SC4612STRT datasheet(9/24 Pages) SEMTECH

9 / 24 page

 2007 Semtech Corp.

www.semtech.com

SC4612

POWER MANAGEMENT

Applications Information

INTRODUCTION

The SC4612 is a versatile voltage mode synchronous

rectified buck PWM convertor, with an input supply (VIN)

ranging from 4.5V to 28V designed to control and drive

N-channel MOSFETs.

The power dissipation is controlled using a novel low volt-

age supply technique, allowing high speed and integra-

tion with the high drive currents to ensure low MOSFET

switching loss. The synchronous buck configuration also

allows converter sinking current from load without losing

output regulation.

The internal reference is trimmed to 500mV with ± 1%

accuracy, and the output voltage can be adjusted by two

external resistors.

A fixed oscillator frequency (up to 1.2MHz) can be

programmed by an external capacitor for an optimized

design.

During the Asynchronous start up, the SC4612 provides a

top MOSFET shut down over current protection, while under

normal operating conditions a low side MOSFET R

DS-ON

current sensing with hiccup mode over current protection,

minimizes power dissipation and provides further

protection.

Other features of the SC4612 include:

Wide input power voltage range (from 4.5V to 28V), low

output voltages down to 500mV, externally programmable

soft-start, hiccup over current protection, wide duty cycle

range, thermal shutdown, asynchronous start-up

protection, and a -40 to 125°C junction operating

temperature range.

THEORY OF OPERATION

SUPPLIES

Two pins (VDD and DRV) are used to power up the SC4612.

If input supply (VDD) is less than 10V (MAX), tie DRV and

VDD together.

This supply should be bypassed with a low ESR 2.2uF (or

greater) ceramic capacitor directly at the DRV to GND pins

of the SC4612.

The DRV supply also provides the bias for the low and the

high side MOSFET gate drive.

The maximum rating for DRV supply is 10V and for

applications where input supply is below 10V, it may be

connected directly to VDD.

START UP SEQUENCE

Start up is inhibited until VDD input reaches its UVLO

threshold. The UVLO limit is 4.5V (TYP).

Meanwhile, the high side and low side gate drivers DH,

and DL, are kept low. Once VDD exceeds the UVLO

threshold, the external soft-start capacitor starts to be

charged by a 25µA current source. If an over current

condition occurs, the SS/EN pin will discharge to 500mV

by an internal switch. During this time, both DH and DL

will be turned off.

When the SS pin reaches 0.8V, the converter will start

switching. The reference input of the error amplifier is

ramped up with the soft-start signal. Initially only the high

side driver is enabled. Keeping the low side MOSFET off

during start up is useful where multiple convertors are

operating in parallel. It prevents forward conduction in the

freewheeling MOSFET which might otherwise cause a dip

in the common output bus.

In case of over current condition which is longer than 10

cycles during the asynchronous start up, SC4612 will turn

off the high side MOSFET gate drive, and the soft-start

sequence will repeat.

When the SS pin reaches 1.3V, the low side MOSFET will

begin to switch and the convertor is fully operational in

the synchronous mode. The soft-start duration is

controlled by the value of the SS cap. If the SS pin is pulled

below 0.5V, the SC4612 is disabled and draws a typical

quiescent current of 5mA.

Bias Generation

A 4.5V to 10V (MAX) supply voltage is required to power

up the SC4612. This voltage could be provided by an ex-

ternal power supply or derived from VDD (VDD >10V)

through an internal pass transistor.

The internal pass transistor will regulate the DRV from an

external supply >10V connected to VDD to produce 7.8V

(TYP) at the DRV pin.

Soft start / Shut down

An external capacitor at the SS/EN pin is used to set up

the soft-start duration. The capacitor value in conjunction

with the internal current source, controls the duration of

soft-start time. If the SS/EN pin is pulled down to GND,

the SC4612 is disabled. The soft-start pin is charged by a

25µA current source and discharged by an internal switch.

When SS/EN is released it charges up to 0.5V as the con-

trol circuit starts up.

SC4612STRT 数据表(PDF) 9 Page - Semtech Corporation

SC4612STRT 数据表(HTML) 9 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

数据表下载