MB39A104
■ SELECTION OF COMPONENTS
• Pch MOS FET
The P-ch MOSFET for switching use should be rated for at least 20% more than the maximum input voltage. To
minimize continuity loss, use a FET with low RDS(ON) between the drain and source. For high input voltage and
high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be considered.
In this application, the Toshiba TPC8102 is used. Continuity loss, on/off switching loss, and total loss are deter-
mined by the following formulas. The selection must ensure that peak drain current does not exceed rated values,
and also must be in accordance with overcurrent detection levels.
Continuity loss : PC
PC = ID 2 × RDS (ON) × Duty
On-cycle switching loss : PS (ON)
VD (Max) × ID × tr × fOSC
PS (ON) =
6
Off-cycle switching loss : PS (OFF)
VD (Max) × ID (Max) × tf × fOSC
PS (OFF) =
6
Total loss : PT
PT = PC + PS (ON) + PS (OFF)
Example: Using the Toshiba TPC8102
CH1
Input voltage VIN (Max) = 19 V, output voltage VO = 5 V, drain current ID = 3 A, Oscillation frequency fOSC = 500 kHz,
L = 15 µH, drain-source on resistance RDS (ON) =: 50 mΩ, tr = tf=: 100 ns.
Drain current (Max) : ID (Max)
VIN − VO
ID (Max) = IO +
= 3 +
=: 3.25 (A)
ton
2L
19 − 5
2 × 15 × 10−6
1
×
× 0.263
500 × 103
Drain current (Min) : ID (Min)
VIN − VO
ID (Min) = IO −
ton
2L
19 − 5
2 × 15 × 10−6
1
= 3 −
×
× 0.263
500 × 103
=: 2.75 (A)
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FUJITSU [ FUJITSU ]
