TPS63900: Output impedance when chip is disabled / shutdown

Hello Nicholas,

Yes, the device will have a high impedance output.

Thank you for your reply; so the TP63900 goes to high impedance state which equals tri-state -is that correct maybe the word 'tri-state' is better.

I want to use device in the following way:

The output of the TPS63900 is used as a source for a charger and I have chosen 2 different levels to match the battery voltage after the charging circuit (inside chip); to save power.

The issue is that the output of the charger is shared with simple communications signals (communications over power line); just consists of pulling GPIO's.

We can’t have additional capacitance (it’ll slow comms down) or significant resistive load on the TPS63900 output line when we’re communicating and the DCDC is off , so if the DCDC chosen can’t do this then we need a pass device on the output to enable isolation of the VCHG comms line from the DCDC.

Do you think we are OK without a pass device ?

Nick

Hello Nick,

The current going into the TPS63900 when it is turned off is in the nA range. We even had the issue that the leakage current of ceramic capacitors is bigger than the current that is going into the device when it is turned off. So TPS63900 is almost an open when the device is turned off.

Nevertheless, the device needs a ceramic output capacitor of 22uF (minimum 10uF capacitance). I understand your system the way that this capacitance would be connected to your communication line, correct?

YES -you are right the output capacitor is connected to the communications line.

Could  I wait for the output to discharge by itself before starting communications on the line -how long would this approximately take ?

If this time is reasonable then I shouldn't need an isolation circuit when communicating using the same output line as the DC -DC regulator output line.

Nick

Hello Nick,

I do not expect that it is important if the capacitor is charged or not charged, it is connected to the line.

OK

So are you recommending to have an isolation circuit anyway ?

So that when the common line (TPS63900 output/ Communication line) is used for communications I need  to make sure that any additional capacitance (such as 10u on TPS63900 output)

is effectively removed by putting in an isolation circuit.

Nick

Hello Nick,

If you mention that additional capacitance on the communication line is not allowed, you need to remove the additional capacitance.

I recommend to check what is happening to the communication if you add the output capacitor to the communication line. If the communication does not work anymore, you need to add something to disconnect the output capacitor.

The efficiency of the converter does not change, but there will be additional losses in the isolation circuit.