MB39A104_06 Datasheet, PDF (24/38 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 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)
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)
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