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This lesson talks about the battery detection circuit of Xiaomi mobile phone

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Here is the battery holder

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The battery holder is responsible for the power detection signal on the third and fourth pins,

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S_IO_J2, and BATT_B+_J2

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These two lines are connected to the switch tube Q5408

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After the switch tube is turned on, the output from the first and fourth pins

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It becomes the I2C bus to detect the battery

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This I2C bus has a pull-up circuit,

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and one end of the pull-up circuit is connected to a 1.8-volt power supply

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Provide a voltage of 1.8 volts to this I2C bus signal through resistor R3202

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In this way, the I2C bus can communicate with the CPU normally

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This I2C bus is also leading to the charging chip

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The steps of its work, first of all,

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there is no abnormality in the pull-up circuit,

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that is, there is no abnormality in the resistance,

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and there is no abnormality in the power supply

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There is 1.8 volts on these two wires

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At this time, the voltage of 1.8 volts can reach here,

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and the high level of 1.8 volts makes the MOS tube turn on

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This is an integrated MOS tube, that is, a composite MOS tube

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The MOS tube must be normally turned on

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During maintenance, if there is 1.8 volts here, there is no 1.8 volts on pin 6 and pin 3 of the MOS tube

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At this time, we can remove the MOS tube, short-circuit its pin 1 and pin 6, and pin 3 and pin 4,

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that is, short-circuit the D and S poles of the MOS tube, and the problem can be eliminated

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Then there will be a situation in the maintenance,

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the right pin of the MOS tube Q5408 does not have 1.8 volts

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We need to test VREG S4 for 1.8 volts

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V_REG_S4 is generally used to power the power-on circuit

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The phone can be turned on, indicating that the voltage is no problem

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The voltage of 1.8 volts here is normal, but there is no voltage here.

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Generally, it is caused by the failure of these two pull-up resistors.

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You only need to replace these two resistors.

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This is Xiaomi's battery detection circuit

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That's all for this lesson