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SC1163

PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER WITH LOW DROPOUT REGULATOR CONTROLLER

Semtech Corporation 

October 25, 1999

PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER WITH LOW DROPOUT

REGULATOR CONTROLLER

SC1162/3

FET type RDS(on) (mΩ) PD (W) Package

BUK556H 22

IRL2203 7.0

2.48

0.79

TO220

D2PAK

Si4410 13.5

1.53 SO-8

BOTTOM FET - Bottom FET losses are almost entirely

due to conduction. The body diode is forced into conduc-

tion at the beginning and end of the bottom switch con-

duction period, so when the FET turns on and off, there

is very little voltage across it, resulting in low switching

losses. Conduction losses for the FET can be deter-

mined by:

INPUT CAPACITORS - since the RMS ripple current in

the input capacitors may be as high as 50% of the out-

put current, suitable capacitors must be chosen ac-

cordingly. Also, during fast load transients, there may

be restrictions on input di/dt. These restrictions require

useable energy storage within the converter circuitry,

either as extra output capacitance or, more usually,

additional input capacitors. Choosing low ESR input

capacitors will help maximize ripple rating for a given

size.

PCOND

=

I

2

O

⋅ RDS (on) ⋅ (1 − δ )

For the example above:

FET type

BUK556H

IRL2203

Si4410

RDS(on) (mΩ) PD (W)

22

1.95

7.0

0.62

13.5

1.20

Package

TO220

D2PAK

SO-8

Each of the package types has a characteristic thermal

impedance, for the TO-220 package, thermal impedance

is mostly determined by the heatsink used. For the sur-

face mount packages on double sided FR4, 2 oz printed

circuit board material, thermal impedances of 40oC/W

for the D2PAK and 80oC/W for the SO-8 are readily

achievable. The corresponding temperature rise is de-

tailed below:

Temperature rise (oC)

FET type Top FET

BUK556H 49.6(1)

Bottom FET

39.0(1)

IRL2203 31.6

24.8

Si4410 122.4

96

(1) With 20oC/W Heatsink

It is apparent that single SO-8 Si4410 are not adequate for

this application, but by using parallel pairs in each posi-

tion, power dissipation will be approximately halved and

temperature rise reduced by a factor of 4.

© 1999 SEMTECH CORP.

11

652 MITCHELL ROAD NEWBURY PARK CA 91320

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