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iPhone backlight circuit repairing

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This video mainly explains how to find the iPhone 8P backlight circuit signals

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and how to find the measurement points

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Step 1, open the component map of the iPhone 8P

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and find the position number of the backlight power supply

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Step 2, search for the position number in the schematic diagram

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Step 3, find the corresponding backlight signals in the circuit diagram

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Step 4, jump to the bitmap according to the pin name to view the connected components

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The voltage and diode value of the signal can be measured at the yellow points in the bitmap

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Let's first open the component map, circuit diagram and bitmap of iPhone 8P

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Find the position number of the backlight power supply in the component map

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The screen of the iPhone 8P is 5.5 inches,

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so there will be two backlight chips in the mainboard

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The search methods of these two backlight chips are the same

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Let's take U5650 as an example to search

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After determining the position number,

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we search for the position number in the schematic diagram

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After finding the backlight chip, find the corresponding signal in the schematic diagram

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Let's first look for the power supply of the backlight power supply

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and the boost inductor of the backlight boost circuit,

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the boost diode and the backlight power supply line

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In the schematic diagram, the D4 pin of U5650 is the power supply pin,

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which is connected to the main power supply

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The main power supply is connected to the composite inductor M5500,

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and after passing through the inductor, it reaches the diode D5650

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M5500 and D5650 are the boost inductor and boost diode used in the backlight boost circuit

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After passing through the diode, it is converted into a backlight power supply

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Jump to the bitmap according to the pin identification,

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the backlight power supply reaches the display seat after passing through the fuse resistor R5730

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We can measure the voltage and diode value of the group of backlight power supply

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on the 39th pin of the display seat

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Next, let's find the turn-on signal of the backlight power supply

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In the schematic diagram, the turn-on signal of U5650 is D3 pin,

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which marks HWEN

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The turn-on signal uses 1.8V power supply to open

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Jump to the bitmap according to the name,

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the power supply will be connected with the power supply of CPU, Wifi and other chips,

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it is not easy to be damaged in actual maintenance

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Let's find the I2C bus of the backlight power supply

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This signal is sent by the CPU to the backlight power supply

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In the schematic diagram, pins B2 and A2 of U5650 are I2C buses

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Jump to the bitmap according to the name,

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we can see that the I2C bus of the backlight is directly connected to the CPU

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This bus will be connected to the bus of other peripherals

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When measuring, we can measure the voltage and diode value at the position of the yellow point in the bitmap

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Another I2C bus is also connected to the CPU

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The yellow position in the bitmap can also measure voltage and diode value

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Let's find the DWI brightness adjustment signal from the CPU to the backlight power supply

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In the schematic diagram, the C2 pin of U5650 is the data of the DWI bus,

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and the C3 pin is the clock of the DWI bus

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Jump to the bitmap according to the name,

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the DWI bus is sent by the CPU and connected to two backlight chips at the same time

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Jump to the bitmap according to the name,

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the DWI bus clock signal is also sent to the two backlight chips by the CPU at the same time

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The DWI bus is rarely damaged during actual maintenance

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If it is damaged, it is usually caused by CPU damage

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Let's find the backlight power frequency adjustment pin, SW pin

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In the schematic diagram, the C4 pin of U5650 is SW1,

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which is mainly used to control the turn-on and cut-off of the backlight boost diode

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Let's take a look at the light-emitting diodes inside the display

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This is the display of the iPhone 11,

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and the internal structure of the LCD screen is basically similar

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After the display is disassembled, we can see that there is an LED light strip at the bottom,

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and there will be two lines on both sides of the light strip

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The connection lines are to provide power and feedback for the light-emitting diodes inside the display

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When the backlight power supply is generated,

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it reaches the light-emitting diode inside the screen through the connection cable,

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and returns to the backlight chip through the feedback line

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When a current flows through the LED,

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the LED will emit light and light up the screen

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Let's take a look at the CAT1 and CAT2 of the backlight chip

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In the schematic diagram, C1 of U5650 is feedback 1,

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and B1 is feedback 2

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Jump to the bitmap according to the name,

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the feedback 1 is connected to the pin 37 of the display seat through the fuse resistor R5731

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Feedback 2 is connected to pin 41 of the display seat through the fuse resistor R5732

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We can judge the quality of the feedback signal by measuring the display seat

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A broken wire in the CAT feedback will lead to a light and dark screen

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If the backlight power supply is not generated, it will cause a dark screen

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In this way, we have searched all the important signals connected to the backlight circuit

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Ok, that's all for this video