MB39A132QN-G-ERE1 Datasheet, PDF (37/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
ΔIL
Ibtm = IOMAX −
2
Itop : Top value of ripple current of inductor [A]
ΔIL
Itop = IOMAX −
2
ΔIL : Inductor ripple current peak to peak value [A]
Tr : FET turn-on time of high-side [s]
Tf : FET turn-off time of high-side [s]
Tr and Tf can be easily found by the following formula.
Qgd × 4
Tr =
5 − Vgs(on)
Qgd × 1
Tf =
Vgs(on)
Qgd : Gate-Drain charge of high-side FET [C]
Vgs(on) : Gate-Source voltage of high-side FET with Qgd [V]
The FET loss of the low-side can be found by the following formula.
VO
PLosideFET = PRON_Loside = IOMAX2 × (1 −
) × Ron_Loside
VIN
PLosideFET : FET loss of low-side [W]
PRON_Loside : FET continuity loss of low-side [W]
IOMAX : Maximum charge current [A]
VIN
: Switching power supply voltage [V]
VO
: Output voltage [V]
Ron_Loside : FET ON resistance of synchronous rectification [Ω]
The FET voltage transiting between drain-source of the low-side is generally small. The SWFET loss is
omitted in this document as it is negligible.
Since the power for driving gate of SWFET is supplied by LDO in IC, the SWFET allowable maximum total
gate charge (QgTotalMax) is determined by the following formula.
0.03
QgTotalMax ≤
fOSC
QgTotalMax : High-side FET allowable maximum total charge [C]
fOSC
: Oscillation frequency [Hz]
DS04–27265–3E
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