PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER, DUAL LOW DROPOUT

REGULATOR CONTROLLER

© 1999 SEMTECH CORP.

PRELIMINARY - January 6, 1999

652 MITCHELL ROAD NEWBURY PARK CA 91320

SC1185

SC1185A

11

)

1

(

R

I

P

)

on

(

DS

2

O

COND

δ

−

⋅

⋅

=

FET type

R

P

Package

IRL34025

15

1.69

D

IRL2203

10.5

1.19

D

Si4410

20

2.26

SO-8

FET type

R

P

Package

IRL34025

15

1.33

D

IRL2203

10.5

0.93

D

Si4410

20

1.77

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:

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 sur-

face mount packages on double sided FR4, 2 oz printed

circuit board material, thermal impedances of 40

for the D

achievable. The corresponding temperature rise is de-

tailed below:

Temperature rise (

FET type

Top FET

Bottom FET

IRL34025

67.6

53.2

IRL2203

47.6

37.2

Si4410

180.8

141.6

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.

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.

Using 1.5X Room temp R

rise.