MB39A104
24
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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 R
DS(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 : P
C
On-cycle switching loss : PS (ON)
Off-cycle switching loss : PS (OFF)
Total loss : PT
PT = PC + PS (ON) + PS (OFF)
Example: Using the Toshiba TPC8102
CH1
Input voltage V
IN (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 R
DS (ON) := 50 mΩ, tr = tf := 100 ns.
Drain current (Max) : ID (Max)
Drain current (Min) : ID (Min)
PC = ID
2
× RDS (ON) × Duty
P
S (ON) =
VD (Max) × ID × tr × fOSC
6
P
S (OFF) =
VD (Max) × ID (Max) × tf × fOSC
6
I
D (Max) = IO +
VIN − VO
ton
2L
= 3 +
19 − 5
×
1
× 0.263
2 × 15 × 10
−6
500 × 10
3
:= 3.25 (A)
ID (Min) = IO −
VIN − VO
ton
2L
= 3 −
19 − 5
×
1
× 0.263
2 × 15 × 10
−6
500 × 10
3
:= 2.75 (A)
