BD9122GUL View Datasheet(PDF) - ROHM Semiconductor
Part Name
Description
Manufacturer
BD9122GUL Datasheet PDF : 19 Pages
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0J3J0AJ00110
Datasheet
Switching Regulator Efficiency
Efficiency ŋ may be expressed by the equation shown below:
η= VOUT×IOUT ×100[%]= POUT ×100[%]=
Vin×Iin
Pin
POUT
POUT+PDα
×100[%]
Efficiency may be improved by reducing the switching regulator power dissipation factors PDα as follows:
Dissipation factors:
1) ON resistance dissipation of inductor and FET：PD(I2R)
2) Gate charge/discharge dissipation：PD(Gate)
3) Switching dissipation：PD(SW)
4) ESR dissipation of capacitor：PD(ESR)
5) Operating current dissipation of IC：PD(IC)
1)PD(I2R)=IOUT2×(RCOIL+RON) (RCOIL[Ω]：DC resistance of inductor, RON[Ω]：ON resistance of FET, IOUT[A]：Output
current.)
2)PD(Gate)=Cgs×f×V (Cgs[F]：Gate capacitance of FET, f[H]：Switching frequency, V[V]：Gate driving voltage of FET)
Vin2×CRSS×IOUT×f
3)PD(SW)=
IDRIVE
(CRSS[F]：Reverse transfer capacitance of FET, IDRIVE[A]：Peak current of gate.)
4)PD(ESR)=IRMS2×ESR (IRMS[A]：Ripple current of capacitor, ESR[Ω]：Equivalent series resistance.)
5)PD(IC)=Vin×ICC (ICC[A]：Circuit current.)
Consideration on Permissible Dissipation and Heat Generation
As this IC functions with high efficiency without significant heat generation in most applications, no special consideration is
needed on permissible dissipation or heat generation. In case of extreme conditions, however, including lower input
voltage, higher output voltage, heavier load, and/or higher temperature, the permissible dissipation and/or heat generation
must be carefully considered.
For dissipation, only conduction losses due to DC resistance of inductor and ON resistance of FET are considered.
Because the conduction losses are considered to play the leading role among other dissipation mentioned above including
gate charge/discharge dissipation and switching dissipation.
1.0
0.8
0.66W
VCSP50L2(2.50×1.10mm□)
ROHM standard 1 layer board
Board size：50mm×58mm
θj-a=189.4℃/W
P=IOUT2×RON
RON=D×RONP+(1-D)RONN
0.6
D：ON duty (=VOUT/VCC)
RCOIL：DC resistance of coil
0.4
RONP：ON resistance of P-channel MOS FET
RONN：ON resistance of N-channel MOS FET
IOUT：Output current
0.2
0
0 25 50 75 100 125 150
Ambient temperature:Ta [℃]
Fig.27 Thermal derating curve
(VCSP50L2)
If VCC=3.3V, VOUT=1.5V, RONP=0.3Ω, RONN=0.2Ω
IOUT=0.3A, for example,
D=VOUT/VCC=1.5/3.3=0.45
RON=0.45×0.3+(1-0.45)×0.2
=0.135+0.11
=0.245[Ω]
P=0.32×0.245≒22.1[mW]
As RONP is greater than RONN in this IC, the dissipation increases as the ON duty becomes greater. With the
consideration on the dissipation as above, thermal design must be carried out with sufficient margin allowed.
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TSZ22111･15･001
11/17
TSZ02201-0J3J0AJ00110-1-2
02.MAR.2012 Rev.001
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