BD95242MUV_10 Datasheet, PDF (12/21 Pages) Rohm – 2ch Switching Regulator for Desktop PC

Technical Note
4. MOSFET Selection
VIN
main switch
L
Co
VOUT
Loss on the main MOSFET
Pmain=PRON+PGATE+PTRAN
=
VOUT
VIN
×RON×IOUT²+Ciss×f×VDD+
VIN²Ã—Crss×IOUT×f
IDRIVE
・・・(10)
(Ron: On-resistance of FET; Ciss: FET gate capacitance;
f: Switching frequency Crss: FET inverse transfer function;
IDRIVE: Gate peak current)
synchronous switch
Loss on the synchronous MOSFET
Psyn=PRON+PGATE
= ^VIN-VOUT ×RON×IOUT²+Ciss×f×VDD ・・・(11)
VIN
5. Setting Detection Resistance (Detect ILIMIT at the peak current)
(A) High accuracy current detective circuit (use the low 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= ^65mV(typ) [A]・・・(12)
R
(R: Detection resistance)
OCP
Current limit ^65mV
(B) Low loss current detective circuit (use the DCR value of inductor)
VIN
IL
When the over current protection is detected by DCR of inductor L,
this parameter (setting value) is determined as in formula (13) below.
(Application circuit:P.18)
L
RL
r
C
VOUT
Co
ILMIT=65mV(typ)×
r×C
L
[A]・・・(13)
L
OCP
(RL= _r×C )
(RL: the DCR value of inductor)
65mV
Current limit
(C) Low loss current detective circuit (the DCR value of inductor : high)
VIN
IL
L (1-k)RL kRL
R1
R2
C
ILIMIT=
65mV(typ)
k×RL
[A]・・・(14)
VOUT
R2
L
( k= _R1+R2 , _RL = kR1C )
Co
(RL: the DCR value of inductor)
65mV
Current limit
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12/20
2010.03 - Rev.C