BD9104FVM datasheet(21/29 Pages) ROHM

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BD9102FVM BD9104FVM BD9106FVM

Datasheet

02.MAR.2012 Rev.001

www.rohm.com

TSZ22111･15･001

TSZ02201-0J3J0AJ00080-1-2

Switching Regulator Efficiency

Efficiency ŋ may be expressed by the equation shown below:

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(I

2R)

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(I

2R)=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)

4)PD(ESR)=IRMS

2×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.

P=IOUT

2×(RCOIL+RON)

RON=D×RONP+(1-D)RONN

D：ON duty (=VOUT/VCC)

RCOIL：DC resistance of coil

RONP：ON resistance of P-channel MOS FET

RONN：ON resistance of N-channel MOS FET

IOUT：Output current

If VCC=5V, VOUT=3.3V, RCOIL

=0.15Ω, RONP=0.35Ω, RONN=0.25Ω

IOUT=0.8A, for example,

D=VOUT/VCC=3.3/5=0.66

RON=0.66×0.35+(1-0.66)×0.25

=0.231+0.085

=0.316[Ω]

P=0.8

2×(0.15+0.316)

≒298[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.

η=

VOUT×IOUT

Vin×Iin

×100[%]=

POUT

×100[%]=

POUT

POUT+PD

×100[%]

Vin

2×CRSS×IOUT×f

IDRIVE

3)PD(SW)=

(CRSS[F]：Reverse transfer capacitance of FET,IDRIVE[A]：Peak current of gate.)

Ambient temperature:Ta [℃]

Fig.50 Thermal derating curves

200

400

600

800

1000

②387.5mW

①587.4mW

①using an IC alone

θj-a=322.6℃/W

②mounted on glass epoxy PCB

θj-a=212.8℃/W

100

125

150

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