TPS54625 5V start up problem but 3.3V works perfectly

What is the input voltage? Ca you post some wavforms during attempted start up? Pleas include Vout, SS and SW.

Hi John,

the input voltage is 18V the maximum but I tried from 1-18. The 3.3V turned on on about 4V but the 5V has never turned on. I will post waveforms on Monday because the device is at the workplace. As I remember, at the moment of turning on the device the Vout is rising until ~4V and than fall down. And it is increasing pretty slowly, the 3.3 is increasing much faster.

But to prevent misunderstandings, I will post waveforms on Monday.

Thanks for your help.

I can usually tell from waveforms what is going on.

Hi John,

I made the waveforms. The green is the SS on both images. The yellow on first is SW and on the second is Vout. For the test, the Vin was 15V.

Thanks for your help.

Albert

The IC is not switching at all. The SS waveform looks abnormal also. How many bad units do you have out of how many total?

This was the second IC that I tried. The low switching voltage can be the effect of low Vin. Because I made some measures and realized that SS capacitor is full but the Vin voltage is only 3.5V and growing. Maybe it can be the reason, that the first switch voltage is only 3.5V. And the FB is below 5V so the IC turns off.

I changed the SS capacitor to 100n, now the Vin is OK, but it seems that the IC is damaged, because the SW is only 2V. I will change the IC and check out how it works.