TPS62912EVM-077: Switching Frequency Changed After Keeping only minimum input/output capacitors

Chenglai FANG

Prodigy 90 points

Part Number: TPS62912EVM-077
Other Parts Discussed in Thread: TPS62912

Tool/software:

Hello experts,

Configuration below:

Vin = 12V, Vout = 5V

SSM: off, S-config : 1MHz

Context:

I am measuring the output ripple if TPS62912EVM, and studying the influence of input/output cap on the buck.

I tried reducing all the cap to the minimum as indicated in the datasheet of TPS62912.

Test results below:

Green curve = SW node

Purple curve = Vout_FLT

Red curve = FFT on Vout_FLT

EVM with minimum cap result:

minimum cap

1MHz switching pulses are present. Additional switching pulses appear.

Output ripple at 530KHz

Tried changed the output voltage to 2V.

The additional switching pulses are gone. The output ripple become at 1MHz, jwhich is normal

INTACT EVM results:

INTACT

Only 1MHz switching pulses are present.

Output ripple at 1MHz

Tried changing Vout to 2V, No difference noted compared with the results above.

Questions below:

Why is the output ripple changing frequency if I remove input/output cap?
Why is output ripple changing frequency if I changed the output regulated voltage for the EVM with minimum cap?

Thank you

Chenglai

1 month ago

0 Rupayan Saha 1 month ago

TI__Intellectual 1220 points

Hello Chenglai,

Please provide the circuit that you have modified as per the datasheet.

0 Chenglai FANG 1 month ago in reply to Rupayan Saha

TI__Prodigy 90 points

Hi Rupayan,

Thank you for your attention on my quesiton !

Here is the circuit, which is modified on the TPS62912EVM

And I met a new problem. The output voltage will drop if I put any load.

I have desoldered the feedback resistors R1 and R2 and replace them with 14k and 28k resistors on a breadboard to create 5V output voltage. But when I add a 18ohm load resistor on the output, the measured output voltage drops to 4V. I have measured the FB node voltage. It equals to 0.8V, the internal Vref.

If I reduce load current the voltage will go higher but never meet 5V set value. This is tested on EVM with minimum input/output caps and the intact EVM, both with changed feedback resistors R1 and R2 on bread board.

0 Rupayan Saha 1 month ago in reply to Chenglai FANG

TI__Intellectual 1220 points

Hi Chenglai,

From the above schematic I observed that you have disconnected the feedback from the output , what do you want to infer by this testing?

0 Chenglai FANG 1 month ago in reply to Rupayan Saha

TI__Prodigy 90 points

Hi Rupayan,

The feedback network is always connected to the VOUT_FILT on the EVM. I did not disconnect it and the output voltage is under regulation with no output current.

0 Rupayan Saha 1 month ago in reply to Chenglai FANG

TI__Intellectual 1220 points

Hi Chenglai,

I will once check and get back to you.

Regards

Rupayan

0 Rupayan Saha 1 month ago in reply to Rupayan Saha

TI__Intellectual 1220 points

Hi Chenglai,

What is the current limit of your input source? Can you share me the details of your input source

Regards

Rupayan

0 Chenglai FANG 1 month ago in reply to Rupayan Saha

TI__Prodigy 90 points

I used an external power supply at 12V and output current limit at 300mA. But during the test, the limit is never reached.

Thank you

0 Rupayan Saha 1 month ago in reply to Chenglai FANG

TI__Intellectual 1220 points

Hi Chenglai,

Can you please change the ic and check

0 Rupayan Saha 1 month ago in reply to Rupayan Saha

TI__Intellectual 1220 points

Hi Chenglai,

To get 5V output please change the R1 to 25.5675Kohm and R2 to 4.87Kohm

Regards

Rupayan

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TPS62912EVM-077: Switching Frequency Changed After Keeping only minimum input/output capacitors