SC1185 Semtech Corporation, SC1185 Datasheet - Page 11

• Manufacturer Part Number: SC1185
• Description: PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER/ DUAL LOW DROPOUT REGULATOR CONTROLLER
• Manufacturer: Semtech Corporation

PRELIMINARY - January 6, 1999
Using 1.5X Room temp R
rise.
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:
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.
FET type
IRL34025
IRL2203
Si4410
FET type
IRL34025
IRL2203
Si4410
FET type
IRL34025
IRL2203
Si4410
P
2
COND
PAK and 80
R
15
10.5
20
R
15
10.5
20
Temperature rise (
Top FET
67.6
47.6
180.8
I
DS(on)
DS(on)
2
O
R
DS
(m ) P
(m ) P
o
(
on
C/W for the SO-8 are readily
)
1 (
DS(ON)
1.69
1.19
2.26
1.33
0.93
1.77
Bottom FET
53.2
37.2
141.6
D
D
(W)
(W)
)
to allow for temperature
o
C)
Package
D
D
SO-8
Package
D
D
SO-8
2
2
2
2
PAK
PAK
PAK
PAK
PROGRAMMABLE SYNCHRONOUS DC/DC
CONVERTER, DUAL LOW DROPOUT
REGULATOR CONTROLLER
o
C/W
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.
652 MITCHELL ROAD NEWBURY PARK CA 91320
SC1185A
SC1185
11