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The relevant circuit of the trigger small current is shown in the block diagram

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To repair the faults of no booting caused by trigger small current,

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we need to check the circuit diagram in the actual maintenance

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Let's take a look at how the circuit diagrams of different brands of mobile phones express the working principle

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We learned earlier that the direct circuit of the trigger small current fault

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is the power supply chip and the power supply provided by the power supply chip,

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the clock signal, the reset signal, the maintain signal,

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and the data communication

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Then let's take a look

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We open Xinzhizao intelligent maintenance system,

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For example, the Xiaomi 10 mobile phone,

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open its mainboard schematic diagram,

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let's see where the circuit related to the trigger small current is

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Open the component map,

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we have to find its power supply first

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The U4700 here is a small power supply

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The U4200 here is the main power chip

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We search for U4200 in the schematic diagram

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It automatically searched for U4200_2,

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which is the second part of the description of the chip U4200

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The model of this chip is PM8250

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But this part is not the description of the main power supply

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We see that there are some boot signals called PHONE_ON_N

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There are also reset signals, maintain signals

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This part is not the power supply part,

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we have to find its power supply part first

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The U4200_4 is the fourth part

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How do we understand this diagram?

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To its left is the power supply input, VPH_PWR,

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which is the main power supply

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The main power supply is supplied to this power chip

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When this power supply chip meets the conditions,

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such as pin 19 and pin 5,

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it outputs a power supply called VREG_S1A_S2A_S3A_0P75 through the energy storage inductor,

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and S means switch

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Is this the CPU power supply?

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We can double click on it and it jumps to page 38

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There is a chip here, U3100,

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so what kind of chip is it?

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SM8250, this is Qualcomm's CPU model,

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its power supply is supplied to the P33 pin of the CPU

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Come back here again

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The power output of this power supply is VREG_S4A_1P8

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1P8 means that the voltage is 1.8V

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Is this 1.8V power supply supplied to the CPU?

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Of course, it gives U4700_7

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Its model number is PM8150A, a small power supply

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We have to find out if it is going to the CPU

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If this power supply goes to the CPU,

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it means that it's the power supply of the CPU

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We searched until we found the CPU,

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which leads to a lot of chips

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Such as audio chip,

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this is a page of the audio chip description

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It leads to a small audio, leads to a hall sensor, and leads to a seat

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Here, it leads to a wifi chip,

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we find it here,

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SM8250 is the CPU

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This VREG_S4A_1P8 is the line just now

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It comes from a large power supply and

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comes through a zero-ohm resistor to the A12 and A29 pins of the CPU

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It is also the power supply of the CPU

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We're going back to where it outputs power,

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so we're going to hurry up here

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That is, the power output from the main power supply of the U4200

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By looking at the line,

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it is actually the power supply of the CPU

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Is the VREG_S5A_1P9 powered by the CPU?

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Here is a method for your reference

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When we see that a power supply starts from the power supply and is finally connected to the CPU,

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then this power supply can be regarded as the power supply of the CPU

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It's just such simple

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Well, we see it coming to the small power supply U4200

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It arrives at U2400 after going to many places

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That's not what we're looking for

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Let's see if it reaches the CPU

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After running around, we found VREG_S5A_1P9,

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it didn't reach the CPU,

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so it is not the power supply of the CPU

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Okay, that's it for this lesson