BQ25176J: Charger remains active when input supply is removed

Hi David

Checking on this question and will have to set up unit in lab.  Try to get you an answer by tomorrow.

Do you have a schematic I can review? 

Hi Bill, my schematic is here::

Hello David,

is the V_SYS node connected directly to an input power supply, nothing else is on that node? Are there any potential pathways from BATT+ to V_SYS which would bypass the charger?

If you put a load on VIN is the battery draining and supplying power to the load?

Sincerely,

Wyatt Keller

Hi Wyatt, the power supply is connected directly to V_SYS. There is a Wired OR circuit connected to V_SYS which enables the switching regulators but no path from battery to V_SYS. In this case V_SYS is sitting at 4.1V (battery voltage) so the regulators are staying enabled (Not the intended functionality). Yes the battery is draining if I load V_SYS with 1K.

Hi David 

Thanks for the additional information.

Still trying to understand the cause of this behavior.  The BQ25176J has a low power SLEEP mode that will turn off the device when VIN < VOUT + VSLEEP.  VIN must be above SLEEP threshold (30mV) for device to be active.  In sleep /PG and STAT pins should be high, even if VIN = VOUT.  

Hi David

I have checked the BQ25176J SLEEP/SHUTDOWN behavior in the lab using your operating conditions on the EVM.   Did not see any problems, unit will power down, /PG and STAT will go High, Vin goes low.

No sign of current flow from output to input, and battery quiescent current is in the 0.35uA range.

How many units are you seeing this behavior on?

Hi Bill, I've seen this behaviour on 2 units. I've done a bit more investigation on this is and have noted that I can get it to operate correctly in 2 ways. 1) Disconnect the power leads from my bench power supply, rather than disabling the output of the power supply. 2) Reducing any loops in the battery leads. (In my setup, I'd cut the positive wire of my battery so I could measure charge/discharge current with a Multimeter.) This makes me think that it's either related to slow fall time on V_SYS due to bulk capacitance from the power supply. (I haven't had a chance to measure it with a scope yet). Alternatively It could be due to noise being induced on the battery leads due to the large loop.

Hi David

If you could capture the slow fall time I could try and duplicate it here, understand it could be difficult.

Hi Bill, here's a scope capture of the power supply during turn off. 

Thanks -- will try this in the lab 

Hi David

I have tested with similar rate of Vin decline in the lab but still cannot duplicate the issue.

To double check, you see STAT pin remain low and current flow from OUT to Vin (about 100mA?).

I am going to look at it more but no success at this point.

Hi Bill, yes the STAT pin remains low. Voltage at the input (V_SYS) decays to battery voltage (see scope screenshot above). Input appears to have a low impedance connection to the battery. I can load the V_SYS with a 1K resistor with no voltage drop.

Hi David

Thanks for the update.

How was the 22nF capacitor feedthrough connected?  In line with the TS pin and grounded to the BQ25176J ground plane? 

Hi Bill, yes connected between the battery NTC and the TS pin. Ground pads connected to the ground plane.