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I want to build a 3-cell battery charger for LiPo batteries using gas-gauge-bq20z65-r1 and bq24753A battery charger IC. But I am having problem understanding how the bq20z65-r1 operates. http://i.stack.imgur.com/wZbaF.png If the adaptor is connected, current should flow from Pack+ to BAT+. If not connected, current should flow from BAT+ to Pack+ via the FETs. Pack+ and Pack- are connected to bq24753A IC which is connected to the adapter and the system.

  1. How is bq20z65-r1 getting powered on? The IC gets power from either battery or Pack+, via Vcc. But since Q2 and Q4 are n-channel FETs, Vgs is low and FETs are off. So the IC cannot be powered on. And to switch on the FETs the IC needs to be on.

  2. Could you also explain me how does the current flows through the FETs. Because Q2 and Q4 are in opposite direction. And as I have understood the FETs allow current to flow in only one direction, don't they?

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The datasheet says this about the "PACK" (2) pin:

Battery pack input voltage sense input. It also serves as device wake up when the device is in shutdown mode

It is also marked as "analog input" and "power".

So, when you connect the charger, the chip gets power from the PACK pin then it turns on the relevant FET. After that it stays on until the batteries discharge completely (after that you need to connect the charger to turn the chip back on).

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  • \$\begingroup\$ Thanks. Could you also explain me how does the current flows through the FETs. Because Q2 and Q4 are in opposite direction. And as I have understood the FETs allow current to flow in only one direction, don't they? \$\endgroup\$ – zud Jun 6 '13 at 14:34

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