXT

OUT

D1

Figure 9. 0.5A Step-Down Converter

3V TO 6V

INPUT

MAX1627

GND

21.5k, 1%

R3

100µF

C5

C6

0.1µF

V+

CS

EXT

FB

Q1 
LOGIC-LEVEL MOSFET

P

2.7µH >8A

D1

R2

20k, 1%

C1–C3: SANYO OS-CON 220µF, 6.3V

C

R2

C4, C5: SANYO OS-CON 100

220pF

RCS1, RCS2: 0.025Ω DALE WSL-2512
Q1: MOTOROLA MTB50PO3HDL
D1: NIEC C10T04Q
L1: SUMIDA CDRH127-2R7NC

C10

0.1µF

100µF

N.C.

C4

OUT
SHDN

REF

Figure 10. 6A Step-Down Converter

R



SENSE

0.15Ω
U1 

LOGIC-LEVEL MOSFET

P

L1 

68µH, 0.7A

L1: SUMIDA CDR1053-680
D1: MOTOROLA MBRS130T3
U1: MOTOROLA MMSF3P02HD

C8

C7

1.0µF

0.1µF

R

, R



CS1

CS2

0.025Ω

L1 

C2

C1

220µF

220µF

OUTPUT

C1
100µF
LOW-ESR
TANTALUM

C9

1.0µF

OUTPUT

2.5V, 6A

C3
220µF

µF, 20V

______________________________________________________________________________________ 13

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

VIA

VIAS

MAX1626/MAX1627

VIA

COMPONENT PLACEMENT GUIDE—COMPONENT SIDE

COPPER ROUTING—BACK SIDE

COPPER ROUTING—FRONT SIDE

Figure 11. Recommended PC Board Design for 6A Step-Down Converter

14 ______________________________________________________________________________________

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

___________________Chip Topography

GNDGNDOUT

EXT

3/5

(FB)



0.105"

(2.63mm)

MAX1626/MAX1627

SHDN

REF

0.081"

(2.06mm)

( ) ARE FOR MAX1627
TRANSISTOR COUNT: 375
SUBSTRATE CONNECTED TO V+

CS

V

CC

______________________________________________________________________________________ 15

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

________________________________________________________Package Information

INCHES MILLIMETERS

DIM

D

A

0.101mm

e

A1

B

MAX1626/MAX1627

0.004in.

HE

C

L

Narrow SO

SMALL-OUTLINE

PACKAGE

(0.150 in.)

0°-8°

A1

DIM

D
D
D



A

B
C
E
e
H
L

PINS



MAX

MIN

0.069

0.053

0.010

0.004

0.019

0.014

0.010

0.007

0.157

0.150

0.228

0.016

8
14
16





0.244

0.050

INCHES MILLIMETERS

MIN

MAX

0.189

0.197

0.337

0.344

0.386

0.394

MIN

1.35

0.10

0.35

0.19

3.80


5.80

0.40

MIN

4.80

8.55

9.80

1.270.050

MAX

1.75

0.25

0.49

0.25

4.00


6.20

1.27

MAX

5.00

8.75

10.00

21-0041A

16 ______________________________________________________________________________________

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