BD9859EFJ Datasheet, PDF (12/16 Pages) Rohm – Simple Step-down Switching Regulators with Built-in Power MOSFET

BD9859EFJ
Technical Note
Selection method of Phase Compensation constant is shown below.
1. Phase Compensation Resistor (RC) is selected in order to set to the desired Cross-over Frequency.
Calculation of RC is done using the formula below.
RC

2π COUT  fc
GEA  GCS

Vout
VFB
＜
2π COUT  0.1
GEA  GCS
fs

Vout
VFB
Here, fc is the desired Cross-over Frequency. It is made about 1/20 and below of the Normal Switching Frequency (fs).
2. Phase compensation Capacitor (CC1) is selected in order to achieve the desired phase margin.
In an application that has a representative Inductance value (about several uH～20uH), by matching zero of
compensation to 1/4 and below of the Cross-over Frequency, sufficient Phase margin can be acquired.CC1 can be
calculated using the following formula.
CC1＞
2π
4
RC

fc
RC is Phase compensation Resistor.
3. Examination whether the second Phase compensation Capacitor CC2 is necessary or not is done.
If the ESR zero of Output Capacitor exists in a place that is smaller than half of the Switching Frequency, a second
Phase compensation Capacitor is necessary. In other words, it is the case wherein the formula below happens.
2π
1
COUT

RESR ^＜
fs
2
In this case, add the second Phase compensation Capacitor CC2, and match the frequency of the third ball to the
Frequency fp3 of ESR zero.
CC2 is looked for using the following formula.
CC2

COUT RESR
RC
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12/15
2010.02 - Rev.A