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iPhone backlight circuit repairing

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This video mainly explains the working principle of the backlight circuit

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Among Apple models, iPhone 6S to iPhone 8P, iPhone XR, and 11 models all use LCD screens

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There are backlight circuits in the mainboards of these models,

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and their backlight circuits work on the same principle,

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and the backlight chips can be replaced with each other

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Step 1

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When the battery is buckled or the adjustable power supply is connected,

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the main power supply will be generated

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One route of the main power supply will provide working voltage for the backlight power supply

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step 2

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After the main power supply passes through an inductor and a diode,

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it is converted into a backlight power supply and connected to the display connector

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At this time, the backlight power supply and the main power supply voltage are the same,

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about 4V

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To light up the light-emitting diodes inside the display,

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a voltage of 17-22V is required

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At this time, the backlight power supply is 4V,

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so the screen is still in a black state.

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step 3

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When the power-on is triggered,

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the main power supply receives the power-on signal

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and will output more than 30 channels of power supply

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The 1.8V power supply will be connected to the backlight power supply

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to provide a turn-on signal for the backlight power supply

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Step 4

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After the CPU working condition is normal,

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it will detect the backlight power supply through the I2C bus

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Step 5

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When booting to the display circuit,

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the CPU sends a brightness adjustment signal for the backlight power supply through the DWI bus

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The signal is a square wave signal,

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and the square wave signal is divided into the first half cycle and the second half cycle

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Step 6

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When the backlight power supply receives the square wave signal,

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it converts through the internal circuit

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Loaded to the SW pin to output a square wave signal,

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used to control the diode on and off

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During the positive half cycle,

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the main power supply charges the rear capacitor through the inductor and diode

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During the negative half cycle,

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the diode is turned off and the capacitor is discharged

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Because the frequency of the square wave signal is relatively fast,

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when the frequency is very fast,

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the charging and discharging process of the capacitor will be superimposed

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After superposition, the backlight power supply will rise to 17-22V

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step 7

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Backlight power supply connected to the display connector

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When the screen is snapped onto the display connector,

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this power supply is connected to the LEDs inside the screen

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Step 8

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After passing through the light-emitting diodes inside the screen,

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the power supply will loop back to the backlight power supply

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The backlight chip internally controls the grounding of the feedback pin,

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allowing the current to flow through the diode inside the screen

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When current flows, it will emit light,

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and the screen will be lit

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Why are there two channels of CAT feedback?

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Because there are two groups of light-emitting diodes inside the screen,

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one group controls half of the brightness of the screen

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One set of power supply circuits will use CAT1,

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and the LEDs on the other half of the screen will use CAT2

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Therefore, after the CAT is disconnected,

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half of the screen will be bright and half will be off

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In the backlight circuit, the green signal is for other functions of the iPhone,

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and the backlight power supply is required for use

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For example, GSM_BURST is the start signal sent by the baseband

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When the screen of the mobile phone is off and a call comes in,

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the baseband will send a signal to turn on the backlight power supply

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The backlight power supply will work, generate a boost power supply,

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and light up the display

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BL_STROBE_EN is the open signal

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When the front camera takes pictures,

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this signal will be used to control the backlight circuit

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Because the front camera of the iPhone does not have a flash,

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it uses the backlight circuit to work as a flash

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The working principle is explained below

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When the front camera is turned on at night to take pictures,

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the CPU will send a turn-on signal to the backlight power supply

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After the backlight power supply receives the turn-on signal,

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it will control the upper inductor and diode to generate instantaneous high voltage

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At this time, the display screen will be white for an instant,

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and then return to the original state

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This is the flash function

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If the inductor and diode used in this group are not installed,

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it will not affect the backlight circuit of the display

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Ok, that's it for this lesson