MB39A104_08 Datasheet, PDF (24/40 Pages) Fujitsu Component Limited. – 2-ch DC/DC Converter IC with Overcurrent Protection

MB39A104
■ SELECTION OF COMPONENTS
• P-ch 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
determined 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)
PS (ON) =
VD (Max) × ID × tr × fOSC
6
Off-cycle switching loss : PS (OFF)
PS (OFF) =
VD (Max)
× ID (Max)
6
× tf × fOSC
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)
ID (Max) = IO +
VIN − VO
2L
ton
=3+
19 − 5
2 × 15 × 10^−6
×
1
500 × 10³
× 0.263
=: 3.25 (A)
Drain current (Min) : ID (Min)
ID (Min)
= IO −
VIN − VO
2L
ton
=3−
19 − 5
2 × 15 × 10^−6
×
1
500 × 10³
× 0.263
=: 2.75 (A)
24