BQ25730: How to build an UPS with BQ25730 and constant output voltage, independent of cell count.

Sorry, USB-OTG switching time is 15ms, not 200ms. But still too long for a UPS......

Hi Tiger.

Thanks for your feedback.

We will check that out.

Best regards

Alex

Hi Tiger.

Meanwhile we experimented a little with the BQ25730EVM and we are a bit confused.

First of all, we set the EN_OTG and the EN_FRS bits in configuration and we set the requested output voltage in OTG_Voltage register (we chose 11.5V in first experiments).

We are doing this while the system is powered by a VBUS source (12V here), the OTG/VAP/FRS pin is already at high level (JP2 is removed).

When we now remove the VBUS power, the VBUS voltage first drops to around 3.2V before starting the OTG mode and delivering 11.5 on VBUS. We guess this is VVBUS_CONVENZ mentioned in datasheet as condition for OTG mode.

Does this mean, OTG does not trigger before VBUS drops below 3.2V?

Or do we have to trigger it externally via OTG/VAP/FRS pin (and would that work as long as VBUS is above VVBUS_CONVENZ)?

And how do we return from OTG/FRS mode i.e. leave it?

Would be great to learn from your expertise here.....

Thanks for your support in advance.

Best regards

Alex

Please refer to below thread. 

e2e.ti.com/.../bq25730-need-info-regarding-the-frs-feature

Hi Tiger.

Thanks for your response, but this does not clarify my question.

In out application it is not possible to monitor VBUS via SW like it is obviously done in the thread you mentioned.

Nor does it really answer the question about what to really do with the OTG/VAP/FRS pin or it's exact behaviour.

I guess we have to do our own experiments and/or chose another IC with more clear documentation of the features.

Thanks for your support.

Best regards

Alex

Hi Tiger.

I think we found a solution for our problem.

Initially we are going to set EN_OTG and EN_FRS permanently to 1.

Additionally we control the level on pin OTG/VAP/FRS by the independend comparator output CMPOUT.

CMPIN is feed from the power source over a resistor voltage divider. This one is adjusted that way, that we will see a voltage of 1.2V on CMPIN when power source delivers 11.5V. If the latter drops below 11.5V, CMPIN will drop below 1.2V and trigger CMPOUT. CMPOUT itself then triggers OTP/VAP/FRS pin and thus the FRS mode.

If input power raises again above 11.5V, CMPOUT will deactivate and stop the FRS mode.....

Seems to work pretty reliable in our flying wire setup.....

What do you think?

Best regards

Alex

Hi Tiger.

Thanks for the feedback.

The circuit you proposed is working, we did almost exactly the same with good results.

And you are right, a little hysteresis is probably necessary, we sometimes see unconditional pulses on CMPOUT.

Therefore and to make the switching level controllable over I2C we decided to use a DAC53701 as comparator,

It's pretty cheap and allows a programmable hysteresis.

We already ordered an evaluation board and will do tests probably by middle of this week.

If you are interested in, I can post our results once we finished evaluation.

Best regards

Alex

Hi, Alex,

Glad to know it works. This could serve as example for others to try and explore the FRS feature. 

Regards,

Tiger

Hi Tiger.

Glad to hear that this can help others as well.

To come up here with more details:

1.) You will definitely need to add an hysteresis to make the solution reliable.

2.) Like mentioned, we are going to use the programmable comparator mode of DAC53701 for this. But no tests yet, still waiting for the evaluation board.....

Best regards, Alex

Thanks for the tip. Glad to know another digital and versatile part DAC53701, outside of my focus area.