MB39A125 Datasheet, PDF (51/64 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB39A125/126
PC = ID2 × RDS (ON) × Duty
= 3² × 0.018 × 0.572
=: 0.093 W
PS (ON) =
VD × ID × tr × fosc
6
22 × 3 × 15 × 10^−9 × 300 × 10³
=
6
=: 0.050 W
PS (OFF) =
VD × ID (Max) × tf × fosc
6
22 × 3.6 × 42 × 10^−9 × 300 × 10³
=
6
=: 0.166 W
PT = PC + PS (ON) + PS (OFF)
=: 0.093 + 0.050 + 0.166
=: 0.309 W
The above power dissipation figures for the µPA2714GR are satisfied with ample margin at 2.0 W.
The Pch MOS FET for switching use must use the one of more than input voltage +20%.
FET which operates when the AC adapter is connected should select FET which satisfies the current decided
by sense resistance R1 enough. Because FET which operates when the AC adapter is not connected becomes
a supply by the battery, it is necessary to select FET which satisfies the current of the system enough.
In this application, the NEC µPA2714GR is used.
• 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 characteristic becomes 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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