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LM34936: Loop compensation / Slope Cap

Part Number: LM34936
Other Parts Discussed in Thread: LM5176

Hello Support Team,

Looking at LM34936 for software programmable Vout, 0v < Vout < 24V @ 500mA max load current
Input voltage ranges from 6v ~ 12v 


#1) Can you please advise suitable values for comp network / slope capacitor, ƒsw = ~350KHz and 10μH 
Is there a TI tool available to verify loop stability? WEBENCH Power Designer calculates values for a specific Vout not across the full range for Vout.



Thanks,
AJ

  • Hi Alex,

    Thanks for reaching out and for considering the LM34936 in your design.

    I suggest using the LM5176 quick-start calculator to come up with the component values. Both devices have similar architectures and the component calculations for the LM5176 are accurate for the LM34936.

    Please let me know if you have any questions.

    Thanks,

    Garrett 

  • Hello Garrett,

    Thank you for the quick response. I downloaded the tool, again it calculates comp loop values for a specific Vout setting

    Assuming fixed inductor size = 12uH and ƒsw = 600 kHz, for Vout = 3.3v the tool yields  

    Rc   =33.2kOhm
    Cc1 =22nF
    Cc2 =56pF

    if I now set Vout = 24v, the tool yields different loop comp value.

    Rc   = 2kOhm
    Cc1 = 120 nF
    Cc2 =1200 pF


    Of course,  once selected and PCB are built, I can't change those value.

    My question is how do I select loop comp values that satisfies Vout across the output range, i.e. 3v < Vout < 24v

    Is it sufficient that 0.3v < COMP voltage < 3.0v for the selected values?



    Thanks!
    AJ

  • Hi AJ,

    The trick to finding compensation that works for both output voltages is compensating the worst case operating condition first. This will be a good starting point and then the compensation components can be adjusted slightly to improve the phase margin as needed.

    For your application start by compensating the 24 V output condition and the minimum input voltage. This should provide a stable loop for the output voltage range. You can check this by changing the output voltage in the calculator to 3.3 V. The loop might not be optimal for the 3.3 V setting but the loop should be stable which is the goal.  

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • Hi Garrett,

    Thank you for the response, makes sense. I will follow your recommendations. 

    Best,
    AJ