LiPo battery charger query

[electrophile]

I'm trying to build a simple battery charger based on Microchip's MCP73831. I could build the schematic as it is which is very simple (as shown here).



The STAT pin can be connected to an LED which shows the charge state (as shown in the table).



With this schematic only the 'Charge in progress' status is shown by the device and when the charge is complete I'm guessing the LED goes off. How can I add another LED which shows that the charge is complete? I tried designing another circuit but I'm not sure if this works (shown here as well). Any ideas anyone?

 

[nsypid]

Hi,

at the last schematic that you show, if either input power or battery not present, the stat pin will go to high impedance meaning that both leds will turn on assuming the forward voltage of the led is 0.7V . you could add a zener in series with the input power like shown in my figure to force both leds to turn off if stat pin is high impedance. So if charge in progress, the top led will turn on, charge complete bottom led will turn on, no battery inserted or no input power, both led will turn off. You have to calculate again the voltage of the zener if the led forward voltage is not 0.7V

 

[electrophile]

Thanks nsypid! How did you calculate the zener voltage to be 2V? The forward voltage of my LEDs is about 2V.

 

[nsypid]

The zener voltage at least has to be bigger than half the input power (2.5V) - led forward voltage and smaller than input power - led forward voltage.

So that if stat pin is high impedance, 5V- 2x2V(LED) = 4V. So 1V left. if zenervoltage > 0.5V, not enough voltage to turn on both leds. And if stat pin high or low, 5V - 2V(LED) = 3V. if zenervoltage < 3V the voltage will be enough to turn led on.

1V zener is enough in this case.

 

[electrophile]

Awesome thanks!