PF3000 Coin Cell Charger Control in OFF state

[billgessaman]

I'm using the PF3000 in an i.MX7D system that has a LiIon main battery that is replaceable by the end user.  When the end user powers off the product, the PF3000 is in the OFF state and the PF3000 data sheet clearly states this:

   "In the OFF state, coin cell charging is not available as the main battery could be depleted unnecessarily."

I appreciate the attempt by the PF3000 designers to help avoid a system issue that might drain the main battery where the capacity is unknown, but this design choice represents an inflexible side effect that I believe is undesirable.  Without at least some minimal charging current in the OFF state, the connected supercap or LiIon button cell is subject to self-discharge and leakage of the LICELL pin on the PF3000.  In our case, the supercap in our system discharges over a period of about 5 days to about 1.8 volts.  If the main battery is removed after the product is powered OFF for these 5 days or more, then the end user gets surprised that their Real-Time Clock in the i.MX7D is lost and needs to be set again.

Each application differs in the capacity of the main battery used, so to hard wire this behavior into the PMIC can lead to undesirable behavior.  If the PF3000 provided the same reduced charging current of 10uA in the OFF state as it does in the Sleep or Standby states then individual application system software could choose how to manage the COINCHEN bit in the COINCTL register of the PF3000.

1. Are there any options available to avoid this behavior in the PF3000 PMIC?
2. If not, would it be reasonable to hope that future PMIC implementations would take this issue into consideration?

Thanks,

Bill Gessaman

[reyes]

Hi Bill,

I understand your concern, and I perfectly understand why would be needed, in some applications like yours, to have the possibility to configure the PMIC to keep the coin cell charger ON while it is in OFF state.

However, as you properly mentioned the PF3000 is designed to disable coin cell charger if the device goes into OFF state, and unfortunately, this setting cannot be changed in the PF3000, so it is not possible to avoid this behavior in the PF3000 PMIC.

I will send your comments to our applications engineers so they can take it in mind for future PMIC implementation.

Regards,

Jose

[billgessaman]

Hi Jose,

I had another thought about how I might have been able to work around this issue with the PF3000.  In the system, I have an "always on" low-power +3.3V LDO that is not part of the PMIC.  By adding a silicon diode and a current limiting resistor, I can source a very small amount of current (less than 10uA maximum) to the super-cap connected to the LICELL pin and compensate for the self-discharge / leakage effect that happens when the PF3000 is turned off.  This is certainly not as elegant as the constant current charging normally done by the PF3000, but may be sufficient to avoid the unexpected loss of the real-time clock.

Does this solution sound acceptable to you and / or your application engineers?

Thanks,

Bill Gessaman

[reyes]

Hi Bill,

I don’t see any problem by adding the diode and the current limiting resistor, it should work properly for your application.

But I would add a solid-state relay instead to fully isolate the PMIC’s coin cell charger of the always on +3.3V power rail when the PMIC is ON. However, as mentioned, the diode + the current limiting resistor should work fine.

Regards,

Jose

[billgessaman]

Jose,

Thanks for the answer which was pretty much what I was anticipating.  This issue is pretty minor and overall I'm certainly happy with the combination of the PF3000 and i.MX7D, but I will hold on to the hope that a future generation of NXP PMICs will allow more flexibility in how the coin cell charger can be managed.

Thanks,

Bill