BD9E151NUX Datasheet, PDF (13/22 Pages) Rohm – 1ch Step-Down Switching Regulator

BD9E151NUX
Datasheet
There are 2 important poles in the Control Loop of this DC/DC.
The first occurs with through the output resistance of Phase compensation Capacitor (C1) and Error amplifier.
The other one occurs with through the Output Capacitor and Load Resistor.
These poles appear in the frequency written below.
・・・ (11)
・・・ (12)
Here, GEA is the trans-conductance of Error amplifier (typ : 250uA/V).
Here, in this Control Loop, one zero becomes important.
With the zero which occurs because of Phase compensation Capacitor C1 and Phase compensation Resistor R3, the
Frequency below appears.
・・・ (13)
Also, if Output Capacitor is big, and that ESR (RESR) is big, in this Control Loop, there are cases when it has an
important, separate zero (ESR zero).
This ESR zero occurs due to ESR of Output Capacitor and Capacitance, and exists in the Frequency below.
・・・ (14)
(ESR zero)
In this case, the 3^rd pole determined with the 2^nd Phase compensation Capacitor (C2) and Phase Correction Resistor
(R3) is used in order to correct the ESR zero results in Loop Gain.
This pole exists in the frequency shown below.
・・・ (15)
(pole that corrects ESR zero)
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.
R3 = ^{2π COUT fc Vout ・・・ (16)}
GEA GCS 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 10uH～22uH), 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.
・・・ (17)
RC is Phase compensation Resistor.
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TSZ22111・15・001
13/19
TSZ02210-0Q3Q0AZ00160-1-2
2013.07.17 Rev.001