Hello,
I'm planning to use a bq24250 as battery charger in a handheld device. I use a single cell Li-Ion battery (3.7V)
I see that there is an interesting command to "self kill" the system leaving the battery with almost zero current drain. I'm talking about the SYSOFF bit that i would like to set when i need to power-off the device (MCU as well)
The question now is: how can i do to put back the system on again? I can't ask final user "to turn on the system again, put in the usb cable :)"
My idea is to use the circuit below to connect Vbatt to Vin when i push the power button. Since VUVLO = 3.5V max, if the battery is not totally death the chip will recognize a valid Vin and wakes up the system. right?
Here a brief explanation of the circuit: first of all, I use 2 NMos (M1,M2) in antiseries to null the effect of body diode. Then a resistor Rb pull the MOS gates (Vb) to the same voltage of the sources (Va) so M1,M2 are off. So normally when the button is open and after the capacitors are charged, Vin is disconnected from Vbatt and the circuit does nothing. In this condition i can kill the power of MCU by setting SYSOFF of the bq24250.
Now the MCU power has gone and after some time I need to power the system on. So i press the button: Point "d" go to Vbatt. C2 (that was charged at Vbatt by Rpd and R2) work as a voltage booster and make the point "c" to go at 2*Vbatt. Cb bypass this impulse, so also point "b" goes to 2*Vbatt. So the two MOS are turned on and the chip now see Vbatt at the Vin pin, so it wake up.
The impulse lenght are determined by the RC value uf the varius capacitor.
What does M3 (and R1,C1)? Well... when someone put an external supply to Vin, I need to avoid the connection of Vin with Vbatt. So when Vin goes up, after a certain time that depends on the R1,C1 values, M3 inhibits the turn on of M1, M2 by connecting the two gates to ground.
I need a time interval (R1,C1) before the inhibit action in order to leave the "power on pulse" to pass.
Am I right? Is there a best way to reach my purpose?
Maybe /CE can be used to reset the SYSOFF condition?
Gianluca