SC1109 Datasheet, PDF (16/17 Pages) Semtech Corporation – SYNCHRONOUS PWM CONTROLLER WITH DUAL LOW DROPOUT REGULATOR CONTROLLERS

SYNCHRONOUS PWM CONTROLLER WITH
DUAL LOW DROPOUT REGULATOR
CONTROLLERS
SC1109
PRELIMINARY - October 16, 2000
Using 1.5X Room temp R_DS(ON) to allow for temperature
rise.
FET type R_DS(on) (mΩ) P_D (W) Package
IRL34025 15
1.69 D²PAK
IRL2203 10.5
1.19 D²PAK
Si4410 20
2.26 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.
P_COND = I_O2 ⋅R_DS(on) ⋅ (1− δ)
For the example above:
FET type
IRL34025
IRL2203
Si4410
R_DS(on) (mΩ) P_D (W)
15
1.33
10.5
0.93
20
1.77
Package
D²PAK
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 40^oC/W
for the D²PAK and 80^oC/W for the SO-8 are readily
achievable. The corresponding temperature rise is de-
tailed below:
Temperature rise (^oC)
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.
© 2000 SEMTECH CORP.
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