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BQ25101: charger chip circuit proposed by TI is confusing.

pragash sangaran78
Genius 3510 points
Part Number: BQ25101

CHG is a dual state. CHG will be low or high impedance. CHG will be Low when the chip is charging and high impedance when the chip stops charging. When it's low, everything looks right. When its charging LED is ON, and the switch is open because CHG is low. Due to this, MCU will be getting voltage to the GPIO.

But when it's not charging but still connected to 5V, CHG is high impedance/open, LED is off, and the switch's gate terminal is open! Since the switch did not toggle to ON, how does microcontroller knows that the charging is already stopped? When the battery is not charging and not connected to 5V, the same scenario happens, I think. Please clarify how does the MCU know when the battery is not charging or stop charging?

  • 0
    TI__Guru* 75906 points

    Hello

    ""But when it's not charging but still connected to 5V, CHG is high impedance/open, LED is off, and the switch's gate terminal is open!"" --During the Hi-Z CHG state Vin voltage will be applied to the switch's gate thru R10 and LED (external mosfet), device will be turned on and MCU-GPIO will be low. Voltage drop thru R10 & LED will be very low due to high impedance mosfet gate (also CHG pin). 

    NOTE -- Max voltage on the CHG pin is 7V while 5V input from the USB can spike higher than this.  Additional protection of the CHG pin is recommend. Another option is to Power the LED from OUT / Battery. 

  • 0 in reply to Bill Johns
    Genius 3510 points

    Could you please explain how else I can protect CHG pin? 

    If I power LED directly from the battery, during CHG = high impedance, how the switch will go to ON? when CHG high impedance, the gate of the switch will see "open" instead of "high" right? So the switch will remain OFF off for both CHG = low and high impedance, and MCU will not know if its finish charging right?

  • 0 in reply to pragash sangaran78
    TI__Guru* 75906 points

    Hello

    "  when CHG high impedance, the gate of the switch will see "open" instead of "high" right?" -- The gate of switch will see "high" in this condition. When CHG is Hi-Z the voltage drop across R10 and LED will be low and gate voltage will be close to V-in, this will turn ON the mosfet / switch.  Gate of mosfet will only need a small amount of current to charge gate capacitance.  This net is high impedance during CHG=Hi-Z so charging the gate is not a problem.  Current does flow thru LED to charge the gate but is low and not illumine.   In this condition MCU-GPIO is Low

    During CHG = low the mosfet gate is low and mosfet / switch is off --- MCU-GPIO signal is High.

    "Could you please explain how else I can protect CHG pin? " -- Need to keep the CHG pin below 7V, a 5V Zener diode would work.  Resistor R10 will limit the current.