TPS6206x loop stability

JonHW

Other Parts Discussed in Thread: TPS62067, TPS62097, TINA-TI

Hello,

I need to smoothen the ramp-up time of Vout to less than 1V/ms for the TPS62067. I am using the Switcher Pro to make a quick design but the recommended Cout is 22uF, which is good enough for the LC network and loop stability, but my external supplied circuit has additional decoupling capacitors and I have to add and additional 470uF capacitor to smoothen even more the ramp-up time when the converter is powered up.

Is the circuit recommended by TI (WEBENCH, Switcher Pro) good enough, taking into account that the external circuitry has additional decoupling capacitors and a 470uF capacitor to smoothen the ramp of the transient power-up? By adding additional capacitors, the stability of the converter is put in jeopardy?

Thanks.

over 9 years ago

0 Chris Glaser over 9 years ago

TI__Guru**** 150950 points

I'm not sure what you mean by 'smoothen the ramp time' but this device and its voltage mode control does usually not tolerate such large output capacitances. See this app note: www.ti.com/.../slva441.pdf Now, you likely have some ESR in that 470uF which changes things and helps.

But I think the TPS62097 is a much better device for you. It offers a SS pin so that you can set the SS time. As well, DCS-Control easily supports stability with large output caps like this. Same package size, same output filter--just higher efficiency and more features.

0 JonHW over 9 years ago in reply to Chris Glaser

Prodigy 100 points

Thank you very much, Chris.

I am not very expert on DC-DC step-down converters, but I have a requirement in the ramp-up time, which is also known as the soft-startup time. In particular, The slope of the ramp shall be under 1V/ms. With this converter TPS62097, I can design the best fitted ramp for my requirement.

Is there any heuristic formula or equation to solve the phase margin for any LC output filter?

0 Chris Glaser over 9 years ago in reply to JonHW

TI__Guru**** 150950 points

No, there is no equation for this--but you can measure it on the EVM. We have an example here: www.ti.com/.../slvuai5.pdf

0 JonHW over 9 years ago in reply to Chris Glaser

Prodigy 100 points

Is there a PSpice model or custom model to simulate the circuit TPS62097 and to show the bode diagram?

0 Chris Glaser over 9 years ago in reply to JonHW

TI__Guru**** 150950 points

Yes, models are available on the 'tools and software' tab on the product folder. Just click on the part number in your last reply.

This feature is also in Webench on some devices.

0 JonHW over 9 years ago in reply to Chris Glaser

Prodigy 100 points

Which minimum version of Orcad do I need? I can't simulate the TPS62097 circuit because the PSpice model is encrypted. I have the Orcad version 10.5.

0 Nikhil Gupta (Product Folder Models) over 9 years ago in reply to JonHW

TI__Mastermind 39210 points

Jon,

You will need at least version 15.7 or above since the model is encrypted. Alternately, try using the TINA-TI model for TPS62067. TINA-TI is a free simulator from TI.

0 JonHW over 9 years ago in reply to Nikhil Gupta (Product Folder Models)

Prodigy 100 points

Hello,

I need to measure the phase margin for the startup response of the TPS62097, using the TINA-TI simulator. How could I represent the Bode diagram?

Regards.

tps62097-startup.tsc

0 Chris Glaser over 9 years ago in reply to JonHW

TI__Guru**** 150950 points

I believe you need an average model to measure the loop.

With a transient model, you can apply a load step and look at the ringing per this app note: www.ti.com/.../slva381b.pdf

0 JonHW over 9 years ago in reply to Chris Glaser

Prodigy 100 points

Chris,

Regarding app note, how do I estimate the phase margin of the coverter if I got the load step response and ringing? I could simulate a response in the TINA-TI but I don't know the phase margin and it is possible that TINA-TI may estimate a phase margin according to the Vout/Vin response.

Regards,

Jon.

0 Chris Glaser over 9 years ago in reply to JonHW

TI__Guru**** 150950 points

Did you read section 2 of the app note? It should explain exactly this.

No, you cannot get phase margin or loop bandwidth with a transient model. You need to run load transients and judge the stability.

What output filter do you want to use?

0 JonHW over 9 years ago in reply to Chris Glaser

Prodigy 100 points

Thank you, Chris.
I am concerned about the stability of my circuit. My LC filter is composed of a L=1uH and C is a set of capacitors that sums about 60uF. All the simulations with TINA-TI show that my circuit behaviour (start-up response and steady-state response) is excellent but a good quantification of the loop stability is the phase margin.

0 Chris Glaser over 9 years ago in reply to JonHW

TI__Guru**** 150950 points

This is well within the normal operating range of the device and is still stable per Table 4 in the D/S.

As well, we have this app note which discusses this topic in more detail: www.ti.com/.../slva710.pdf

Power management

Power management forum

TPS6206x loop stability