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TPS53126: VFB1(ripple) voltage for calculating feedback resistors

Tino Huljev
Prodigy 10 points
Part Number: TPS53126
Other Parts Discussed in Thread: TEST2
Hi,
I'm trying to select feedback resistors values for 0.85V and various other output voltages.
In datasheet section 9.2.2.6 Choose Output Voltage Set Point Resistors there is Equation 12: R1 =  {VO1 / [0.758 + VFB1(ripple)/2] -1} * R2.
There is no given value for VFB1(ripple) in datasheet, only equation 4 for calculating IL1(ripple).
Is there internally generated always present ripple voltage on VFB1 pin or do I calculate it from IL1(ripple), output capacitor ESR and feedback resistors?
What confuses me is D-CAP mode control which is explained in datasheet and states that there is no need for ESR induced output ripple.
If it is calculated from IL1(ripple), does it mean if there is, hypothetically speaking, 0 Ohm ESR there will be no VFB1(ripple)?
Thanks in advance
  • 0
    TI__Mastermind 29920 points

    Hello Tino, 

    scanning thru the datasheet, it look to me the VFV Ripple is the expect ripple at the FB that corespend the VOUT. 

    For example for ripple of 30mV ripple and with VO=1.8V, R2=10K, i can calculate R1=13K 

    Tomoya is traveling today, he can provide more inputs later on 

    Thanks! 

    Tahar 

  • 0
    TI__Expert 6926 points

    Hi Tino,

    The VFB1(ripple) in the datasheet corresponds to the VOUT ripple divided down by the ratio of the feedback resistors plus the internal ripple injection voltage. The RC time constant of the internal ripple injection circuit is 175us for 350kHz operation and 88us for 700kHz operation. To calculate the internal ripple injection voltage, replace L1 in Equation 4 from the datasheet with the RC time constant. To calculate VOUT ripple, solve Equation 10 in the datasheet for Vo1(ripple), where C1 is the output capacitance. To reflect VOUT ripple to FB pin, multiply VOUT ripple by VREF divided by VOUT. VREF=0.765 if TEST2=GND and VREF=0.758 if TEST2=V5FILT.