BD9151MUV Datasheet, PDF (11/17 Pages) Rohm – Output 1.5A or Less High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9151MUV
Technical Note
3. Selection of input capacitor (Cin)
VCC
Cin
VOUT
L
Co
Fig.35 Input capacitor
Input capacitor to select must be a low ESR capacitor of the capacitance
sufficient to cope with high ripple current to prevent high transient voltage. The
ripple current IRMS is given by the equation (5):
√_{VOUT(VCC-VOUT)}
IRMS=IOUT×
VCC
[A]・・・(5)
< Worst case > IRMS(max.)
When Vcc=2×VOUT, IRMS=
IOUT
2
If VCC=5.0V, VOUT=1.8V, and IOUTmax.=0.4A, (BD9151MUV)
IRMS=2×
√_1.8(5.0-1.8)
5.0
=0.48[ARMS]
A low ESR 22µF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
4. Determination of RITH, CITH that works as a phase compensator
As the Current Mode Control is designed to limit a inductor current, a pole (phase lag) appears in the low frequency area
due to a CR filter consisting of a output capacitor and a load resistance, while a zero (phase lead) appears in the high
frequency area due to the output capacitor and its ESR. So, adding a zero to the power amplifier output with C easily
compensates the phases and R as described below to cancel a pole at the power amplifier.
A
Gain
[dB] ⁰
Phase ⁰
[deg]
-90
fp(Min.)
fp(Max.)
IOUTMin.
IOUTMax.
fz(ESR)
fp=
1
2π×RO×CO
fz(ESR)=
1
2π×ESR×CO
Pole at power amplifier
When the output current decreases, the load resistance Ro
increases and the pole frequency lowers.
Fig.36 Open loop gain characteristics
fp(Min.)=
1
2π×ROMax.×CO
[Hz]←with lighter load
fp(Max.)=
1
2π×ROMin.×CO
[Hz] ←with heavier load
A
Gain
[dB]
0
0
Phase
[deg]
-90
fz(Amp.)
Zero at power amplifier
Increasing capacitance of the output capacitor lowers the pole
frequency while the zero frequency does not change.
(This is because when the capacitance is doubled, the capacitor
ESR reduces to half.)
fz(Amp.)=
1
2π×RITH×CITH
Fig.37 Error amp phase compensation characteristics
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11/16
2009.08 - Rev.A