BD9533EKN_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
Current limit
VOUT
Co
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)
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