BD9111NV_09 Datasheet, PDF (10/14 Pages) Rohm – Output 2A or More High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9111NV
Technical Note
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, the phases are easily compensated by adding a zero to the
power amplifier output with C 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)
Fig.30 Open loop gain characteristics
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.
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.31 Error amp phase compensation characteristics
VCC
Cin
L
EN
VCC,PVCC
SW
VOUT
VOUT
VOUT
ESR
RO
ITH
GND,PGND
CO
RITH
CITH
Fig.32 Typical application
Stable feedback loop may be achieved by canceling the pole fp (Min.) produced by the output capacitor and the load
resistance with CR zero correction by the error amplifier.
fz(Amp.)= fp(Min.)
1
2π×RITH×CITH
=
1
2π×ROMax.×CO
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10/13
2009.05 - Rev.A