TPS65251: PFM mode error.

Ashwin S1

Part Number: TPS65251

Hi,

My input voltage is 12V, O/p of buck1 = 3.3V, Buck2 = 2.5V, Buck3 = 1.8V.

1. During the bringup, the EN of Buck1 is toggling i have checked the input ripple which is around 500mV.

2. When i am checking the datasheet there where two modes PFM and PWM. During the light load condition it will be PFM mode.

3. There is an graph which says that Transition from PFM to PWM mode.

4. LOW_P pin is pulled down to ground.

5. Load is connected.

Issues :

1. Buck1 o/p voltage is coming around 1.8V but it should be 3.3V.(Cross verified the resistance)

2. Buck2 and Buck3 is turned off.

We want to Know why it is in PFM mode and How to change to PWM mode. Is there anything to do with the LOW_P pin?

over 5 years ago

0 Zhao Ma over 5 years ago

TI__Genius 15135 points

Hi, Ashwin

Could you send me the schematic and layout files? PDF format is okay.

0 Ashwin S1 over 5 years ago in reply to Zhao Ma

Prodigy 90 points

Hi,

Please find the attached image of the schematic. Regarding layout file will share once i get from layout person.

Regards,

Ashwin S

0 Zhao Ma over 5 years ago in reply to Ashwin S1

TI__Genius 15135 points

Hi, Ashwin

The schematic screenshot is not clear and i didn't see the compensation circuit.

Could you send me the clear and complete schematic?

0 Ashwin S1 over 5 years ago in reply to Zhao Ma

Prodigy 90 points

Hi,

Attached the clear image with comp circuit.

Regards,

Ashwin S

0 Zhao Ma over 5 years ago in reply to Ashwin S1

TI__Genius 15135 points

Hi, Ashwin

Some comments:

1. Suggest changing compensation circuit:

Change C43=4.7nF, R79=20kohm.

2. After that, measure Vout1, LX1 waveforms and zoom in.

3. Please double check the location of C180, C192, C181 in layout, they should be placed close to VIN_BUCK1/2/3 pin.

If no, please make a method to add a 10uF caps close to every VIN_BUCK1/2/3 pin in layout, then test again.

0 user5399698 over 5 years ago in reply to Zhao Ma

Prodigy 10 points

Hi Zhao,

We have tuned the C43 and R79, the board starts to power on and again its shuts down.

Regards,

Muthukumar Paramasivam

0 Zhao Ma over 5 years ago in reply to user5399698

TI__Genius 15135 points

Hi, Paramasivam

Could you send me some waveforms? including VIN1 pin voltage, Vout1, LX1, and zoom in.

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TPS65251: PFM mode error.