BD9540EFV Datasheet, PDF (11/18 Pages) Rohm – 2ch Switching Regulators for Desktop PC

BD9540EFV
4. MOSFET Selection
VIN
main switch
L
Co
VOUT
synchronous switch
Technical Note
Main MOSFET power dissipation is computed as follows:
Pmain = PRON + PGATE + PTRAN
=
VOUT
VIN
×RON×IOUT²+Qg(High)×f×5VReg+V^{IN 2×Crss×IOUT×f}
IDRIVE
・・・(9)
(Ron: On-resistance of FET; Qg: FET gate capacitance;
f: Switching frequency; Crss: FET inverse transfer function;
I_DRIVE: Gate peak current)
Synchronous MOSFET power dissipation is computed as follows:
Psyn = PRON + PGATE
VIN-VOUT
=
×RON×IOUT²+5VReg×f×VDD
・・・(10)
VIN
Qg loss is also incurred as internal power dissipation in the IC:
= PIC(DRIVE) = Qg(High)×f + Qg(Low)×f ×(VIN-5VReg) ・・・(11)
For example:
If Qg(High) = 20nq, Qg(Low) = 50nq, f = 300kHz,
PIC(DRIVE) = 20n×300k +50n×300k ×(12-5)
= 0.147W
5. Determining Detection Resistance
RILIM
VIN
L
SW
PGND
VOUT
Co
The over-current protection function is controlled via the voltage detected
between the SW and PGND pins – i.e., the ON-resistance of the
synchronous FET. The current limit value is determined by formula (12)
below:
ILIM=
10k
[A]・・・(12)
RILIM ×RON
(RILIM: Resistance for setting over-current protection limit,
RON: Low side FET On-resistance)
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11/17
2009.04 - Rev.B