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I was working on a design of Li-ion battery charger using LTC4001. Here a pin is called CHRG, which indicates the different state of charging like charging cycle begins, threshold reach(i.e 4.2V or C/10), Fully Charged. I'm bit confused with the operation of this circuit. So can anyone help with this circuit? enter image description here

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  • \$\begingroup\$ What is it exactly you are confused about? How the pin works? Or how the LEDs are working? \$\endgroup\$ – MCG Sep 5 '18 at 12:50
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From the datasheet:

When a charge cycle starts, the CHRG- pin is pulled to ground by an internal N-channel MOSFET which is capable of driving an LED. When the charge current drops below the end-of-charge (IDET) threshold for at least 4ms, and the battery voltage is close to the float voltage, the N-channel MOSFET turns off and a weak 30µA current source to ground is connected to the CHRG- pin. This weak pull-down remains until the charge cycle ends. After charging ends, the pin will become high impedance.

It isn't high/low/hi-Z, it's strong pulldown, weak pulldown and hi-Z. So,

  • When the strong pulldown is enabled, Q1 and Q2 are turned on, but Q3 is cut off. Only D2 lights up
  • When the 30 µA pulldown is enabled, there's about 1.46 V across the BE junction of Q1 plus R1. Q1 is still on, but Q2 is cut off. Q3 turns on as well. Only D1 lights up.
  • when the CHRG- output is high-impedance, Q1 and Q2 are cut off. Q3 is on, but both D1 and D2 are off.
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