BQ25125: Reverse current and MR pin protection

Florian Zierer

Part Number: BQ25125

Tool/software:

We are currently designing a layout using the BQ25125 to charge a Li-Ion battery and power our product with 1.8V. We have some questions regarding pin protection.

The IN, GND, and /MR pins are accessible from outside our product to connect a charging cable that includes an integrated pushbutton (on /MR) to GND.

What would happen if GND and IN are unintentionally inverted by the user, resulting in 5V being applied to GND and GND to the IN pin? Is there built-in reverse current protection on the power supply side or only on the battery side? I did not find any information on this in the FAQ document or the datasheet. The only mention is regarding reverse current from BAT to IN.
Regarding the /MR pin, the absolute maximum ratings indicate 6.6V for "BAT Operating Voltage - VBAT, MR." Section 9.3.22 of the datasheet states that "The MR input has an internal pull-up to BAT." What would happen if 5V is externally connected to the /MR pin, considering we are using a Li-Ion battery with a voltage range of 3.7-4.2V?

Attached, is our schematics for the charger. Is there anything we should add to protect against the scenarios described above? Do you see any other issues with our schematics?

electronic-Power Management.pdf

3 months ago

0 Juan Ospina 3 months ago

TI__Genius 13785 points

Hi,

Florian Zierer said:
What would happen if GND and IN are unintentionally inverted by the user, resulting in 5V being applied to GND and GND to the IN pin? Is there built-in reverse current protection on the power supply side or only on the battery side? I did not find any information on this in the FAQ document or the datasheet. The only mention is regarding reverse current from BAT to IN.

This would be essentially applying -5V to the VIN pin, which exceeds the minimum voltage in the Absolute Maximum. This risks device damage and is not recommended.

Florian Zierer said:
Regarding the /MR pin, the absolute maximum ratings indicate 6.6V for "BAT Operating Voltage - VBAT, MR." Section 9.3.22 of the datasheet states that "The MR input has an internal pull-up to BAT." What would happen if 5V is externally connected to the /MR pin, considering we are using a Li-Ion battery with a voltage range of 3.7-4.2V?

Due to the internal pull-up to BAT, it is always recommended that this pin be taken into consideration as an alternate path to BAT. Applying a 5V voltage to /MR while the battery is less than that can result in unregulated charging. Similarly, if a voltage less than battery voltage is applied, this will pull some current from the battery as well. Best practice is to leave the MR floating and only apply a pull-down when it needs to be pressed.

Florian Zierer said:
Is there anything we should add to protect against the scenarios described above? Do you see any other issues with our schematics?

A protection diode for the reverse current being applied might be helpful in protecting against -5V on VIN. For /MR, its best not to apply an external voltage, rather just having a pull-down circuit.

Your schematics look good, although I can't tell the size of the PMID Cap. Additionally, some of the digital signals (I2C, /PG, /CD) require a pull-up, though I imagine those resistors aren't pictured here.

Best Regards,

Juan Ospina

0 Florian Zierer 3 months ago in reply to Juan Ospina

Prodigy 50 points

Hi Juan,

what would be a good solution to protect VIN/GND? When using a diode with ~0.7V voltage drop (or a schottky with 0.3V) and 5V Vin, would the remaining 4.3V/4.7V be enough to reliably charge a LI-ION battery (3.7-4.2V) or would a P-Channel reverse protection be better?

And how can I protect the /MR pin? A diode would not work (/MR needs < 0.3V and the forward voltage on the diode is higher). Do you have a suggestion? Unfortunately I can't protect it mechanically so it needs to be an electrical solution.

Thanks for the hint to the PMID cap. I set it to 4.7µF.
The I2C lines have 10kR pull ups to 1.8V not visible here and /CD is connected to a MCU pin with internal pullups.

Best,

Florian

0 Juan Ospina 3 months ago in reply to Florian Zierer

TI__Genius 13785 points

Hi Florian,

Florian Zierer said:
would the remaining 4.3V/4.7V be enough to reliably charge a LI-ION battery (3.7-4.2V)

4.7V from a schottkey diode would certainly be enough to continue charging the battery, 4.3V does run into dropout voltage issues needed for fully charging up to 4.2V so I don't recommend that. A P-Channel reverse protection would also work in this scenario.

Florian Zierer said:
And how can I protect the /MR pin?

Typically, for circumstances where a signal to control the /MR can't be connected directly to the /MR due to unintended current flow, the signal would be used to drive a FET to pull down the /MR pin when needed. Would that work for your application?