TPS65251: Unstable output voltage during load

Konrad Sierotowicz

Part Number: TPS65251

Hi!

I am facing a big problem with unstable voltage on the outputs.

I have got my converter calculated for 12V/3A on BUCK1, 7V/2A on BUCK2, and 5V/2A on BUCK3.

When there is no load all output voltages are correct. But when i try to drain more than 450-500mA from for example BUCK3 the converter starts to be ustable.

During unstable operation all voltages on the output drops and BUCK3 voltage looks like square wave.

I will try to put some load on other outputs as well but for know i need help.

over 4 years ago

0 Jason Wang (BSR-MV) over 4 years ago

TI__Genius 9550 points

hi,

TPS65251 has 3 output rails. Please help do below test:

EN1, EN2, pull low, check BUCK3, stable or not

EN1, EN3 pull low, check BUCK2, stable or not

EN2, EN3 pull low, check BUCK1, stable or not

share your test results for analysis

0 usama abbasi over 4 years ago in reply to Jason Wang (BSR-MV)

Prodigy 20 points

Hello Jason

We are collegue with Konrad. The suggestion you gave did not yield results yet. Kindly advise on the stability. The time period is like 40msec

Thanks

0 Jason Wang (BSR-MV) over 4 years ago in reply to usama abbasi

TI__Genius 9550 points

Hi,

seems enter OCP status. Check R_LIM1/2/3 value.

0 usama abbasi over 4 years ago in reply to Jason Wang (BSR-MV)

Prodigy 20 points

Hello Jason

We checked the resistance and it meets the specs as per datasheet. So it should not be reaching Over Current Protection. The load is hardly 400mA(when we are testing) and it should be able to go till 3A

Any other suggestions?

Thanks

0 Zhao Ma over 4 years ago in reply to usama abbasi

TI__Genius 15135 points

Hi, Sir

Try to remove the feed forward cap C211, C214, C215, then test again.

BTW, what is the input voltage?

0 Konrad Sierotowicz over 4 years ago in reply to Zhao Ma

Prodigy 40 points

i have pulled up all EN inputs and now it looks better.

BUT i am still facing some problems.

I have changed Rlim1 resistor 100k to program 3A maximum current for output 1 at 12V.

I am suppling circuit with 14.4V.

I am able to reach around 1.3A of current on the Ouput 1 with 12V but then the voltage stars to drop.

Any ideas?

I am checking other outputs as well.

0 Konrad Sierotowicz over 4 years ago in reply to Konrad Sierotowicz

Prodigy 40 points

Update:

Buck 1 settings: 12V/3A (100k Rlim1 resistor) loses stability at around ~1.3A of load current

Buck 3 settings: 5V/2A (120k Rlim3 resistor) loses stability at around ~1.02A of load current

I didn't have a chance to check buck 2 because after loosing stability during BUCK 3 tests the converter seems to have broken.

0 Zhao Ma over 4 years ago in reply to Konrad Sierotowicz

TI__Genius 15135 points

Hi, Konrad

It sounds like IC triggered over temperature protection.

In your application, the output voltages(5V, 7V, 12V) are high, when Vin=14.4V, the duty cycle will be big , which means high-side MOSFET will turn on more time in every period.

You can see in EC table/below, the high-side switch has bigger turn-on resistance than low-side switch, so there will be big power loss for your application, it would cause IC triggered over temperature protection.

Do you have the waveforms at big loading when IC is unstable? including Vout1, LX1 and zoom in.

Power management

Power management forum

TPS65251: Unstable output voltage during load