TPS548A20: -no output with load

Hi Ruben,

   Here are a few things that you can try.

Can you try using a resistive load of 2A and try starting the converter with this load and check if it continues to work with 2A?

When you meant by connecting the load, is that an electronic load and if so what is the rise time of the load? 

the TPS548A20 has undervoltage protection - so when the output drops because you are increasing the load suddenly, this can trigger UVP and cause the device to turn off the high side and low side FET and essentially shutting down the device.

If the device is shutting down with a resistive load at the output, can you please send us oscilloscope shots of the output voltage, switch SW node and VIN before the load is applied and during the time the load is applied?  I want to see if the SW node is turned off after output voltage drops to a sufficient voltage.

Regards,

Gerold

Hello Gerol,

Thanks in advance for your help,

The tests were made with resistive loads: 5 ohm,10 ohm and 20 ohm (power resistors of 15W) .

At the moment, only the FB was measured for the two cases:

-Without load:  600mV (correct value) since the output is 5V.

-With any load: 0V since the output is 0V -> this activate the undervoltage protection as you said.


Tomorrow I will do the tests that you suggested and then I will put the results

Best regards,

Ruben

Hi Ruben

     Also, are you starting up with a resistive load or connecting the load after it startsup?

If the shutdown happens even before the device starts up, try changing the Rtrip resistor as well to see if that helps.

Regards,

Gerold

Hi Gerold,

It was tested for both cases and the results were always 0V at output.

The Rtrip sensor was changed to 20k too and nothing changed.

Best regards,
Ruben

Hello Gerold,

The oscilloscope shots of the output voltage, SW pin and VIN (before the load is applied and after the load is applied) are displayed bellow.

VIN=12V

- Without load : the output voltage and the SW pin have the value of 5V.

- With load (4.4ohms), at the moment that its connected , the output voltage and SW pin drop (instantly) to 0V.

Best regards,

Ruben

Hi Ruben

    I was expecting switching to be seen on the SW pin when the converter is working. Can you increase the time scale in the oscilloscope to 2us/division or if necessary increase it further till I can see enough SW node pulses.

Regarding the waveform with 4.4ohm, I see that Vout is already sitting at 0V even before the load is applied. Why?

Regards,

Gerold

Hello there,

The SW pin is constant (don t know why)... it was measure at cbst and csnub pin and if you notice the scale used was 480ns/div...

The waveform with 4.4 ohm is the print that we get after the load is applied so that's why it is always zero.

Don't know what to do now..

best regards,

Ruben

Hi Ruben

   That is not possible as this is a switching converter and SW node has to switch - atleast during startup. Can you capture the waveform during startup ?

Can you please try replacing the IC or try a different board?

Regards,

Gerold

Hi Gerold,

Thanks again for your help.

One note about the pcb layout: I did not used inner layers, just top and bottom , can originate this kind of problem?

I will do some more tests and capture the waveform.

Best regards,

Ruben 

Hi Ruben,

Any updates on this? If you are still having problems, can you please provide a schematic from your EDA, as well as your PCB layout?

Hello Kevin,

Yes the problem persists, in the next days I will do some more tests and post the results here.

As soon as I can I will provide the schematic and layout too.

Best regards,

Ruben