BD9673AEFJ Datasheet, PDF (18/23 Pages) Rohm – Flexible Step-down Switching Regulators with Built-in Power MOSFET

BD9673AEFJ
Datasheet
The target of Phase compensation design is to create a communication function in order to acquire necessary band and
Phase margin.
Cross-over Frequency (band) at which Loop gain of Return Loop becomes “0” is important.
When Cross-over Frequency becomes low, Power supply Fluctuation Response, Load Response, etc worsens.
On the other hand, when Cross-over Frequency is too high, instability of the Loop can occur.
Tentatively, Cross-over Frequency is targeted to be made 1/20 or below of Switching Frequency.
Selection method of Phase Compensation constant is shown below.
1. Phase Compensation Resistor (R3) is selected in order to set to the desired Cross-over Frequency.
Calculation of RC is done using the formula below.
2 π× COUT × fc
R3 =
GEA × GCS
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 (C1) is selected in order to achieve the desired phase margin.
In an application that has a representative Inductance value (about several µH～20µH), by matching zero of
compensation to 1/4 and below of the Cross-over Frequency, sufficient Phase margin can be acquired. C1 can be
calculated using the following formula.
4
C1 _{＞ 2π× R3 × fc}
RC is Phase compensation Resistor.
3. Examination whether the second Phase compensation Capacitor C2 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.
1
＜ ^fs
2π× COUT × RESR 2
In this case, add the second Phase compensation Capacitor C2, and match the frequency of the third pole to the
Frequency fp3 of ESR zero.
C2 is looked for using the following formula.
C2 ^{= COUT × RESR}
R3
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TSZ22111・15・001
18/20
TSZ02201-0Q1Q0AJ00190-1-2
28.JAN.2014 Rev.003