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TPS54538EVM: Board does not power up correctly under moderate load.

Bernd Stoeckel

Prodigy 10 points

Part Number: TPS54538EVM

Tool/software:

Hi Experts,

I bought a TPS54538EVM to gain experience with this IC. I'm using it as is (no modifications).

Operating conditions

Vin = 24V

Rload = 1...28Ohm

Everything is working fine as long as I connect the load resistor after powering the board. It delivers 5A at 1Ohm as stated in the data sheet.

Do I connect a load smaller than 18Ohm during power-up then the circuit does not reach the output voltage of 5V and the control loop is getting unstable. A short disconnection of the load fixes the problem.

Output voltage for different load resistors:

No load

Rload = 28Ohm

Rload = 18Ohm

Rload = 15Ohm

Does somebody have any suggestion what I did wrong?

I tried to modify C12, C7, C10 and R9 (180kOhm) afterwards. Nothing changed the behavior at all.

Thanks for your help,

Bernd

0 Manish Gautam 26 days ago

TI__Intellectual 2450 points

Hi Brend,

Did you limit the current at input side, can you please check it once.

If you limit the current value, then it won't be reaching the desired output.

Regards,

Manish

0 Bernd Stoeckel 26 days ago in reply to Manish Gautam

Prodigy 10 points

Hi Manish,

Thanks for your reply.

The current limit was set to 3A. The pictures below show the output voltage (red) and the input voltage (24V, yellow). There is no drop of the input voltage during power up.

Rload = 15Ohm

Vin turned on by 24V supply power button.

Vin turned on by toggle switch.

The final output voltage is around 3.6V.

Regards,

Bernd

0 Manish Gautam 26 days ago in reply to Bernd Stoeckel

TI__Intellectual 2450 points

Hi,

Also, can you please check the current limit at output side.

Regards,

Manish

0 Bernd Stoeckel 26 days ago in reply to Manish Gautam

Prodigy 10 points

Hi,

I do not use an active load. I'm using a bunch of 40W power resistors as load.

Regards,

Bernd

0 Manish Gautam 26 days ago in reply to Bernd Stoeckel

TI__Intellectual 2450 points

Hi,

Can you please tell me the connection of J3, J4 and J5 connector.

Regards,

Manish

0 Bernd Stoeckel 26 days ago in reply to Manish Gautam

Prodigy 10 points

Hi,

Here are my jumper configurations

J5 1-2: OPEN (soft start)

J4 1-2, 2-3: all OPEN (floating EN pin)

J3 1-2: OPEN (no UVLO)

Thanks.

Best regards,

Bernd

0 Manish Gautam 26 days ago in reply to Bernd Stoeckel

TI__Intellectual 2450 points

Hi,

I did the test on EVM at load resistance 150ohm and Vin = 24V,

I got 5V vout

Regards,

Manish

0 Manish Gautam 26 days ago in reply to Manish Gautam

TI__Intellectual 2450 points

Hi

For Rload 180ohm

Vout = 5.00678V

Regards,

Manish

0 Bernd Stoeckel 26 days ago in reply to Manish Gautam

Prodigy 10 points

Hi,

Thanks for your help.

I works on my side too for Rload > 15 Ohm. Did you try 15 or 150 Ohm?

Regards,

Bernd

0 Bernd Stoeckel 26 days ago in reply to Bernd Stoeckel

Prodigy 10 points

Hi Manish,

I really appreciate you help. It doesn't work definitely for Rload = 10 Ohm. Do you have a chance to try it with a similar resistance?

The output voltage 2.3V.

I will give up if it works for your EVM.

Thanks.

Regards,

Bernd

Rload = 10 Ohm

0 Manish Gautam 26 days ago in reply to Bernd Stoeckel

TI__Intellectual 2450 points

Hi,

I did it for 150ohm and 180ohm.

Regards,

Manish

0 Bernd Stoeckel 24 days ago in reply to Manish Gautam

Prodigy 10 points

Hi Manish,

I think I found the problem.

The converter doesn't start properly under load if the PG/SS pin is configured for soft-start functionality.

I modified R9 (Rmode). It works for 18 kOhm, 330 kOhm, but does not for 0 Ohm, 180 kOhm and 680 kOhm.

Yellow: 24V in

Red: 5V out

Rload = 3 Ohm, R9 = 0 Ohm (soft-start configuration, doesn't work)

Rload = 3 Ohm, R9 = 18 kOhm (power-good configuration, works properly)

Thank you very much for your help.

Best regards,

Bernd