PROGRAMMABLE SYNCHRONOUS DC/DC
CONVERTER, DUAL LOW DROPOUT
REGULATOR CONTROLLER
SC1182/3
August 25, 1998
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.
FET type
BUK556H 22
PD (W) Package
RDS(on) (m )
Ω
2.48
0.79
1.53
TO220
D2PAK
SO-8
IRL2203
Si4410
7.0
13.5
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:
PCOND I2 RDS(on ) (1
)
− δ
=
O
For the example above:
FET type
BUK556H 22
PD (W) Package
RDS(on) (m )
Ω
1.95
0.62
1.20
TO220
D2PAK
SO-8
IRL2203
Si4410
7.0
13.5
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
40oC/W for the D2PAK and 80oC/W for the SO-8 are
readily achievable. The corresponding temperature rise
is detailed below:
Temperature rise (oC)
FET type Top FET
BUK556H 49.6(1)
Bottom FET
39.0(1)
24.8
96
IRL2203
Si4410
31.6
122.4
(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 po-
sition, power dissipation will be approximately halved and
temperature rise reduced by a factor of 4.
11
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