BD9532EKN_08 Datasheet, PDF (12/21 Pages) Rohm – Switching Regulators for DDR-SDRAM Cores

4. MOSFET Selection
VIN
main switch
L
Co
VOUT
synchronous switch
Loss on the main MOSFET
Pmain=PRON+PGATE+PTRAN
=
VOUT
VIN
×RON×IOUT²+Ciss×f×VDD+
VIN²Ã—Crss×IOUT×f
IDRIVE
・・・(11)
(Ron: On-resistance of FET; Ciss: FET gate capacity;
f: Switching frequency Crss: FET inverse transfer function;
I_DRIVE: Gate peak current)
Loss on the synchronous MOSFET
Psyn=PRON+PGATE
= ^VIN-VOUT ×RON×IOUT²+Ciss×f×VDD ・・・(12)
VIN
5. Setting Detection Resistance
VIN
L
R
IL
VOUT
Co
The over current protection function detects the output ripple current
peak value. This parameter (setting value) is determined as in
formula (13) below.
ILMIT= ^VILIM×0.1 [A]・・・(13)
R
(VILIM: ILIM voltage; R: Detection resistance)
Current limit
VIN
IL
L RL
r
C
VOUT
Co
Current limit
IL
ILIMIT
detect point
0
t
When the over current protection is detected by DCR of coil L, this
parameter (setting value) is determined as in formula (14) below.
(Application circuit:P20)
ILMIT=VILIM×0.1×
r×C
L
[A]・・・(14)
(RL=
L
r×C
)
(VILIM:ILIM voltage
RL: the DCR value of coil)
As soon as the voltage drop between Is+ and Is- generated by the
inductor current becomes specific threshold, the gate voltage of the
high side MOSFET becomes low.
Since the peak voltage of the inductor ripple current is detected, this
operation can sense high current ripple operation caused by
inductance saturated rated current and lead to high reliable systems.
12/20