TPS54340B: PWM is not working

Raghuwar Jha

Prodigy 10 points

Part Number: TPS54340B
Other Parts Discussed in Thread: TPS54340

Sir,

I have designed a buck converter using TPS54340 where the specifications are given below

Vin= 22V-30V

Vout = 5V

Iout= 2A

Switching Frequency=100kHz

L=100uH

Cout=220uf

the problem is that i am not getting any switching pulses (PWM is not working) although the output voltage at no-load is 5V but as soon as i connect a load it drops to some mili-volts.

Fig-1 is Switching waveform(SW to Gnd)

Fig-2 indicates the voltage between pin VIN and SW

over 1 year ago

0 Richard Fu22 over 1 year ago

TI__Mastermind 31315 points

Hi,

I suspect that there may be an issue with the layout, so I would first check that the layout matches the recommended layout in the datasheet.

You may post your your schematic and layout if you would like a review.

Thanks,

Richard

0 Raghuwar Jha over 1 year ago in reply to Richard Fu22

Prodigy 10 points

Respected sir,

I have attached the ppt which includes schematic as well as layout for the given converter...

Schematic_and_layout.pdf

Awaiting for your response..

Thanks and Regards

0 Raghuwar Jha over 1 year ago in reply to Raghuwar Jha

Prodigy 10 points

0 Richard Fu22 over 1 year ago in reply to Raghuwar Jha

TI__Mastermind 31315 points

Hi Raghuwar,

Sorry for the delay in response.

For the schematic, can you explain why you need the bottom feedback capacitor? It is usually not needed, as only the feedforward capacitor C6 is used to improve the loop response.

I suspect the issue is probably in the layout, in that the boot cap should be routed closely to the switch node instead of through the via. I suggest you follow the layout guidelines in the datasheet in section 36 using polygon pours for the power signals and traces for the analog signal.

Thanks,

Richard

0 Raghuwar Jha over 1 year ago in reply to Richard Fu22

Prodigy 10 points

Thank you for your response sir,

I have made two changes in the given circuit .Firstly, I removed C7 and secondly, I have connected a boot capacitor directly between BOOT pin and SW pin.

But, there is no change in the waveforms.

also in the given layout example, they used the same configuration to connect the boot capacitor.

Thanks and Regards,

Raghuwar Jha

0 Richard Fu22 over 1 year ago in reply to Raghuwar Jha

TI__Mastermind 31315 points

Hi Raghuwar,

Can you also confirm if the layout has the polygon pours for the power signals and GND and follows the guidelines exactly? It is critical for good operation. The reason I say this is that the SW pulses should not look so irregular.

Thanks,

Richard

0 Richard Fu22 over 1 year ago in reply to Richard Fu22

TI__Mastermind 31315 points

HI Raghuwar,

Due to lack of activity in the last two weeks, I will close this thread.

Thanks,

Richard

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TPS54340B: PWM is not working