是LTC4006的datasheet,2管脚（CHG），有三种输出状态，
1、the CHG pin is pulled low by aninternal N-channel MOSFET
2、a 25µA current source is connected from the CHG pin to GND
3、a 25µA current source is connected from the CHG pin to GND
三种输出状态与常规说的三态有明显不同。
CHG* (Pin 2): Open-Drain Charge Status Output.
1 When the battery is being charged, the CHG* pin is pulled low by an internal N-channel MOSFET.
充电过程中，直接被拉低----可以是较大的电流
2 When the charge current drops below 10% of programmed current, the N-channel MOSFET turns off and a 25µA current source is connected from the CHG* pin to GND.
充电电流小于预置电流10%时，停止拉低（开关断开）但通过一25µA电流源接到地。意即提供一种新的状态。
3 When the timer runs out or the input supply is removed, the current source will be disconnected and the CHG* pin is forced into a high impedance state. A pull-up resistor is required. The timer function is defeated by forcing this pin below 1V (or connecting it to GND).
切断电源或定时器超时，开关被关闭也没有25uA电流源，引脚呈高阻状态。通过一只上拉电阻，可以使定时功能失效（低于1V或接地）
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