BD9161FVM_12 Datasheet, PDF (11/19 Pages) Rohm – 2.5V to 4.5V, 0.6A 1ch Synchronous Buck Converter Integrated FET

BD9161FVM
Datasheet
Switching Regulator Efficiency
Efficiency ŋ may be expressed by the equation shown below:
η= ^VOUT×IOUT ×100[%]= ^POUT ×100[%]=
POUT _×100[%]
Vin×Iin
Pin
POUT+PDα
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)=IOUT²Ã—(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)
Vin²Ã—CRSS×IOUT×f
3)PD(SW)=
IDRIVE
(CRSS[F]：Reverse transfer capacitance of FET, IDRIVE[A]：Peak current of gate.)
4)PD(ESR)=IRMS²Ã—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.
1000
800
②587.4mW
600
①387.5mW
400
①Using an IC alone
θj-a=322.6℃/W
②mounted on glass epoxy PCB
θj-a=212.8℃/W
P=IOUT²Ã—(RON)
RON=D×RONP+(1-D)×RONN
D：ON duty (=VOUT/VCC)
RONP：ON resistance of P-channel MOS FET
RONN：ON resistance of N-channel MOS FET
IOUT：Output current
200
0
0 25 50 75 85 100 125 150
Ambient temperature:Ta [℃]
Fig.27 Thermal derating curve
(MSOP8)
If VCC=3.3V, VOUT=2.5V RONP=0.35Ω, RONN=0.37Ω
IOUT=0.6A, for example,
D=VOUT/VCC=2.5/3.3=0.758
RON=0.758×0.35+(1-0.758)×0.37
=0.2653+0.08954
=0.35484[Ω]
P=0.6²Ã—0.35484
≒127.7[mV]
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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