BD9123MUV_11 Datasheet, PDF (13/18 Pages) Rohm – Output 1.5A or Less High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9123MUV
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
fp(Min.)
fp(Max.)
IOUTMin.
IOUTMax.
fz(ESR)
1
fp= _{2π×RO×CO}
1
fz(ESR)= _{2π×ESR×CO}
Pole at power amplifier
0
Phase
[deg]
When the output current decreases, the load resistance Ro
increases and the pole frequency lowers.
-90
fp(Min.)=
1
2π×ROMax.×CO
[Hz]←with lighter load
Fig.37 Open loop gain characteristics
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.38 Error amp phase compensation characteristics
VCC
VCC
Cin
EN
VCC,PVCC
RPG
VOUT
VOUT
PGOOD
VID<2:0>
VID<2:0)
L
ITH
SW
GND,PGND
RITH
ESR
CITH
CO
Fig.39 Typical application
VOUT
RO
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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13/17
2011.01 - Rev.A