SC1185_05 Datasheet, PDF (10/14 Pages) Semtech Corporation – Programmable Synchronous DC/DC

SC1185 & SC1185A
POWER MANAGEMENT
Layout Guidelines
sider conduction losses to determine FET suitability.
For a 5V in; 2.8V out at 14.2A requirement, typical FET
losses would be: Using 1.5X Room temp R_DS(ON) to allow for
temperature rise.
FET type
IRL34025
IRL2203
Si4410
R_DS(on) (mΩ)
15
10.5
20
P_D (W)
1.69
1.19
2.26
Package
D²Pak
D²Pak
S0-8
BOTTOM FET - Bottom FET losses are almost entirely due
to conduction. The body diode is forced into conduction 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:
P_COND = I_O2 ⋅ R_DS(on) ⋅ (1− δ)
For the example above:
position, 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 accordingly.
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 out-
put capacitance or, more usually, additional input capaci-
tors. Choosing low ESR input capacitors will help maximize
ripple rating for a given size.
FET type
IRL34025
IRL2203
Si4410
R_DS(on) (mΩ)
15
10.5
20
P_D (W)
1.33
0.93
1.77
Package
D²Pak
D²Pak
S0-8
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 achiev-
able. The corresponding temperature rise is detailed be-
low:
FET type
IRL34025
IRL2203
Si4410
Temperature rise (^oC)
Top FET
Bottom FET
67.6
53.2
47.6
37.2
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
 2005 Semtech Corp.
10
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