MB39C308 Datasheet, PDF (32/60 Pages) Fujitsu Component Limited. – 6ch DC/DC Converter IC for LPIA Platform VR

MB39C308
High-side FET conducting power dissipation
VO
PRON = (IoMAX) ² ×
VDD
× RON
PRON : High-side FET conducting power dissipation [W]
IoMAX : Maximum load current [A]
VDD : Switching system power supply voltage [V]
VO : Output setting voltage [V]
RON : High-side FET on resistance [Ω]
High-side FET switching power dissipation
VDD × fOSC × (Ibtm × tr + Itop × tf)
PSW =
2
PSW : Switching power dissipation [W]
VDD : Switching system power supply voltage [V]
fOSC : Switching frequency (Hz)
Ibtm : Inductor ripple current bottom value [A]
Itop : Inductor ripple current top value [A]
tr : High-side FET turn-on time [s]
tf : High-side FET turn-off time [s]
tr and tf can be found simply from the following formula.
Qgd × 12
Qgd × 12
tr =
tf =
5 − Vth
Vth
Qgd : Gate-Drain charge of High-side FET [C]
Vth : High-side FET threshold voltage [V]
The power dissipation of the Low-side FET can be found from the following formula.
PLosideFET = PRon = (IOMAX) ² × (1
VO
VDD
) × Ron
PRon : Low-side FET conducting power dissipation [W]
IoMAX : Maximum load current [A]
VDD : Switching power supply voltage [V]
VO : Output setting voltage [V]
Ron : Low-side FET on resistance [Ω]
Note : The transition voltage of the voltage between the drain and source of the Low-side FET is generally small
and the switching power loss is negligible. Therefore it has been omitted from this formula.
32
DS04-27261-6E