TL783: How to measure phase marign of TL783

Hi Koichi,

Thank you for this great question. To calculate phase margin, I would recommend doing a simple load transient or you can do the noninvasive test Zout test. I would read through both of these documentation for a better understanding of this test. 

Please let me know if you have any questions. 

https://www.ti.com/lit/ug/snoa826/snoa826.pdf

https://www.ti.com/lit/an/snva020b/snva020b.pdf

Thank you,

Josh  Nachassi 

Hello Josh-san,

Thank you for the prompt reply.

I have already done load transient test and looks good but want to see phase/gain margin on the FRA equipment to understand the margin  quantitatively.

I put 50 ohm to inject sine wave between Vo and R1 which are described in the datasheet but could not see probable results then asked question.

Your answer seems that phase/gain margin cannot be measured with FRA in the case of TL783, am I correct ?

Hello Josh-san,

Thank you very much for the reply.

I am a bit confused where the loop should be broken.

In the schematic below, Which of A or B is appropriate location ? 

The pdf you attached states  portion "A" , but you mentioned that "Breaking the loop through R2 and the ADJ" (="B") 

Hi Josh-san,

Thank you.

I measured phase/gain margin by FRA equipment as below, but cannot judge if the measurement succeeded or not.

Phase margin is 40 degree or around but I could not see gain margin because the point where phase equal to zero.

Could you please advise if the is suitable data for TL783 and what is  phase/gain margin on this chart ?

Note: red line:Gain   , blue line : Phase

Hi Koichi,

A 40-degree phase margin is in the realm of what to expect. As far as the gain margin goes, I will have to take a closer look. Can you provide me with additional shots of the gain margin? This will point you in the direction of stability. 

Thank you,

Josh Nachassi

Hi Josh-san,

Thank you for the comments.

I measured phase/gain by changing Vpeak of sinewave, 20mVpeak,100mVpeak and 200mVpek,

but could not see big difference on those.

The point of my question is , how judge about gain margin on those charts ?

Please advice.

Dear Koichi,

Give me some time to review these phase margins. 

Thank you,

Josh Nachassi