TPS63900: Inductor

Hi,

It should not be caused by inductor, according parameters of this inductor, it is ok for TPS63900.

Did you check if the Vin is stable or with a big ripple? 2.2V is close to UVLO_OFF. 

Regards

Tao

I'd expect 2.2V is still quite far off from 1.75V UVLO. We can see the same behavior - when input voltage drops down to ~2.2V (even with a stable power source) then TPS63900 stops working properly. It seems that this happens when the output capacitance is higher than the input capacitance.

Thanks for the inputs, I've relayed your insights back to the customer and will let you know when I hear back!

Hi   Abigail, Rastislav,

Anyway, it is easy to capture a waveform when this issue happening, right? Then, we can easier to approch the potential root cause.

Regards

Tao

Hi Tao,

it seems that this weird behavior depends on output capacitance. If I connect 1F supercapacitor (e.g. SCCQ12E105SRB) to TPS63900EVM output and set output voltage to 3V, with 2V input it won't start - the output will stay around 1.7V or 1.8V. I need to increase the input voltage to above ~2.2V for the EVM to start charging the supercap.

And actually it doesn't have to be a supercapacitor, using a combination of 220uF MLCC and 330uF tantalum is enough. And the problem is more apparent when there's some current drawn, i.e. MCU starts doing something immediately after power-up. Then we can see constant restarts, because the output voltage drops quickly.

Is there any difference on using Vo1 or Vo2, if unlimited current setting is used? We use SEL=LOW and RCFG3=13.3k. RCFG1=RCFG2=GND.

Hi Rastislav,

There should be same behavior no matter Vo1 or Vo2 used. 

With 2V input, device won/t start to charge your output cap? it is strange because device can start once Vin is higher than 1.8V.

Regards

Tao

There are multiple posts on the forum that state the same thing:

* https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/985500/tps63900-soft-start-voltage

* https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/999642/tps63900-issue-with-the-operation-at-low-voltage-2-4v-with-max-60ma-current

We had similar issue on our board - once the output capacitance was bigger (e.g. 10uF MLCC + 220uF MLCC + 330uF tanatalum), TPS63900 had problems starting with lower voltage (<2.2V) and we had to increase input capacitance a lot (e.g. 10uF MLCC + 330uF tantalum). Even then it didn't work properly below 2V.

That's why I tested the same situation on EVM. The easiest test is to just connect the supercap to EVM, select 3V output voltage, discharge (short) the supercap and apply 1.8V to the input. Measuring output voltage you'll see that it gets stuck at ~1.8V. Then slowly increase input voltage and look for the moment when the output voltage starts rising. Once you reach the tripping point the outout voltage starts increasing slowly (probably inrush current protection or something) and after a short while it jumps to 3V quickly. You'll need to go way above 2V for this startup to happen.

Hi Rastislav,

Got it, then, i think it is necessary to capture waveforms including Vin, Vout and IL when unexpected behaviors happened.

As you know, when device is switching, there will be some ripple at Vin, if ripple is high enough, the valley voltage of Vin may trigger UVLO_OFF.  This phenomenon has big relationship with input cap and your power supply, we can figure out the root cause by capture the waveform mentioned.

Regards

Tao