MAX8704 Просмотр технического описания (PDF) - Maxim Integrated

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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

Pin Description

PIN

NAME

FUNCTION

1

VIN

Input Voltage Sense. The MAX8704 senses the voltage across the external MOSFET (VIN - VCSN) to

determine the MOSFET’s power dissipation.

2

VCC

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

analog ground with a 1µF or greater ceramic capacitor.

Open-Drain Power-Good Output. PGOOD is low when the output voltage is more than 8% (typ) below

3

PGOOD

the nominal regulation voltage. PGOOD is also pulled low during soft-start and in shutdown.

Approximately 3ms (typ) after the LDO reaches the regulation voltage, PGOOD becomes high

impedance as long as the output remains in regulation.

Power-Limit Adjustment. The PLIM output sources a current directly proportional to the MOSFET’s

power dissipation. If the PLIM voltage exceeds the 1.0V power-limit threshold, the regulator reduces

4

PLIM

the power dissipation by folding back the current limit. An external resistor between PLIM and GND

sets the maximum MOSFET’s power dissipation. Additionally, an external capacitor filters the PLIM

voltage, allowing short high-power transients to occur periodically.

Soft-Start and Enable Input. Connect SS/EN to an open-drain output. When SS/EN is pulled low, the

5

SS/EN linear regulator shuts down and pulls the output to ground. Connect a soft-start capacitor from SS/EN

to GND to slowly ramp up the current limit during startup (see the Soft-Start and Enable section).

Feedback Input. Connect FB to VCC for a fixed 1.5V output, or connect FB to GND for a fixed 1.2V

6

FB

output. For an adjustable output, connect FB to a resistive divider from the output voltage. The FB

regulation level is 0.5V.

Negative Current-Sense Input and Output Sense Input. Connect to the negative terminal of the

7

CSN

current-sense element as shown in Figure 1. CSN serves as the feedback input in fixed-voltage mode

(FB = GND or VCC). When the MAX8704 is disabled, the output is discharged through a 10Ω resistor

to GND.

Positive Current-Sense Input. Connect to the positive terminal of the current-sense element as shown

8

CSP

in Figure 1. The MAX8704 driver reduces the gate voltage when the current-limit threshold is

exceeded.

9

GND

Ground

10

DRV

Gate Drive for the External n-Channel MOSFET

Detailed Description

The MAX8704 is a low-dropout, external n-channel

MOSFET linear regulator for low-voltage notebook

power supplies. The regulator uses two separate sup-

plies—the notebook’s 5V bias supply (VCC) for driving

the external n-channel MOSFET, and the lowest system

supply available for the power input (VIN). By using

separate bias and power inputs, the MAX8704 maxi-

mizes the gate drive while minimizing the power loss.

The regulator provides an accurate (-2% typ load regu-

lation) output that delivers up to 5A for powering the

low-voltage (1.0V, 1.2V, 1.5V, and 1.8V) supplies

required by notebook chipsets.

Figure 1 shows the standard application circuit, and

Figure 2 shows the functional diagram. The MAX8704

standard application circuit delivers up to 5A and oper-

ates with input voltages up to 5.5V, but not simultane-

ously. Continuous high output currents can only be

achieved when the input-to-output differential voltage is

low (Figure 1).

5.0V Bias Supply (VCC)

The VCC input powers the control circuitry and provides

the gate drive to the external n-channel MOSFET. This

improves efficiency by allowing VIN to be powered from

a low-voltage system supply. Power VCC from a well-

regulated 5V supply. Current drawn from the VCC sup-

ply remains relatively constant with variations in VIN and

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