Technical Note

BD8960NV

10/14

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2010.04 - Rev.B

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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.

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.

Fig.28 Open loop gain characteristics

Fig.29 Error amp phase compensation characteristics

fp=

1

1

Pole at power amplifier

When the output current decreases, the load resistance Ro

increases and the pole frequency lowers.

1

[Hz]←with lighter load

1

[Hz] ←with heavier load

Zero at power amplifier

1

GND,PGND

SW

EN

ITH

Cin

L

ESR

Fig.30 Typical application

1

=

1

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.)

Gain

[dB]

Phase

[deg]

A

0

0

-90

A

0

0

-90

fz(Amp.)

fp(Min.)

fp(Max.)

fz(ESR)

Gain

[dB]

Phase

[deg]