bq24086 stability and dropout voltage with remote battery

Hello Jeremy,

What exactly are you trying to protect the battery from? You are concerned about things such as reverse polarity you could use something like a P-FET which would be more efficient and then you would not have to be as worried about stability. I would place the BAT connection on the pack side of the FET to improve the measurement accuracy so that you are not losing any of you capacity by including the P-FET resistance in your battery voltage for charge termination.

Regarding the voltage drop you could gain a ball park by using the charging current of 0.75A resulting in a voltage drop of 600mV which would give a FET resistance of about 0.8 ohms which would give you an approximation of about 100mV.

Let me know if you have any questions.

Hi Ryan,

The battery needs to be protected from accidental discharge when the terminals are shorted, as well as reverse polarity. The terminals are only used for charging as the active circuit is contained with the battery in a small portable item (a similar operational model is a DECT phone, but this has much lower power consumption). The battery is only 120mAh, so efficiency is not a big issue. Charging current is less than 100mA. The cost (to the nearest cent!) is crucial and the size is also important.

If I used a PFET I would need two back-to-back to prevent discharge. I chose a Schottky because it is is cheaper and smaller.

Jeremy

Hello Jeremy,

I had another thought. What about placing an NPN BJT on the low side of the battery between the charger minus and battery minus.

Tie the base to battery + through a resistor. Emitter to charger minus. Collector to battery minus.

When there is a positive potential on the charging leads the BJT is placed in saturation and when the charger is disconnected the inherent opposing PN junctions within the device eliminate reverse conduction.

This method also addresses the revers polarity issue.

Ryan,

I tested this. The transistor seems to be permanently on. A MOSFET seems a better idea to me. Any suggestions?

Jeremy

Hello Jeremy,

I have been working with a project that needs to have bi direction protection which will inherently provide short circuit protection. It uses two PFETs in common source configuration. While you had expressed an interest in avoiding additional components if you are wanting such comprehensive protection I can not think of any other way.

http://www.ti.com/lit/an/slva139/slva139.pdf

What I am thinking you could do would be to configure using the PFET design but instead of using a single PFET you could use a single package common source such as the CSD75207W15 which offers a small solution or two independent PFETs. While this is bringing the brainstorming back to beginning I think that will be the simplest solution.

On a different note or line of thinking:

Regarding short circuit protection what sort of connector are you using? Is it possible to mitigate the short risk with the mechanical connector design?

Ryan,

Thanks. The dual PFET is the same idea we had here. I will investigate further.

I have found the NPN is biased by the battery voltage across the base resistor and the b-c junction, causing a constant current drain which is unacceptable.

Regarding protection, the input connector is a SIM contact pad configuration, so the risk of s/c is quite high. Hence the need to prevent battery discharge by accidental contact with a metal object.

Jeremy

Hello Jeremy,

Thank you for clarifying the mechanical limitations and risks you are encountering. I will keep looking for other options but I think two sub $0.10 FETs will be hard to beat.

Let me know if you come up with any ideas that you want to bounce around.