Asus N550LF power management circuit failed (TI BQ24735 QFN20)

Discussion in 'Technical Repair' started by BlackOps, Jul 17, 2016.

  1. BlackOps

    Thread Starter Member

    May 23, 2016
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    Hi, first off, the reason i post this on this forum instead of tomsguide is because, i can't get any hardware related auestions there when it comes down to micro and IC's. So i got an Asus N550LF. It doesn't power on. When i plug in an fully charged battery, the board makes an slightly high pitch noise. Its near the 1E28 capacitor. The 2R2 coils are making it.the 1E28 capacitor is heating up very slightly. Coils to. Now, the BQ24735 TI charging IC failed. So i soldered an wire between Vcc (Pin1) to ACDRV (Pin4), so the board is receiving the required power to turn on. But it still makes the buzzing noise. Note, the capacitor on the Vcc line of the IC is missing. Don't care about that, its just a noise filter for the IC and since its dead its just doing nothing.

    But why is it making a buzzing noise? A dead mosfet? How do i test a fet here? I got an DMM but i couldn't find any schematics and boardviews for this laptop. Btw i don't care if this board ends up with releasing magic smoke. I got this laptop for free (long story, from a good friend) and i'm planning on getting a new PCB anyways. But i want this PCB to operate at least if possible.

    Any help is higly appriciated
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  3. BlackOps

    Thread Starter Member

    May 23, 2016
    32
    0
    Hi, yea i already know that but i want to troubleshoot the computer while power is going trough it and i was hoping if anyone was familiar with an BQ24735 laptop charging circuit. But the main question is why it makes an high pitch noise, what could it be? What is the part, that makes the inductor whine? And i want to know what the purpose of the L3533 IC is. If i touch it, the tone of the noise lowers and rattles very precisely.

    And the L3533 is getting slightly hot, hotter then it should when its off.

    Thanks anyways.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    The power supply circuitry in laptops is invariably switch-mode to get best efficiency. They use high speed switching - too high to hear. Yours is faulty so it may be running at the wrong frequency, pulsing on and off, or just unstable and making noises you can hear. Without a schematic and an oscilloscope the rest is guesswork.
     
  5. BlackOps

    Thread Starter Member

    May 23, 2016
    32
    0
    Hi, thanks for your response. So from what i get, is that this PCB is trash? It has some very expensive and working chips on there like the i7 4500U and an uncommon north bridge. But yea they are BGA mounted.

    But back to the point, is there no way i could bypass this entire circuit? I don't care about efficiency and the power supply can supply more then enough power to power it on.

    With bypass, i mean 2 wires directly to the VRM or the PMIC.

    And btw, i will get an new board anyways, regardless of the condition of this board. Even when it works. But i wan't to get this to work and there must be a way without an schematic.

    Still curious what the L3533 is doing in this laptop as the tone is lowering when touched (with related passives).

    http://www.anpec.com.tw/ashx_prod_f...021185525322.pdf&original_name=APL3533QBI.pdf
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    That chip is probably switching between battery and mains power.
    Laptops go to a lot of trouble with power management. This kind of 'bypass' likely wouldn't be simple even with a schematic and knowledge of the power management system.
    I think that's a 'no'. :(
     
    BlackOps likes this.
  7. BlackOps

    Thread Starter Member

    May 23, 2016
    32
    0
    Yea, i think i will sell it for parts. I mean its not worth it. Have been working on it since march this year and it became worse. I will sell the CPU and such.

    I'm planning on getting a new board anyways.

    However thanks for your assistance!
     
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