PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER, DUAL LOW DROPOUT

REGULATOR CONTROLLER

© 1998 SEMTECH CORP.

August 25, 1998

652 MITCHELL ROAD NEWBURY PARK CA 91320

SC1182/3

11

)

1

(

R

I

P

)

on

(

DS

2

O

COND

δ

−

⋅

⋅

=

FET type

R

P

Package

BUK556H 22

2.48

TO220

IRL2203

7.0

0.79

D

Si4410

13.5

1.53

SO-8

FET type

R

P

Package

BUK556H 22

1.95

TO220

IRL2203

7.0

0.62

D

Si4410

13.5

1.20

SO-8

BOTTOM FET - Bottom FET losses are almost entirely

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

duction at the beginning and end of the bottom switch

conduction 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

determined by:

For the example above:

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 surface mount packages on double sided FR4, 2

oz printed circuit board material, thermal impedances of

40

readily achievable. The corresponding temperature rise

is detailed below:

Temperature rise (

FET type

Top FET

Bottom FET

BUK556H 49.6

(1)

39.0

(1)

IRL2203

31.6

24.8

Si4410

122.4

96

(1) With 20

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

for this application, but by using parallel pairs in each po-

sition, power dissipation will be approximately halved and

temperature rise reduced by a factor of 4.

INPUT CAPACITORS - since the RMS ripple current

in the input capacitors may be as high as 50% of the

output 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.