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I am in need of your advice on a BMS design. I am designing a circuit which gets powers from battery or USB input (with Power Path management). After searching I found BQ24232 IC (http://www.ti.com/lit/ds/symlink/bq24232.pdf) will work for me.

Here, I need to have an LED to indicate charging complete (Battery Full). The IC has a ~CHG pin which goes to VSS while charging and get high impedance when getting full, according to the datasheet. 

enter image description here

As the datasheet is only giving words on using it to indicate charging status,by connecting LED between output and this pin, I would like to know if this will work if I connect LED between this pin and VSS to indicate battery-full status.  I will appreciate every single possible advice from you all. 

Edit: In summary, I need to understand what does this mean by "~CHG is high impedance when charging is complete". And how can I make use of this pin for Battery-full indicator LED?

Thank you

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  • \$\begingroup\$ ~CHG is high impedance when charging is complete ... here is an equivalent circuit ... imagine a push button switch ... connect one side to GND ... label other side ~CHG ... \$\endgroup\$
    – jsotola
    Dec 23, 2019 at 17:55

2 Answers 2

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You can connect it like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The /CHG pin will sink up to 5mA with less than 400mV drop so it will definitely turn any LED off. Here Vusb is "IN" on the bq2423x (5V power from the USB bus).

The /CHG pin is the drain of an N-channel MOSFET (source is grounded) so it either pulls the output low, or it is turned off and the output can assume any value of output voltage (from 0V up to the supply voltage of the chip).

When the battery is charging the MOSFET is "on" and it shunts the LED, so the current flows through R1 to ground through the chip so the LED remains dark. When charging is complete the MOSFET turns "off", allowing the /CHG output voltage to rise to 2-3V depending on the LED, and the LED illuminates.

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If the pin goes low when it's charging, the LED must be connected to a voltage source above its forward voltage to illuminate in that condition. For example the battery positive terminal, or a regulated voltage in your system that deives from it. If you connect it to VSS it will never illuminate. VSS is ground.

As far as a full indication, the light going off would indicate that. Otherwise, you was ct some kind of logic circuit to take that as an input, and whether the charging power source is present to drive a full indication. You don't want that LED to be in whenever got are not charging or you will kill batteries very efficiently.

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