BD9528AMUV Datasheet, PDF (19/30 Pages) Rohm – High Performance Regulators for PCs Main Power Supply for Notebook PCs(With Built-in Linear Regulator)

BD9528AMUV
Technical Note
4. MOSFET Selection
MOSFET may cause the loss as below, so please select proper FET for each.
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
(Ron: On-resistance of FET; Ciss: FET gate capacitance;
f: Switching frequency Crss: FET inverse transfer function;
I_DRIVE: Gate peak current)
・・・(10)
＜Loss on the synchronous MOSFET＞
Psyn=PRON+PGATE
= ^VIN-VOUT ×RON×IOUT²+Ciss×f×VDD
VIN
5. Setting output voltage
This IC is operated that output voltage is REF≒FB.
And it is operated that output voltage is feed back to FB pin.
・・・(11)
<Output Voltage>
VOUT =
(R1+R2)
R2
×REF(0.7V)+
⊿VOUT =⊿I_L×ESR
1
2
⊿VOUT
(⊿V_OUT: Output ripple voltage)
(⊿I_L: ripple current of coil)
⊿I_L =( VIN - VOUT)×
VOUT
(L×VIN×f)
※(Notice) Please set ⊿FB more than 10mV
(L: inductance[H] f: switching frequency[Hz])
Ex. VIN=20V,VOUT=5V,f=300kHz,L=2.5uH,ESR=20mΩ,R1=56kΩ,R2=9.1kΩ
⊿IL =(20V-5V)×5V/(2.5×10^-6H×20V×300×10³Hz)=5[A]
⊿V_OUT=5A×20×10^-3Ω=0.1[V]
V_OUT=0.7V×(56kΩ+9.1kΩ)/9.1kΩ+1/2×0.1V=5.057[V]
⊿I_L =(20V-5V)×_{(2.5×10-6H×205VV×300×103Hz)} =5(A)
⊿VOUT =5A×20×10^-3Ω=0.1(V)
VOUT =0.7V×
(51kΩ+ 9.1kΩ)
9.1kΩ
+
VIN
1
2
×0.1V=5.057(V)
VIN
REF
FB
H³REG
CONTROLLA
RQ
S
SLLM
SLLM
Driver
Circuit
Output voltage
R1
R2
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19/29
2011.11 - Rev.A