SC420 데이터 시트보기 (PDF) - Semtech Corporation

부품명

상세내역

일치하는 목록

SC420

High Speed, Combi-Sense™, Synchronous Power MOSFET Driver for Mobile Applications

Semtech Corporation 

SC420

POWER MANAGEMENT

Applications Information

Combi-Sense (Lossless current sense)

The Combi-Sense is a way to sense the output current

on a combination of power devices. There is no sense

resistor and the current is sensed on: Top MOSFET, bot-

tom MOSFET and output inductor.

An internal phase node VPN sends a signal which is inte-

grated by the Combi-Sense network. This network con-

sists of a resistor and capacitor in series, connected

between VPN and the DRN pins. The resulting signal is

large, clean and not duty cycle sensitive. It can be used

directly for close loop current mode control and current

limit.

Fast Switching Drives

As the switching frequency of PWM controllers is increased

to reduce power supply volume and cost, fast rise and

fall times are necessary to minimize switching losses (TOP

MOSFET) and reduce dead-time (BOTTOM MOSFET)

losses. While low Rds_On MOSFET’s present a power

saving, the MOSFETs die area is larger and the effective

input capacitance of the MOSFET is increased. Often a

50% decrease in Rds_On doubles the effective input

gate charge, which must be supplied by the driver. The

Rds_On power savings can be offset by the switching

and dead-time losses with a suboptimum driver. While

discrete solution can achieve reasonable drive capabil-

ity, implementing shoot-through, programmable delay and

other housekeeping functions necessary for safe opera-

tion can become cumbersome and costly. The SC420

presents a total solution for the high-speed, high power

density applications. Wide input supply range of 4.5V-

25V allows use in battery powered applications, new high

voltage, distributed power supplies.

Shoot Through Protection

The control input (CO) to the SC420 is typically supplied

by a PWM controller that regulates the power supply out-

put. The timing diagram demonstrates the sequence of

events by which the top and bottom drive signals are

applied. The shoot-through protection is implemented

by holding the bottom FET off until the voltage at the

phase node (intersection of top FET source, the output

inductor and the bottom FET drain) has dropped below

1V. This assures that the top FET has turned off and

that a direct current path does not exist between the

input supply and ground, a shoot-through condition dur-

ing which both the top and bottom FET’s could be on

momentarily. The top FET is also prevented from turn-

ing on until the bottom FET is off. The top FET turn-on

delay is internally set to 20ns (typical) and may be

programmably extended by an external capacitor on the

Cdelay pin, the delay is increased by 1ns/pf.

The EN (enable) pin may be used to turn both TG and BG

drives off. This lowers power consumption by reducing

the quiescent current draw of the SC420 to IQsd.

Tri-State

If the CO pin is undriven it will float to an internally de-

fined voltage of 1.4V. This will switch the TG and BG

pins low and also tri-states the VPN node.

Over Temperature Shutdown

The SC420 will shutdown by pulling both driver’s low if

its junction temperature, TJ, exceeds 165°C. The driv-

ers will resume operation when TJ declines below 155oC.

Layout Guidelines

As with any high speed , high current, switching regula-

tor circuit, proper layout is critical in achieving optimum

performance of the SC420. The Combi-SenseTM Con-

troller Evaluation board schematic shows a three-phase

synchronous design with all surface mountable compo-

nents.

Tight placement and short, wide traces must be used in

layout of The gate drives, DRN, and especially PGND pin.

The top gate driver supply voltage is provided by

bootstrapping the boost supply and adding it to the phase

node (DRN) voltage. Since the bootstrap capacitor sup-

plies the charge to the top gate, it must be less than

0.5in away from the SC420. Ceramic X7R capacitors

are a good choice for supply bypassing near the chip.

Supply Voltage

The SC420 can operate from 4.75V to 6V. The VIN pin

bypass capacitor must also be less than 0.5in away from

the SC420. The ground node of this capacitor, the

SC420 PGND pin and the Source of the bottom FET must

© 2003 Semtech Corp.

7

United States Patent No. 6,441,597

www.semtech.com

Share Link: