TPS2121: VOUT not switching to Vin1 if VIN2 is higher

Hi Frank,

Are you using a resistor divider from VIN1 -> PR1 -> GND? And, do you have a schematic you can share? Please let me know if you'd prefer to share it over forum chat instead.

Thanks,

Patrick

Hi Patrick

Please see attached schematic and simulation 

Hi Frank,

The schematic is proper based on what you are looking to accomplish. One thing I'd like to note is that it looks like the model is stuck in the diode mode, outputting whichever input is higher, as the part switches back to IN1 at the end. Can you probe PR1 as well as a sanity check? Based on this I'm betting there's a bug in the sim model when ported to LTSPICE, since the part should be switching back to IN1 when it becomes valid (when PR1 > 1.04V).

And did you see this behavior in a board or just in sim?

Thanks,

Patrick

Hi Patrick

We checked PR1 and confirmed it goes above the VREF. I'll Double-check and provide a screenshot on Monday 

So far we have not ordered any boards since we wanted to confirm it's suitability before ordering.

Can you support with a simulation in the Native format? Or otherwise verify the behavior?

Thanks for the swift support.

Kindest regards 

Frank Severinsen 

Hi Frank,

I think it would be best if I verify this in the lab and send you the waveform so you have the most accurate representation of the behavior - I can do so on Monday. Will that work for you? Please let me know if your board schematic differs from the one you sent above.

Thanks,

Patrick

Hi Patrick

Sounds great, I'll look forward to your results. I think we'll be tweaking some resistor values for the final board, but we'll mainly just need the IN1 priority confirmed 

Hi Patrick

Any update on the lab test?

Hi Patrick

Thanks for the response.

It looks like VIN1 and VIN2 are both around 12V in your test. The main question is if the chips works when VIN2 > VIN1.

Could you please test it with VIN1 =12V and VIN2 =14V?

Please let me know if I missed anything 

Kindest regards 

Hi Frank,

Sure, I mistakenly set VIN2 to 12V when switching power supplies - apologies for that. I'll test with VIN2 = 14V today.

Thanks,

Patrick