Large 1 kHz oscillation in TPS5430 output when used as buck-boost converter

William Parker75

Other Parts Discussed in Thread: TPS5430EVM-342, TPS5430

I am using a TPS5430 in inverting buck-boost configuration with enable logic level shifted with BJT controlled voltage dividers.

My application is V_in=15.7 V, V_out= -11V I_load= 40mA to 2A

I have significant high frequency noise on the PH node and V_out (which is unexpected since I have substantial low ESR filter capacitors), but the really strange behavior is a 1V, 1kHz triangular oscillation in the output voltage when powering a 5W resistive load.

More scope captures of this phenomenon are shown in the attached pdf.

I also included a schematic of my power supply circuit and screen captures of the PH node voltage and V_out high frequency noise.

All filter caps (330 uF and 10uF are aluminum electrolytic low ESR).

The compensation network values were computed using the procedure outlined in TI app note SLVA237C

(http://www.ti.com/lit/an/slva237c/slva237c.pdf).

I have no idea why the large 1kHz oscillation in V_out occurs. Any advice would be appreciated.

Thank you.

Oscilloscope captures of 1kHz large oscillation in Vout and noise on Vout and PN nodes.pdf Additional scope captures

n_11V_schematic.TSC - TINA.pdf Circuit schemaic

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

You may be reaching current limit. Your circuit can only supply 1.76A load current (remember you have to derate current by a factor of 1-D). You may also have layout issues. Can you post your layout? Another thing to look at is to add 10uF in parallel with C1

0 William Parker75 over 9 years ago in reply to JohnTucker

Prodigy 100 points

That may have been part of the problem. I am still prototyping on a perf board, but after reading the E2E wiki article on output ripple ( http://e2e.ti.com/support/power_management/simple_switcher/w/simple_switcher_wiki/2243.understanding-measuring-and-reducing-output-voltage-ripple ), I chaned my layout to minimize the distance to the input capacitor. With that change and and reducing the load, I was able to get a stable output.

As you can see in the screen capture below, the output is stable with only switching noise, but I have been unable to get it to go below -7.5V.

I used R1 = 10k Ohms and R2 = 1250 Ohms initially, but this set the output at -7.4V.

I have tried varying R1 from 5-15k and R2 from 200-5000 Ohms, but the output does not seem to respond. Any thoughts on why this might happen?

Thanks.

0 JohnTucker over 9 years ago in reply to William Parker75

TI__Guru**** 191445 points

I'm not sure why you can't get lower voltage output. You might want to try ordering the inverting buck boost EVM:

http://www.ti.com/tool/tps5430evm-342

it has a known good layout. You ca modify it for your output voltage.

0 William Parker75 over 9 years ago in reply to JohnTucker

Prodigy 100 points

John,

Thank you.

I was able to get the TPS5430 to o to -11V with some more tweaking of the feed back resistors and cleaning up my layout.

I'll move to a PCB for further testing and pattern it after the EVM you mentioned.

Here's a scope capture of the final output.

And the switching noise:

0 William Parker75 over 9 years ago in reply to JohnTucker

Prodigy 100 points

John,

Thank you for your help with this project. Can I email you my PCB layout for a quick review?

I patterned it after the TPS5430 buck-boost EVM board layout as best I could, but the pictures of the layout in the EVM user guide don't make the layout entirely clear.

Thank you.

0 JohnTucker over 9 years ago in reply to William Parker75

TI__Guru**** 191445 points

my email is j-tucker@ti.com. I will delete this post in 24 hrs.

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Large 1 kHz oscillation in TPS5430 output when used as buck-boost converter