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MAX8704

High-Current, Low-Voltage Linear Regulator with Power-Limited, External MOSFET

Maxim Integrated 

High-Current, Low-Voltage Linear Regulator

with Power-Limited, External MOSFET

1V

PLIM

SS/EN

2V

CSP

CSN

VCC

CURRENT

LIMIT

TO CONTROL

BLOCK

Figure 3. Current-Limit Functional Diagram

where the feedback threshold (VFB) equals 0.5V, as

specified in the Electrical Characteristics table. The

minimum adjustable output voltage is 0.5V (FB = CSN).

The maximum adjustable output voltage is limited by

the gate driver’s output-voltage swing range (see the

Electrical Characteristics table) and the gate threshold

of the selected n-channel MOSFET.

Fault Protection

Current Limit

The MAX8704 features a current limit (Figure 3) that

monitors the voltage across the current-sense resistor,

typically limiting the CSP to CSN voltage to 50mV.

When the CSP to CSN voltage reaches the current-limit

threshold, the MAX8704 regulates the output current

rather than the output voltage. During startup, the soft-

start circuit ramps the current limit to reduce the input

surge current (see the Soft-Start Capacitor Selection

section).

MOSFET Power-Limit Protection

The MAX8704 includes a proprietary power-limit circuit

to protect the external n-channel MOSFET, especially

under short-circuit conditions. The MAX8704 uses an

internal multiplier circuit to generate an output current

(IPLIM) that is directly proportional to the MOSFET

power dissipation. When the PLIM voltage exceeds

1.0V, the MAX8704 folds back the current limit to

reduce the power dissipation across the external com-

ponents (Figure 3). The power limit allows an output

short for an indefinite period of time without damaging

the MAX8704 or its external components.

Thermal-Overload Protection

Thermal-overload protection prevents the MAX8704

from overheating. When the junction temperature

exceeds +140°C, the linear regulator automatically

pulls PGOOD low and enters shutdown—the MAX8704

pulls SS/EN low with an internal 1kΩ pulldown resistor.

This disables the driver and discharges the output,

allowing the device to cool. Once the junction tempera-

ture cools by 20°C, the thermal protection circuit

releases the SS/EN input, allowing the MAX8704 to

automatically power up using the soft-start sequence. A

continuous thermal-overload condition results in a

pulsed output.

Power-Good

The MAX8704 provides an open-drain PGOOD output

that goes high 3ms (typ) after the output initially reach-

es regulation. PGOOD transitions low immediately after

the output voltage drops below 92% (typ) of the nomi-

nal regulation voltage, or when the MAX8704 enters

shutdown. Connect a pullup resistor from PGOOD to

VCC for a logic-level output. Use a 100kΩ resistor to

minimize current consumption.

Design Procedure

External MOSFET Selection

The external MOSFET selection depends on the gate

threshold voltage, input-to-output voltage, and package

power dissipation. The MAX8704 uses an external n-

channel MOSFET controlled by a 5V driver, so the max-

imum gate-to-source voltage across the MOSFET

(VGS(MAX)) is equivalent to:

VGS(MAX) = VDRV(MAX) - VCSP

where the maximum drive voltage is approximately

VCC - 1V. The selected MOSFET’s on-resistance must

be low enough to support the minimum input-to-output

differential voltage (dropout voltage) and maximum

load required by the application:

RDS(ON)(MIN)

=

VIN(MIN)

− VCSLIMIT

IOUT(MAX)

−

VOUT

For output voltages less than 1.5V, standard MOSFETs

that provide on-resistance specifications with 2.5V

gate-to-source voltages are sufficient. For output volt-

ages greater than 1.5V, use low-threshold MOSFETs

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