TPS62135: BUCK converter retains voltage on the input even after it is disconnected

Quick follow up to this.....

Is the mosfet inside a buck converter normally closed when no input power is supplied?  This would lead to the voltage at the output feeding back to the input

Thanks

Steve

Hi Steve,

I wrote this app note, found on the device product page, to address your second question: http://www.ti.com/lit/an/slyt689/slyt689.pdf

From your first post, it sounded like the diode was after the TPS62135 and before the bq2057?  Can you post a schematic or block diagram and a waveform of Vout, Vin, SW, and EN?

Hi Chris....

Your app note confirmed what I saw in datasheet....body diode is conducting...Yes I have a diode feeding output of buck to BQ...the battery is leaking through that diode most likely.  I have exceptionally large feedback resistors on the buck per datasheet , like 1.5 Meg....in any event I placed a diode between input connector and buck such that the battery feeds back thru the body diode but then gets blocked by the external diode I put in...it costs me about a half of volt on the input side but I have it to spare...thanks for the help...I will definitely read your app note

Steve

Chris....

My apologies for the long delay....Working on multiple projects at this end and just getting back to this....So I do have a question....Hoping you can resolve....

I don't have the luxury of trying this on my current design so I'd like to get it right before next board spin...I would like to place a diode in series with the input power that feeds the TPS62135....I've experimented here and it is working nicely....Recall I have an N-FET on the front end (at Vin) that needs to detect the loss of input power and the battery charger that hangs off the output of this buck, when battery is connected, feeds back through the body diode to the input causing a false positive on the N-FET making the micro think Vin power is still applied.....Anyhow my circuit has the ENABLE pin of the Buck connected directly to Vin...I was THINKING about placing the Enable on the anode / Vin side of the added diode and the input to the Buck (ie Vin pin) to the cathode....My concern is if I don't do this the switcher will switch without Vin applied due to a fed back output voltage....So questions.....

If Output Voltage of Switcher is 8.4V due to Battery connection and it feeds back to the Vin and Enable pin of the Switcher (i.e. 8V), will the switcher draw a higher quiescent current? Will it switch? (doesn't appear to be switching, by scope probing)

If I do what I stated above about splitting the Enable and Vin across the diode will the output discharge function try to discharge my battery? (because effectively I would still have Vin pin at 8V and datasheet says anything above 2V will activate the discharge)  I was thinking about doing this to reduce my quiescent current from 18uA to 1uA

Looking forward to hearing your thoughts!

Thanks for your patience....

Steve

Hi Steve,

Maybe you can draw a sketch of your system and what signals you need where.

Is it possible to get a less leaky diode for the output side?  Would this solve all your issues?  Maybe a standard silicon diode would be better (on leakage) than a schottky.

Also, the TPS62135 has a precise EN input.  This can be used to turn off the IC at a specific level of Vin.  When it is disabled (and Vin is still present), PG will be low.  Could this alert your MCU somehow?

Even if there is no switching, if the IC is enabled with a sufficient Vin, it will draw higher current in 100% mode: http://www.ti.com/lit/an/slyt412/slyt412.pdf  If you disable it, it will draw some current through the output discharge.  However, the pin to pin TPS621351 disables the output discharge.

Chris....

Yes agreed a pic is worth 1000 words...Do you have a direct TI email where we can take this offline?

Thanks

Steve

Yep, you can click on my name and start a (private) 'conversation'.

Hi Steve,

Here is an FAQ on how to do private messaging: https://e2e.ti.com/support/site-support/f/1024/p/693323/2555102

I should have my settings set so that you don't need to friend me first, but it won't hurt.

Hi Steve,

Yes, I think if you tie EN (through a voltage divider) to the anode of your input diode, this will keep the IC off even if there is some leakage back through all 3 diodes.  The resistors on the EN pin would be needed to sink any leakage current that does make it back through the diodes.  The resistors would also reduce the EN voltage a little bit to keep it below the Vin + 0.3V abs max rating.

Then, you also need to use the TPS621351 IC version, which does not have output discharge.

With these changes, the IC will be off (EN = low) when the input power is removed.  Without any output discharge, there will be very little leakage current.

I also recommend using a SS cap, as a good design practice.