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SN1106041 FRA & C4 Feedback effect

virgan yusuf
Prodigy 10 points

Good afternoon,

Hi my name is Virgan and now I am evaluating a main board with SN1106041 DC/DC

We use this on 5V, 1.5V, and 1.2V

What i want to ask is:

1.What is the proper approach for small signal analysis, for stability test?

2. What is the effect if C4 47pF is installed in 1.5V and 1.2V, is it improving low voltage phase or not ?

Thank you

  • 0
    TI__Guru**** 191445 points

    To understand the small signal analysis, i would start here:

    http://www.ti.com/litv/pdf/slva546

    C4 is a "feed forward" capacitor (Cff).  It produces a zero at 1/2*pi*C4*R1 and a pole at 1/2*pi*C4*(R1||R2).  This produces some phase boost.  Since the relationship between R1 and R2 is fixed by the output voltage, the frequency difference between the zero and pole is fixed by the output voltage.  So for Vout = Vref, Fz = Fp and there is no boost available. As Vout increases, the frequency difference between the zero and pole increases, and the phase boost increases also.