MB39A132QN-G-ERE1 Datasheet, PDF (36/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
• SWFET selection
If MB39A132 is used for the charger for a notebook PC, since the output voltage of an AC adapter, which is
the input voltage of an SWFET, is 25 V or less, in general, a 30 V class MOS FET can be used as the SWFET.
Obtain the maximum value of the current flowing through the SWFET in order to determine whether the
current flowing through the SWFET is within the rated value. The maximum current flowing through the
SWFET can be found by the following formula.
ΔIL
IDMAX ≥ IoMAX +
2
IDMAX : Maximum SWFET drain current [A]
IOMAX : Maximum charge current [A]
ΔIL : Inductor ripple current peak to peak value [A]
In addition, find the loss of the SWFET in order to determine whether the allowable loss of the SWFET is
within the rated value. The allowable loss of the high-side of FET can be found by the following formula.
P = P + P HisideFET
RON_Hiside
SW_Hiside
PHisideFET : FET loss of high-side [W]
PRON_Hiside: FET continuity loss of high-side [W]
PSW_Hiside : FET switching loss of high-side [W]
FET continuity loss of high-side
VO
PRON_Hiside = IOMAX2 ×
× RON_Hiside
VIN
PRON_Hiside : FET continuity loss of high-side [W]
IOMAX : Maximum charge current [A]
VIN
: Switching power supply voltage [V]
VO
: Output voltage [V]
RON_Hiside : FET ON resistance of high-side [Ω]
FET switching loss of high-side
VIN × fOSC × (Ibtm × Tr + Itop × Tf)
PSW_Hiside =
2
PSW_Hiside : FET switching loss of high-side [W]
VIN
: Switching power supply voltage [V]
fOSC : Switching frequency (Hz)
Ibtm : Bottom value of ripple current of inductor [A]
36
DS04–27265–3E