BD9111NV_09 Datasheet, PDF(8/14 Page) Rohm – Output 2A or More High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

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BD9111NV_09 Datasheet, PDF (8/14 Pages) Rohm – Output 2A or More High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

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BD9111NV

Technical Note

âSwitching regulator efficiency

Efficiency Å may be expressed by the equation shown below:

Î·= VOUTÃIOUT Ã100[%]= POUT Ã100[%]=

VinÃIin

POUT

POUT+PDÎ±

Ã100[%]

Efficiency may be improved by reducing the switching regulator power dissipation factors PDÎ± as follows:

Dissipation factors:

1) ON resistance dissipation of inductor and FETï¼PD(I2R)

2) Gate charge/discharge dissipationï¼PD(Gate)

3) Switching dissipationï¼PD(SW)

4) ESR dissipation of capacitorï¼PD(ESR)

5) Operating current dissipation of ICï¼PD(IC)

1)PD(I2R)=IOUT2Ã(RCOIL+RON) (RCOIL[â¦]ï¼DC resistance of inductor, RON[â¦]ï¼ON resistance of FET, IOUT[A]ï¼Output current.)

2)PD(Gate)=CgsÃfÃV (Cgs[F]ï¼Gate capacitance of FET,f[Hz]ï¼Switching frequency,V[V]ï¼Gate driving voltage of FET)

3)PD(SW)=

Vin2ÃCRSSÃIOUTÃf

IDRIVE

(CRSS[F]ï¼Reverse transfer capacitance of FET,IDRIVE[A]ï¼Peak current of gate.)

4)PD(ESR)=IRMS2ÃESR (IRMS[A]ï¼Ripple current of capacitor,ESR[â¦]ï¼Equivalent series resistance.)

5)PD(IC)=VinÃICC (ICC[A]ï¼Circuit current.)

âConsideration 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.

â 3.9W

4.0

3.0

2.0

1.0 â¡0.90W

â¢0.64W

â for SON008V5060

JEDEC 4 layer board 76.2Ã114.3Ã1.6mm

Î¸j-a=32.1â/W

â¡for SON008V5060

ROHM standard 1 layer board 70Ã70Ã1.6mm

Î¸j-a=138.9â/W

â¢ IC only

Î¸j-a=195.3â/W

P=IOUT2ÃRON

RON=DÃRONP+(1-D)RONN

Dï¼ON duty (=VOUT/VCC)

RCOILï¼DC resistance of coil

RONPï¼ON resistance of P-channel MOS FET

RONNï¼ON resistance of N-channel MOS FET

IOUTï¼Output current

0 25 50 75 100105 125 150

Ambient temperature:Ta [â]

Fig.26 Thermal derating curve

(SON008V5060)

If VCC=5V, VOUT=3.3V, RONP=0.2â¦, RONN=0.16â¦

IOUT=2A, for example,

D=VOUT/VCC=3.3/5.0=0.66

RON=0.66Ã0.20+(1-0.66)Ã0.16

=0.132+0.0544

=0.1864[â¦]

P=22Ã0.1864ï¼0.7456W]

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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2009.05 - Rev.A

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