LM70880: LM70880 stability issue

Helmuth Senn

Part Number: LM70880

Hello,
I'm using the LM70880 as an inverting regulator. Input Voltage is 10 to 27 V, Output -15 V 1.3 A.
The design works, but at 24 V input I see stability issues at loads above 200 mA.
Is there more information for designing inverting regulators with the LM70880 available? How to improve stability and optimize the compensation network?
Kind regards,
Helmuth

10 months ago

0 EricLee 10 months ago

TI__Guru 68961 points

Hello Helmuth

You can simulate your design using PSPICE. The model is available at https://www.ti.com/lit/zip/snvmcd7

To check stability, you can use Power Stage Designer https://www.ti.com/tool/POWERSTAGE-DESIGNER and select CMC Inverting Buck-Boost as a control scheme.

Also please refer the Buck Boost section of this article. /cfs-file/__key/communityserver-discussions-components-files/196/0412.slup340.pdf

-EL

0 Helmuth Senn 10 months ago in reply to EricLee

Prodigy 20 points

Hello EricLee

Thank you for your answers. Unfortunately, the simulation model is only partially helpful in this case, as it cannot be used for inverting regulators. The binary model has an internal connection to GND.

The article is very good, but it seems that my problem is not related to stability in general, but rather to the specific regulator.

It turns out that the LM70880 in my application goes to PFM mode instead of CCM. As I set the PFM / SYNCIN pin correctly to AGND/-Vout, I have no idea why this happens. With a 12 V supply, switching from DCM to CCM works, although I want to start and stay with CCM. However, with a 24 V supply, it appears that the regulator remains in DCM mode, which is why the output voltage is unstable for higher loads. What could cause this behaviour?

I found a TI reference design for using the LM70880 as an inverting regulator for -7 to -11 V, so I have a new question: is -15 V also possible? PMP31291 reference design | TI.com

Kind regards,

Helmuth

+1 EricLee 10 months ago in reply to Helmuth Senn

TI__Guru 68961 points

Hello Helmuth

You can simulate inverting buck boost using buck model. just move your ground symbol from t https://www.ti.com/lit/an/snva856b/snva856b.pdf

About your board, please check the operation again in buck configuration. Sometimes e-load is not isolated from earth ground, and causes a problem.

Yes, LM70880 supports +27V to -15V conversion.

-EL

0 Helmuth Senn 9 months ago in reply to EricLee

Prodigy 20 points

Hello EricLee,

Thank you for your help. The simulation now works! I was put off by the remark in the model that it is 'not for inverting design'.

I also tested an inverter with the Eval Board, and it worked as expected.

It finally turned out that my sense connection layout was poor. Improving it resolved the problem.

Kind regards,

Helmuth

0 EricLee 9 months ago in reply to Helmuth Senn

TI__Guru 68961 points

Thanks for choosing TI. Please feel free to contact us if you have any question.
-EL

Power management

Power management forum

LM70880: LM70880 stability issue