MB39A125 Datasheet, PDF (49/64 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB39A125/126
■ SELECTION OF COMPONENTS
• Pch MOS FET
The Pch MOS FET for switching use should be rated for at least +20% more than the 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-cycle switching loss will be higher so that power dissipation must be considered. In this
application, the NEC µPA2714GR 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.
Continuity loss : Pc
PC
= ID2 × 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) × tf × fosc
6
Total loss : PT
PT
= PC + PS (ON) + PS (OFF)
Example) Using the µPA2714GR
16.8 V setting
Input voltage VIN (Max) = 25 V, output voltage VO = 16.8 V, drain current ID = 3 A, oscillation frequency fosc = 300 kHz,
L = 15 µH, drain-source on resistance RDS (ON) =: 18 mΩ, tr=: 15 ns, tf=: 42 ns
Drain current (Max) : ID (Max)
VIN − Vo
ID (Max) = Io +
tON
2L
= 3+
25 − 16.8
2 × 15 × 10^−6
=: 3.6 A
1
×
× 0.672
300 × 10³
Drain current (Min) : ID (Min)
ID (Min) = Io−
VIN − Vo
tON
2L
= 3−
25 − 16.8
2 × 15 × 10^−6
=: 2.4 A
1
×
300 × 10³
× 0.672
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