MB39A119 Datasheet, PDF (36/53 Pages) Fujitsu Component Limited. – DC/DC Converter IC of Synchronous Rectification for charging Li-ion battery

MB39A119
■ SELECTION OF COMPONENTS
• N-ch MOS FET
The N-ch 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 µPA2752 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)
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)
• Inductor
In selecting inductors, it is of course essential not to apply more current than the rated capacity of the inductor,
but also to note that the lower limit for ripple current is a critical point that if reached will cause discontinuous
operation and a considerable drop in efficiency. This can be prevented by choosing a higher inductance value,
which will enable continuous operation under light loads. Note that if the inductance value is too high, however,
direct current resistance (DCR) is increased and this will also reduce efficiency. The inductance must be set at
the point where efficiency is greatest.
Note also that the DC superimposition characteristics become worse as the load current value approaches the
rated current value of the inductor, so that the inductance value is reduced and ripple current increases, causing
loss of efficiency. The selection of rated current value and inductance value will vary depending on where the
point of peak efficiency lies with respect to load current. Inductance values are determined by the following
formulas.
The L value for all load current conditions is set so that the peak to peak value of the ripple current is 1/2 the
load current or less.
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