BD9123MUV_11 Datasheet, PDF (10/18 Pages) Rohm – Output 1.5A or Less High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9123MUV
Technical Note
・Short-current protection circuit with time delay function
Turns OFF the output to protect the IC from breakdown when the incorporated current limiter is activated continuously for
the fixed time (TLATCH) or more. The output thus held tuned OFF may be recovered by restarting EN or by re-unlocking
UVLO.
VCC
Output Short circuit
Threshold Voltage
VOUT
IL Limit
IL
Output voltage OFF Latch
Output voltage
OFF
t1<TLATCH
Operated mode
t2=TLATCH
Output voltage
OFF
Operated mode
UVLO
Timer Latch
UVLO
Fig.32 Short-current protection circuit with time delay timing chart
●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 P_Dα 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.)
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10/17
2011.01 - Rev.A