General Description

The MAX8737 dual high-power linear regulator con-
trollers use external n-channel MOSFETs to generate
two independent low-voltage supplies for notebook
computers. The MAX8737 delivers low output voltages
from 0.5V to 2.5V (±5mV no-load accuracy). The exter-
nal components allow scalable current design with
loads up to 5A with excellent load regulation (1%). The
regulator operates from a low input voltage, which also
reduces the power dissipation in the external n-channel
MOSFET. The controller powers the external MOSFET
gate driver from the standard 5V system supply.

The MAX8737 includes current and thermal limits to
prevent damage to the linear regulator. The MAX8737
uses an external resistive divider to fold back the cur-
rent limit, reducing the overall power dissipation. The
MAX8737 uses an external resistive-divider in series
with the current-sense input (CS_), providing foldback
current-limit protection, and effectively reducing the
short-circuit power dissipation.

An output undervoltage timeout is available for low-cost
applications that omit the current-sense resistor. The
output undervoltage (UVP) timing depends on the mag-
nitude of the voltage at V

OUT

. The UVP detects and

shuts down the LDO if the output voltage drops out of
regulation. The controller uses an adjustable reference
input (REFIN_) to set the nominal output voltage
(V

OUT_

), which minimizes the cost and makes the sta-

bility independent of the output voltage.

Each linear regulator features an adjustable soft-start
function, and generates a delayed power-good
(PGOOD) signal that signals when the linear regulator
is in regulation. The MAX8737 is a low-cost solution
requiring few external components and is available in a
small, 4mm x 4mm, 16-pin thin QFN package.

Applications

Notebook and Desktop Computers

Point-of-Load Regulators

MCH

and V

CCP

CPU Supplies

Low-Voltage Bias Supplies

Servers

Features

Low-Cost Dual Linear Regulators
Output Voltage Accuracy ±5mV
Independent 0.5V to 2.5V Reference Inputs
Foldback Current-Limit Protection
Output Undervoltage-Lockout Protection
Thermal Limit (Internal Sensor)
1.0V to 5.5V Input Supply Voltage (External FET

Drain)

5V Bias Supply Voltage
Independent Power-Good Open-Drain Outputs
Independent Enable Inputs
Soft-Shutdown Output Discharge
Low Supply Current (0.5mA)
5µA (max) Shutdown Supply Current

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

________________________________________________________________ Maxim Integrated Products

DRV1

GND

N.C.

DRV2

N.C.

CS2

OUT2

REFIN2

CS1

OUT1

REFIN1

PGOOD2

PGOOD1

EN2

EN1

TOP VIEW

MAX8737

4mm x 4mm TQFN

Pin Configuration

19-3705; Rev 0; 5/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

Ordering Information

PART

TEMP RANGE PIN-PACKAGE

MAX8737ETE

-40°C to +85°C 16 Thin QFN-EP* 4mm x 4mm

MAX8737ETE+ -40°C to +85°C 16 Thin QFN-EP* 4mm x 4mm

*EP = Exposed pad.

+Denotes lead-free packaging.

MAX8737

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

_______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

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.

to GND ..............................................................-0.3V to +6V

OUT1, OUT2 to GND................................................-0.3V to +6V
REFIN1, REFIN2, PGOOD1, PGOOD2, EN1,

EN2 to GND..........................................................-0.3V to +6V

DRV1, DRV2, CS1, CS2 to GND.................-0.3V to (V

+ 0.3V)

Continuous Power Dissipation (T

= +70°C)

16-Pin 4mm x 4mm Thin QFN (derated 25mW/°C
above +70°C).............................................................2000mW

Operating Temperature Range

MAX8737ETE ...................................................-40°C to +85°C

Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C

ELECTRICAL CHARACTERISTICS

= 5V, EN_ = CS_ = V

, V

REFIN

= 1.0V, T

= 0°C to +85°C, unless otherwise noted. Typical values are at T

= +25°C.)

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

Supply Voltage Range

4.75

5.50

Undervoltage Lockout

Threshold

Rising edge, 200mV hysteresis (typ)

4.1

4.35

4.6

Quiescent Supply Current

EN1 = EN2 = V

0.5

Shutdown Supply Current

EN1 = EN2 = GND

0.1

µA

REFIN to OUT Offset Voltage

OUT

-5

OUT_ Input Bias Current

OUT

-1

µA

DRIVERS

Output high; V

OUT

_ = V

REFIN

_ - 25mV,

LOAD

= 1mA

0.3

0.05

DRV_ Output Voltage Swing
(Note 1)

Output low; V

OUT

_ = V

REFIN

_ + 25mV,

LOAD

= 1mA

0.03

0.3

DRV_ Maximum Sourcing Current

OUT

_ = V

REFIN

_ - 25mV; V

DRV

= 3V

DRV_ Maximum Sinking Current

OUT

_ = V

REFIN

_ + 25mV; V

DRV

= 3V

OUT_ to DRV_ Transconductance
(Large Signal)

MDRV

0.8

DRV_ Power-Supply Rejection
Ratio

10Hz < f < 10kHz, I

DRV

= 1mA, C

DRV

10nF

-80

DRV_ Soft-Start Charging Current

SOFT

170

400

µA

REFERENCE INPUT

REFIN_ Voltage Range

REFIN

= 4.75V to 5.5V

0.5

2.5

REFIN_ Input Bias Current

REFIN

_ = 0 to 2.5V

-100

-10

+100

FAULT PROTECTION

Thermal Shutdown Threshold

SHDN

Hysteresis = 20°C

+125

°C

= 0°C to +85°C

Current-Limit Threshold

ILIM

_ - V

OUT

= +85°C

7.5

12.5

CS_ Input Current

-1

µA

Linear Regulator UVP Threshold
(Slow)

UVP

(SLOW)

With respect to V

REFIN

; CS_ = V

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

= 5V, EN_ = CS_ = V

, V

REFIN

= 1.0V, T

= 0°C to +85°C, unless otherwise noted. Typical values are at T

= +25°C.)

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

Linear Regulator UVP Threshold
(Fast)

UVP

(FAST)

With respect to V

REFIN

; CS_ = V

Slow Short-Circuit Timer Duration

UVP(SLOW)

With respect to V

REFIN

; CS_ = V

µs

Fast Short-Circuit Timer Duration

UVP(FAST)

With respect to V

REFIN

; CS_ = V

µs

Discharge-Mode On-Resistance
OUT_ Pin

OUT

INPUTS AND OUTPUTS

EN_ Input Low Level

0.6

EN_ Input High Level

Rising edge, 200mV (typ) hysteresis

1.6

Enable Leakage Current

-1

µA

Power-Good Trip Threshold
(Lower)

With respect to error comparator threshold,
hysteresis = 4% (falling edge)

-15

-12

-9

Power-Good Startup Delay

Power-Good Propagation Delay

PGOOD

OUT_ forced 2% beyond PGOOD_ trip
threshold

µs

Power-Good Output Low Voltage

SINK

= 4mA

0.3

Power Good Leakage Current

GOO

OUT

_ = 1.0V (PGOOD_ high impedance),

µA

ELECTRICAL CHARACTERISTICS

= 5V, EN_ = CS_ = V

, V

REFIN

= 1.0V, T

= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

Supply Voltage Range

4.75

5.50

Undervoltage Lockout

Threshold

Rising edge 200mV hysteresis (typ)

4.1

4.6

Quiescent Supply Current

EN1 = EN2 = V

1.5

Shutdown Supply Current

EN1 = EN2 = GND

µA

REFIN to OUT Offset Voltage

OUT

-7

DRIVERS

Output high; V

OUT

_ = V

REFIN

_ - 25mV;

LOAD

= 1mA

0.3

DRV_ Output Voltage Swing
(Note 1)

Output low; V

OUT

_ = V

REFIN

_ + 25mV:

LOAD

= 1mA

0.3

DRV_ Maximum Sourcing Current

OUT

_ = V

REFIN

_ - 25mV; V

DRV

= 3V

3.5

DRV_ Maximum Sinking Current

OUT

_ = V

REFIN

_ + 25mV; V

DRV

= 3V

3.5

DRV_ Soft-Start Charging Current

SOFT

400

µA

MAX8737

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

_______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

= 5V, EN_ = CS_ = V

, V

REFIN

= 1.0V, T

= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

REFERENCE INPUT

REFIN_ Voltage Range

REFIN

= 4.75V to 5.5V

0.5

2.5

FAULT PROTECTION

Current-Limit Threshold

ILIM

_ - V

OUT

6.5

13.5

Linear Regulator UVP Threshold
(Slow)

UVP

(SLOW)

With respect to V

REFIN

; CS_ = V

Linear Regulator UVP Threshold
(Fast)

UVP

(FAST)

With respect to V

REFIN

; CS_ = V

INPUTS AND OUTPUTS

EN_ Input Low Level

0.6

EN_ Input High Level

1.6

Power-Good Trip Threshold
(Lower)

With respect to error comparator threshold,
hysteresis = 4% (falling edge)

-15

-9

Power-Good Output Low Voltage

SINK

= 4mA

0.3

Note 1: Low threshold n-channel MOSFET is required for 2.5V (±2%) output.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.

Typical Operating Characteristics

(Circuit of Figure 1, T

= +25°C, unless otherwise noted.)

OUTPUT-VOLTAGE DEVIATION

vs. LOAD CURRENT

MAX8737 toc01

LOAD CURRENT (A)

OUTPUT-VOLTAGE DEVIATION (mV)

1.5

1.0

0.5

-4

-3

-2

-1

-5

2.0

OUT

= 1.5V

FOLDBACK CURRENT LIMIT

vs. OUTPUT VOLTAGE

MAX8737 toc02

OUTPUT VOLTAGE (V)

CURRENT LIMIT (A)

1.0

0.5

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

1.5

1ms/div

SOFT-START

(EN RISING EDGE)

MAX8737 toc03

1.5V

A. EN1, 5V/div
B. DRV1, 2V/div
NO LOAD

C. LDO1 OUTPUT, 1V/div
D. PGOOD1, 5V/div

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

_______________________________________________________________________________________

100

µs/div

SOFT-STOP

(EN FALLING EDGE)

MAX8737 toc04

1.5V

A. EN1, 5V/div
B. DRV1, 2V/div
NO LOAD

C. LDO1 OUTPUT, 1V/div
D. PGOOD1, 5V/div

2ms/div

SOFT-START

(UVLO RISING EDGE)

MAX8737 toc05

1.5V

A. 5V BIAS (V

), 5V/div

B. DRV1, 2V/div
NO LOAD, EN = V

C. LDO1 OUTPUT, 1V/div
D. PGOOD1, 5V/div

2ms/div

SOFT-STOP

(UVLO FALLING EDGE)

MAX8737 toc06

1.5V

A. 5V BIAS (V

), 5V/div

B. DRV1, 2V/div
NO LOAD, EN = V

C. LDO1 OUTPUT, 1V/div
D. PGOOD1, 5V/div

µs/div

LOAD TRANSIENT

(0.1A TO 2.1A)

0.1A

MAX8737 toc07

1.50V

1.49V

2.1A

3.2V

2.8V

A. CONTROL SIGNAL
B. LOAD CURRENT, 2A/div

C. DRV1, 500mV/div
D. LDO1 OUTPUT VOLTAGE,
10mV/div

1.51V

µs/div

LOAD TRANSIENT

(NO LOAD TO 2A)

MAX8737 toc08

1.50V

1.45V

3.2V

2.7V

A. CONTROL SIGNAL
B. LOAD CURRENT, 2A/div

C. DRV1, 1V/div
D. LDO1 OUTPUT VOLTAGE,
50mV/div

1.7V

µs/div

LOAD TRANSIENT

(NO LOAD TO 2A)

MAX8737 toc09

1.50V

1.45V

3.2V

2.7V

A. CONTROL SIGNAL
B. LOAD CURRENT, 2A/div

C. DRV1, 1V/div
D. LDO1 OUTPUT VOLTAGE,
50mV/div

Typical Operating Characteristics (continued)

(Circuit of Figure 1, T

= +25°C, unless otherwise noted.)

MAX8737

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

_______________________________________________________________________________________

DRV TRANSCONDUCTANCE

DISTRIBUTION

MAX8737 toc13

TRANSCONDUCTANCE (S)

SAMPLE PERCENTAGE (%)

1.3

1.0

0.8

0.5

1.5

SAMPLE SIZE = 150

OUT1
OUT2

OUTPUT-VOLTAGE DEVIATION

vs. TEMPERATURE

MAX8737 toc14

TEMPERATURE (

°C)

OUTPUT-VOLTAGE DEVIATION (mV)

-15

-2

-1

-3

-40

0.1

0.001

0.01

GAIN AND PHASE

(OUT1)

MAX8737 toc15

GAIN (dB)

PHASE (

°
)

-20

180

-90

-180

FREQUENCY (MHz)

1.5V OUTPUT, 1A LOAD, C

OUT

= (1) 10

µF 1206 16V CERAMIC

0.1

0.001

0.01

GAIN AND PHASE

(OUT2)

MAX8737 toc16

GAIN (dB)

PHASE (

°
)

-20

180

-90

-180

FREQUENCY (MHz)

1.05V OUTPUT, 2A LOAD, C

OUT

= (1) 22

µF 1206 6V CERAMIC

Typical Operating Characteristics (continued)

(Circuit of Figure 1, T

= +25°C, unless otherwise noted.)

µs/div

FOLDBACK CURRENT LIMIT

(SHORT-CIRCUIT RESPONSE)

10V

MAX8737 toc10

1.50V

20A

A. GATE OF FET LOAD, 10V/div
B. DRV1, 1V/div
C. MOSFET CURRENT, 20A/div

D. LD01 OUTPUT
VOLTAGE, 2V/div
E. PGOOD1, 5V/div

1.5V

OUTPUT OFFSET VOLTAGE

DISTRIBUTION

MAX8737 toc11

OUTPUT OFFSET VOLTAGE (mV)

SAMPLE PERCENTAGE (%)

-3

-5

SAMPLE SIZE = 150

OUT1
OUT2

CURRENT-LIMIT THRESHOLD

DISTRIBUTION

MAX8737 toc12

CURRENT LIMIT (mV)

SAMPLE PERCENTAGE (%)

11.3

10.0

8.8

7.5

12.5

SAMPLE SIZE = 150

OUT1
OUT2

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

_______________________________________________________________________________________

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

Pin Description

PIN

NAME

FUNCTION

Analog and Driver Supply Input. Connect to the system supply voltage (+5.0V). Bypass V

to analog

ground with a 1µF or greater ceramic capacitor.

CS1

Positive Current-Sense Input for LDO1. To enable (foldback) current limit, connect CS1 to the positive
terminal of the current-sense element as shown in Figure 1. The MAX8737 driver reduces the gate
voltage when the 10mV (typ) current-limit threshold is exceeded. When CS1 is connected to V

, the

MAX8737 disables the current-limit protection and enables the output undervoltage protection (see the
UVP Short-Circuit Protection section).

OUT1

Outp ut Feed b ack- S ense, N eg ati ve C ur r ent- S ense, and D i schar g e Inp ut for LD O 1. C onnect d i r ectl y to the
l i near r eg ul ator outp ut. W hen LD O1 i s d i sab l ed , OU T1 i s d i schar g ed thr oug h an i nter nal 10

FE T to GN D .

REFIN1

External Reference Input for LDO1. REFIN1 sets the main output regulation voltage (V

OUT1

= V

REFIN1

PGOOD2

Open-Drain Power-Good Output for LDO2. PGOOD2 is low when the output voltage is more than 12%
(typ) below the normal regulation point, during soft-start, and in shutdown. Approximately 2ms (typ) after
OUT2 reaches the regulation voltage (REFIN2), PGOOD2 becomes high impedance as long as the
output remains in regulation.

PGOOD1

Open-Drain Power-Good Output for LDO1. PGOOD1 is low when the output voltage is more than 12%
(typ) below the normal regulation point, during soft-start, and in shutdown. Approximately 2ms (typ) after
OUT1 reaches the regulation voltage (REFIN1), PGOOD1 becomes high impedance as long as the
output remains in regulation.

EN2

Enable Input for LDO2. Connect EN2 to Vcc for always ON. When EN2 is pulled low, the linear regulator
shuts down and pulls the output to ground.

EN1

Enable Input for LDO1. Connect EN1 to Vcc for always ON. When EN1 is pulled low, the linear regulator
shuts down and pulls the output to ground.

REFIN2

External Reference Input for the Secondary Regulator (LDO2). REFIN2 sets the main output regulation
voltage (V

OUT2

= V

REFIN2

OUT2

Output Sense, Negative Current-Sense Input, and Discharge Input for the Secondary Regulator (LDO2).
Connect directly to the linear regulator output. When the LDO2 is disabled, OUT2 is discharged through
an internal 10

FET to GND.

CS2

Positive Current-Sense Input for LDO2. To enable (foldback) current limit, connect CS2 to the positive
terminal of the current-sense element as shown in Figure 1. The MAX8737 driver reduces the gate
voltage when the 10mV (typ) current-limit threshold is exceeded. When CS2 is connected to V

, the

MAX8737 disables the current-limit protection and enables the output undervoltage protection (see the
UVP Short-Circuit Protection section).

12, 14

N.C.

Not Internally Connected

DRV2

External N-Channel Gate Drive for LDO2

GND

Ground. Connect the thin QFN backside pad to GND.

DRV1

External N-Channel Gate Drive for LDO1

Exposed Pad. Connect the thin QFN backside pad to GND.

MAX8737

Detailed Description

The MAX8737 is a dual, low-dropout, external n-chan-
nel linear regulator controller for low-voltage notebook
computer power supplies. The linear regulator provides
a 0.5V to 2.5V (±5mV no-load) output for powering the
low-voltage supplies to desktop and notebook CPU
chipsets (V

CCP

and V

MCH

). The regulator operates

from low input voltage, which also reduces the power
dissipation in the external n-channel MOSFET. The con-
troller powers the external MOSFET gate driver from the
standard 5V system supply.

The controller features independent enable inputs
(EN_), PGOOD outputs (PGOOD_), input undervoltage
lockout (UVLO), and output undervoltage protection
(UVP). The controller uses an adjustable reference
input (REFIN_) to set the nominal output voltage
(V

OUT

), which minimizes the cost and makes the stabili-

ty independent of the output voltage. An output UVP
timing depends on the magnitude of the voltage at
V

OUT

. The UVP detects and shuts down the LDO if the

output voltage drops below the nominal output voltage
(V

REFIN

). Each linear regulator features an adjustable

soft-start function, and generates a delayed PGOOD
signal that signals when the linear regulator is in regula-
tion. The MAX8737 uses an external resistor-divider in
series with the current-sense input (CS_), providing
foldback current-limit protection, and effectively reduc-
ing the short-circuit power dissipation. The MAX8737 is
available in a thin QFN package to reduce the thermal
impedance, and improve the thermal coupling between
the controller and the external MOSFETs.

REFIN Input

The low-cost linear regulator uses an adjustable refer-
ence input (REFIN_) to set the nominal output voltage,
which minimizes cost and simplifies the stability--the
stability calculation is independent of V

OUT

. The output

voltage accuracy depends on the accuracy of the
source generating the REFIN voltage. Multiple accurate
references are typically available elsewhere in the sys-
tem (such as the switching regulator providing the low-
voltage input supply). If lower output accuracy is
acceptable, divide down and filter another regulated
output voltage supply.

To set output voltage, select R2 = 100k

and select R1

using the following formula:

Soft-Start

When the LDO is activated, the respective DRV_ is
pulled up from GND with a typical soft-start current of
170µA. The soft-start current limits the output voltage
slew rate and also limits the initial current spike through
the external n-channel MOSFET. The slew rate is also
limited by the compensation capacitance used at the
DRV_ pin.

The maximum drain current during startup is the ratio of
C

OUT

to C

COMP

, multiplied by the soft-start current

SOFT

of 170µA (typ).

Enable and Power Good

The MAX8737 has independent enable control inputs
(EN1, EN2). Drive EN1 high to enable output 1. Drive
EN2 high to enable output 2. When EN_ is driven low,
the corresponding DRV_ and PGOOD_ pins are pulled
to GND, and the output is discharged through a 10

switch.

There are two independent PGOOD_ outputs indicating
the supply status. PGOOD_ is pulled high 2ms after the
controller is enabled (EN_ is pulled high and V

exceeds its UVLO threshold), and the output is in regu-
lation. If either output is out of regulation, the respective
PGOOD_ goes low immediately. The MAX8737 pulls
PGOOD_ low if the output voltage drops below the
lower trip threshold of -12% (typ) or when V

is in

UVLO or when EN_ is pulled low.

Soft-Stop

The MAX8737 enables a soft-stop function that dis-
charges the output through an internal 10

switch

when EN_ is driven low or V

is in UVLO. The dis-

charge time of the output depends on the output
capacitance, output load, and the exact resistance of
the internal discharge switch. To slow down the dis-
charge rate, add resistance in series with the OUT_ pin.

5.0V Bias Supply (V

)

The linear regulator operates with very low input volt-
ages. V

may be as low as 1.2V, so a secondary 5V

supply is required to provide sufficient bias to the gate
drivers. Locally decouple the V

input with 1µF or

greater of ceramic capacitance.

Current Limit

The MAX8737 features a current limit that monitors the
voltage across the current-sense resistor, which limits
V

_ - V

OUT

_ to 10mV (typ). However, in case of a

short-circuit condition, the power dissipation across the
external FET will be extremely high. To protect the
external FET, the MAX8737 uses an external resistive
divider (see Figure 1) to fold back the current limit,
reducing the overall power dissipation. The foldback

REF

REFIN

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

_______________________________________________________________________________________

resistor network is calculated using the short-circuit
current (I

SHORT

), the maximum load current (I

MAX

), cur-

rent-sense resistor (R

), the 10mV (±3mV) current-

limit threshold (V

ILIM

), and the external reference input

(REFIN_). See Figure 3:

1) Pick the R

requirement for maximum short-cir-

cuit current:

2) Select R1 = 10

and select R2 using the follow-

ing formula:

UVP Short-Circuit Protection

There are two levels of short-circuit UVP available in the
controller. When the current-limit protection is not used
(CS_= V

), the output undervoltage timeout protection

is enabled, which protects the regulator against short
circuits. Output UVP timing depends on the magnitude
of the output voltage drop. To clear the UVP fault latch,
toggle the respective EN_ input, or cycle V

below its

UVLO threshold.

REFIN

ILIM

MAX CS

ILIM

(

)

ILIM SHORT

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

_______________________________________________________________________________________

R6B
100k

POWER
GOOD 2

* A LOCAL 10

µF CERAMIC CAPACITOR WILL BE SUFFICIENT

FOR MOST APPLICATIONS. IF THE MAX8737 IS POWERED
FROM A HIGH-IMPEDANCE SOURCE, ADDITIONAL LOW-ESR
POLYMER CAPACITORS ARE RECOMMENDED ON THE INPUT.

R3B
33

C2B
0.22

µF

IN2

µF

SYS2

100

µF

OUT2

µF

1.05V
3A (MAX)

INPUT
1.25V TO 1.5V

OFF

R2A

100k

R1A

33.2k

OFF

R6A

100k

POWER

GOOD 1

IN1

µF

SYS1

100

µF

OUT1

µF

1.5V

2A (MAX)

1.0

µF

5V BIAS
SUPPLY

INPUT

1.8V TO 2.5V

R3A

C2A

0.1

µF

GND

REFIN1

EN1

OUT1

DRV1

PGOOD1

PGOOD2

DRV2

OUT2

EN2

MAX8737

NOTE: THE SYSTEM REFERENCE IS TYPICALLY
GENERATED BY THE STEP-DOWN CONVERTER
USED TO POWER THE DUAL LOW-VOLTAGE
LINEAR REGULATORS.

R4B
10

R5B
150

R1B

90k

R2B
100k

SYSTEM REF (2.0V)

R4A

R5A

340

REFIN2

CS2

20m

CS1

20m

CS1

CS2

N1/N2: Si 4922DY

Figure 1. Typical Operating Circuit with Current Limit

Slow UVP

If the output drops below 80% of the nominal output
voltage (V

REFIN

) for 75µs, the MAX8737 shuts down the

LDO and pulls the DRV_ pin to ground. If the output
voltage returns above 80% of the nominal output volt-
age (V

REFIN

) within the 75µs, the controller ignores the

load transient.

Fast UVP

If the output voltage drops below 60% of the nominal
output voltage (V

REFIN

) for approximately 5µs, the

MAX8737 immediately shuts down and pulls the DRV_
pin to ground. If the output voltage returns above 80%
of the nominal output voltage (V

REFIN

) within the 5µs,

the controller ignores the load transient.

Thermal Protection

The MAX8737 is available in a thin QFN package to
reduce the thermal impedance, and improve the ther-
mal coupling between the controller and the external
MOSFETs. When the controller's junction temperature
exceeds T

= +125°C (max), a thermal sensor turns off

the external pass transistor, allowing the system to
cool. The thermal sensor turns the pass transistor back
on once the controller's junction temperature drops by
approximately 20°C.

Design Procedure

Input Capacitor Selection (C

)

Typically, the MAX8737 is powered from the output of a
step-down regulator, effectively providing a low-imped-
ance source. A local 10µF ceramic capacitor at V

and

a 1.0µF ceramic capacitor at V

BIAS

should be sufficient

for most applications. If the linear regulator is connect-
ed to a high-impedance input, low-ESR polymer capac-
itors are recommended on the input.

Output Capacitor Selection (C

OUT

)

To maintain stability and provide good transient
response, the MAX8737 requires 4.7µF/A (4.7µF mini-
mum) of low ESR ceramic capacitor at the output. The
regulator remains stable with capacitances higher than
the minimum. When selecting the output capacitor to

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

______________________________________________________________________________________

OUT

INPUT

OUT

DRV

MAX8737

OUT

INPUT

OUT

DRV

10mV

MAX

OUT

10mV

MAX

OUT

SIMPLE CURRENT-LIMIT PROTECTION

FOLDBACK CURRENT-LIMIT PROTECTION

Figure 3. Current-Limit Protection

MAX8737

provide good transient response, the capacitor's ESR
should be minimized:

OUT

x ESR

where

OUT

is the maximum peak-to-peak load current

step, and

OUT

is the transient output-voltage tolerance.

Regulator Compensation

The compensation network (R3_, C2_) is customizable
and depends on load and MOSFET characteristics:

· Use of ceramic output capacitors with low R

ESR

ensure stability and minimize ESR voltage drop at
load step

· Strength of the external n-channel MOSFET (g

), its

forward transconductance (g

), and the gate-to-

source capacitance (C

)

· The driver transconductance (G

MDRV

) of the inte-

grated circuit driver

· Load current range (including the minimum load):

MIN

to I

MAX

Recommended Procedure

Use the C

, g

FS,

from the chosen transistor data

sheet and use the equation below to translate the mea-
sured g

to g

for normal operation:

1) Determine the LDO transconductance using the

MOSFET's forward transconductance (g

), and the

drain current (I

) used to test the selected MOSFET:

2) Calculate the compensation resistor based on the

output capacitor (C

OUT

), the MOSFET's gate-to-

source capacitance (C

= C

ISS

- C

RSS

), and the

minimum driver transconductance:

3) Calculate the compensation capacitance using the

minimum load current (I

MIN

) and compensation

resistor value calculated above:

where V

= 25mV.

Example: The example below is used to demonstrate
the stability calculation for the application circuit in
Figure 1.

1) Choose V

OUT

= 1.05V and I

MAX

= 3A and the mini-

mum load can be determined from the foldback cur-
rent-limit resistance:

2) For the selected MOSFET (Si4922DY), C

2000pF at 1.5V, and g

= 30S at I

= 8.8A:

3) The output capacitor must be at least 4.7µF/A.

Therefore the design must use a minimum 14.1µF
capacitor. The closest standard capacitor value is
22µF.

4) Based on the above operating conditions and com-

ponent selection, the compensation resistor value
should be:

5) Finally, select the compensation capacitor value:

External MOSFET Selection

The MAX8737 uses an n-channel MOSFET as the
series pass transistor instead of a p-channel MOSFET
to reduce cost. The selected MOSFET must have a
gate threshold voltage (at the required max load) that
meets the following criteria:

where V

is the controller bias voltage, and V

GS_MAX

is the maximum gate voltage required to yield the on-
resistance (R

DS_ON

) specified by the manufacturer's

data sheet. Make sure that input-to-output voltage
meets the condition below to avoid entering dropout,
where output voltage starts to decrease and any ripple
on the input also passes through to the output. R

DSON

has a positive temperature coefficient (approximately

GS MAX

OUT

2 25

0 15

× ×

(

)

17 5

0 5

8 8

17 5

MIN

OUT

V C

T OUT

MIN MDRV

(

)

OUT

GS M

0 5

MAX

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

______________________________________________________________________________________

0.5%/°C); therefore, the value of R

DSON

at the highest

operating junction temperature should be used:

where V

IN_MIN

is the minimum input voltage at the drain

of the MOSFET.

MOSFET Power Dissipation

The maximum power dissipation of the MAX8737
depends on the thermal resistance of the external n-
channel MOSFET package, the board layout, the tem-
perature difference between the die and ambient air,
and the rate of airflow. The power dissipated in the
MOSFET is:

DIS

= I

OUT

- V

CSP

)

The maximum allowable power dissipation is deter-
mined by the following formula:

where T

J(MAX)

is the maximum junction temperature

(+150°C), T

is the ambient temperature,

is the

thermal resistance from the die junction to the package
case, and

is the thermal resistance from the case

through the PC board, copper traces, and other materi-
als to the surrounding air. Standard 8-pin SO MOSFETs
are typically rated for 2W, while new power packages
(PowerPAKTM, DirectFETTM, etc.) can achieve power
dissipation ratings as high as 5W. For optimum power
dissipation, use a large ground plane with good ther-
mal contact to ground and use wide input and output
traces. Extra copper on the PC board increases ther-
mal mass and reduces the thermal resistance of the
board. See Figure 4.

PC Board Layout Guidelines

Due to the high-current paths and tight output accuracy
required by most applications, careful PC board layout
is required. An evaluation kit (MAX8737EVKIT) is avail-
able to speed design. It is important to keep all
traces as short as possible to minimize the high-
current trace dimensions to reduce the effect of
undesirable parasitic inductance. The MOSFET dissi-
pates a fair amount of heat due to the high currents
involved, especially during large input-to-output voltage
differences. To dissipate the heat generated by the
MOSFET, make power traces very wide with a large
amount of copper area. An efficient way to achieve
good power dissipation on a surface-mount package is
to lay out copper areas directly under the MOSFET
package on multiple layers and connect the areas
through vias. Use a ground plane to minimize imped-
ance and inductance.

In addition to the usual high-power considerations, here
are four tips to ensure high output accuracy:

· Ensure that the feedback connection to C

OUT

short and direct.

· Place the reference input resistors next to the

REFIN_ pin.

· Place RC and CC next to the DRV_ pin.

· Ensure REFIN_ and DRV_ traces are away from

noisy sources to ensure tight accuracy.

DIS MAX

J MAX

(

)

(

)

IN MIN

OUT MAX

MAX

DSON MAX

(

)

MAX8737

Dual, Low-Voltage Linear Regulator Controllers

with External MOSFETs

______________________________________________________________________________________

PowerPAK is a registered trademark of Vishay Siliconix.

DirectFET is a trademark of International Rectifier Corp.

MAX8737

Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs

______________________________________________________________________________________

R6B
100k

POWER
GOOD 2

R3B
33

C2B
0.22

µF

IN2

µF

SYS2

100

µF

OUT2

µF

1.05V
3A (MAX)

INPUT
1.25V TO 1.5V

OFF

100k

42.2k

47.5k

OFF

R6A

100k

POWER

GOOD 1

IN1

µF

SYS1

100

µF

OUT1

µF

1.5V

2A (MAX)

1.0

µF

5V BIAS
SUPPLY

INPUT

1.8V TO 2.5V

R3A

C2A

0.1

µF

GND

REFIN2

REFIN1

EN1

OUT1

DRV1

PGOOD1

CS1

CS2

PGOOD2

DRV2

OUT2

EN2

MAX8737

NOTE: THE SYSTEM REFERENCE IS TYPICALLY
GENERATED BY THE STEP-DOWN CONVERTER
USED TO POWER THE DUAL LOW-VOLTAGE
LINEAR REGULATORS.

SYSTEM REF (2.0V)

N1/N2: Si 4922DY

*A LOCAL 10

µF CERAMIC CAPACITOR IS SUFFICIENT

FOR MOST APPLICATIONS. IF THE MAX8737 IS POWERED
FROM A HIGH-IMPEDANCE SOURCE, ADDITIONAL LOW-ESR
POLYMER CAPACITORS ARE RECOMMENDED ON THE INPUT.

Figure 4. Typical Operating Circuit with Output Undervoltage Protection

Chip Information

TRANSISTOR COUNT: 1562

PROCESS: BiCMOS

Part Number MAX8737