TPS54618C-Q1: The output of TPS54618C-Q1 abnormal when Enable pin goes up slowly

Hello User,

Can you share with us the schematic you are using for the implementation of TPS54618C ?

Would you also be able to provide the SW node waveform when this event occurs ?

What is the 1.15V volt on the screenshot you sent ?

Let me know if you more questions or comments.

Thanks a lot!

Regards,

Dorian

Hi :

1.15V is using TPS62811QWRWYRQ1 and unable is also from 5V.

The enable pin of TPS62811QWRWYRQ1 connects to 5V without divider resistors.

But it looks like no the same issue occurs.

The SW node waveform will be uploaded later.

TPS54618C-Q1.pdf

Hello,

Could you also capture the SS/TR Waveform ?

Let me know once you have the waveform.

Thanks a lot!

Regards,

Dorian 

Hi

Waveform is attached. Thanks

Hello,

Looking at schematic and values, I do not see anything which you should create this behavior.

If you could have the EN waveform added to the above capture , this would be very helpful.

From first picture you sent, it seems that when VIN crosses 2.5V, the soft starts and try to reach 3.3V.

As VIN goes up slowly, Soft-start is very quickly close to 100%.

Can you verify also the soldering of R67/R68 ?

Let me know.

Thanks alot!

Regards,

Dorian  

hi

update two figure.

this one is with soft-start input (so is enable pin)

the enable pin waveform is also presented.

When the enable reaches 1.22V, TPS54618C-Q1 starts to output. However it stop generating until enable reaches 1.78V.

This one is without soft-start input (so is enable pin)

it works normally.

Please help to figure out what happen ! Thank you very much.

Hello,

I am not sure to understand what you mean by "with or without soft-start input (so is enable pin)" here, can you rephrase ?

I simply think what you are seeing is normal behavior of TPS54618C.

As you have a 10k/10k Ohm resistor bridge from VIN to EN to GND. It means that V_Enable=VIN/2 at all times.

You also have a 2700pF on SS/TR, which gives 1.08ms for softstart.

At first Bump on VOUT (Yellow curve), V_EN= 1.25V Rising, VIN=2.5V above UVLO Rising. TPS54618C start switching to operate softstart in 1.08ms. VOUT is directly limited by VIN being too small VIN=2.5V. 

You encounter same sort of issue at second bump before systems enters conditions where it can operate a normal soft start. V_EN is above threshold and VIN is above 3.3V. 

To avoid this problem you need to reconsider your sequencing up. Either by enabling the TPS54618C later once VIN is above 3.3V.

Or change R67 & R68 resistor values so you would have VIN above 3.3V once enable is triggered to start the softstart sequence.

Could you try to replace R67 by a 18k Ohm resistor and keep R68 as 10k ? You should have enable triggered when VIN is 3.5V. Your softstart should work correctly like this.

Let me know once you have try this or if you have any comments.

Thanks a lot!

Regards,

Dorian 

Hello,

I haven't heard back from you.

Were you able to make it work by replacing R67 by a 18k Ohm resistor ?

Please, let me know.

Thanks a lot!

Regards,
Dorian