TPS63002: IC locking up when ramping input voltage

Hi Marko.

i think you test with a long cable from source and board, right?

make the cable as short as you can, and increase the input cap. i think it will be different.

Regards

Tao

Ok I'll try adding more capacitance on the input to address the inrush / transient current and report back. 

Could you please advise on the scenario when the device is slightly power starved on the input? Why is not able to continue operating with the output voltage slightly lower and then recover when sufficient power is available? 

Hi  Marko,

i do no know what do you mean about slightly and sufficient power, with a lower or higher current limit?

Regards

Tao

Meaning the output is trying to draw more power than it's available on the input. As an example, let's say there is a 25 ohm load on the output so it needs 200 mA / 1W. If the input can only source 980 mW, the TPS63002 output drops down to 2V and stays at 2V even if the available power at the input goes back above 1W. So my question is, why does the output drop to 2V instead of respective ~4.95V (assuming 100% efficiency for the sake of the discussion) and why is it not able to recover to 5V when the input available power goes above 1W in this example?  

Hi  Marko,

that seems not make sense. do you have waveforms?

Regards

Tao

Here is another example with waveforms.

Output has a 100 Ohm load. Input has 4V and drawing 75 mA. As soon as I change the current limit on the input down to 74mA (1mA difference than what's being consumed) the voltage on the input and output drop down to 1.6V average. See waveforms attached, Input (CH2) Output (CH3). Could you please advise on why this is happening?

Hi Marko,

that is much clearer with a waveform.

After input current limit drop to 74mA, input can not support your output and input voltage is pulled down till UVLO OFF triggered. then device shunt down and input rise up with 74mA current till UVLO ON. finally device will work between UVLO ON/OFF repeatedly.

Regards

Tao

To me this seems like a design oversight. Why isn't there some margin here? If the input is slightly power starved, to me it would make sense if the output slightly dips, the whole point of a regulator is to to try to keep the output steady while there is fluctuations on the input. The even bigger concern here is that the device doesn't recover after the current limit is adjusted back to greater than 75mA instead it keeps sinking whatever current is available till eventually reaches thermal breakdown. Could you please comment if this is intended by design? 

Hi   Marko,

what this device do is try to keep the output voltage as a set one and never care about your status of input.

actually, we have some devices with input current limit function, their peformance will be difference with 63002.

during your start-up with a small output voltage, device should not only support your output current but also have to charge your output cap(main part). then it will need a much more enregy to support. May be you can set a big delay on EN, this will make your device start to switching with a much higher input voltage and reduce your risk.

Regards

Tao

Could you please explain what you mean by input current limit function? Which devices are you referring to?

Hi  Marko,

input current limit means device will limit the averange input current. you can take TPS63900 as ref.

1731.tps63900.pdf

Regards

Tao

Taking this device as an example what would happen in the same scenario as I described above when the device is power starved? And how does it respond / recover after a such an event when sufficient input power is available in regards to the output voltage? 

Hi  Marko,

output voltage will drop down till a balance achieved. and it can not recover unless you change the limit of input current.

Regards

Tao

Ok that makes sense and that's what I was hoping is the case with TPS63002. Could you please explain from architecture stand point why TPS63900 will recover when the current limit is increased beyond the required input current while TPS63002 stays at the low voltage 1.6V in this case and does not recover?

Hi  Marko,

they will start-up with a different bias-output voltage. 

actually, this is not only depend on device, but also components external. EX, you have a large enough input cap, you also can start-up as expected for UVLO OFF will not be triggered.

Regards

Tao

How much capacitance is needed on the input for TPS63002 to recover after it has hit a current limit? Currently I have 60 uF and the output voltage stays low even after current limit has been increased as I have described before. 

increase till UVLO-OFF not been triggered during start-up.

also, you should keep  cable from power supply to your board as short as possible