FAN5234 查看數據表（PDF） - Fairchild Semiconductor

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FAN5234

Mobile-Friendly PWM/PFM Controller

Fairchild Semiconductor 

FAN5234

PRODUCT SPECIFICATION

EN

FPWM

SS

VIN

VSEN

POR/UVLO

HYST

OVP

OSC

CLK

RAMP

Q

SR

PWM

EA

DUTY

CYCLE

CLAMP

5V

VDD

FPWM

HYST

ADAPTIVE

GATE

VDD

CONTROL LOGIC

RAMP

Σ

PWM

S/H

PWM/HYST

ILIM det. MODE

CURRENT PROCESSING

I

OU T

SS

ILIM

VREF

PGOOD

Reference and

Soft Start

PWM/HYST

REF2

Figure 2. IC Block Diagram

BOOT

C

BOOT

VIN

Q1

HDRV

SW

Q2

LDRV

PGND

VOUT

L OUT

COUT

ISNS RSENSE

Circuit Description

Overview

The FAN5234 is a PWM controller intended for low voltage

power applications in modern notebook, desktop, and

sub-notebook PCs. The output voltage of the controller can

be set in the range of 0.9V to 5.5V by an external resistor

divider.

The synchronous buck converter can operate from either an

unregulated DC source (such as a notebook battery) with

voltage ranging from 2V to 24V, or from a regulated system

rail. In either mode of operation the IC is biased from a +5V

source. The PWM modulator uses an average current mode

control with input voltage feed-forward for simplified feed-

back loop compensation and improved line regulation. The

controller includes integrated feedback loop compensation

that dramatically reduces the number of external compo-

nents.

Depending on the load level, the converter can operate either

in fixed frequency PWM mode or in a hysteretic mode.

Switch-over from PWM to hysteretic mode improves the

converters' efficiency at light loads and prolongs battery run

time. In hysteretic mode, a comparator is synchronized to the

main clock that allows seamless transition between the oper-

ational modes and reduced channel-to-channel interaction.

REV. 1.0.10 5/3/04

The hysteretic mode of operation can be inhibited indepen-

dently using the FPWM pin if variable frequency operation is

not desired.

Oscillator

Table 1. Converter Operating modes

Mode

Battery

Fixed

300

Fixed

600

FSW

(Khz)

300

300

600

Converter

Power

2 to 24V

< 5.5V Fixed

< 5.5V Fixed

VIN Pin

Battery (>5V)

100KΩ to GND

GND

When VIN is from the battery, the oscillator's ramp ampli-

tude is proportional to VIN, providing voltage feed-forward

control for improved loop response. When in either of the

Fixed modes, oscillator's ramp amplitude is fixed. The oper-

ating frequency is then determined according to the connec-

tion on the VIN pin (Table 1).

Initialization and Soft Start

Assuming EN is high, FAN5234 is initialized when VCC

exceeds the rising UVLO threshold. Should VCC drop below

the UVLO threshold, an internal Power-On Reset function

disables the chip.

5

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