SC4612 데이터 시트보기 (PDF) - Semtech Corporation
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SC4612 Datasheet PDF : 24 Pages
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SC4612
POWER MANAGEMENT
Application Information (Cont.)
that the stored charge on the bottom FET body diode will
be moved through the top FET as it starts to turn on. The
resulting power dissipation in the top FET will be:
P RR = Q RR ⋅ V IN ⋅ f OSC
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:
To a first order approximation, it is convenient to only
consider conduction losses to determine FET suitability.
For a 5V in, 2.8V out at 14.2A requirement, typical FET
losses would be:
FET Type
IRL3402S
Temperature rise ( 0C)
Top FET
Bottom FET
67.6
53.2
FET Type
IRL3402S
RDS(on) (mΩ)
15
PD(W)
1.69
Package
D2PAK
IRL2203
Si4410
47.6
180.8
37.2
141.6
IRL2203
10.5
Si4410
20
1.19
2.26
D2PAK
SO-8
Using 1.5X Room temp RDS(ON) to allow for temperature rise.
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 very low switching losses.
Conduction losses for the FET can be determined by:
PCOND = I2O ⋅ RDS(on) ⋅ (1− D)
It is apparent that single SO-8 Si4410 are not adequate for
this application, By using parallel pairs in each 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 output
capacitance or, more usually, additional input capacitors.
Choosing low ESR input capacitors will help maximize ripple
rating for a given size.
For the example above:
FET Type
IRL3402S
IRL2203
Si4410
RDS(on) (mΩ)
15
10.5
20
PD(W)
1.33
0.93
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 surface
2007 Semtech Corp.
16
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