Lenovo Thinkpad Helix 2 unknown chip

[IRmska]

Hi, im new on the forum, I hope im posting the question at the right place:

Im trying to repair my 2nd gen Thinkpad Helix, model no. 20CH. The screen didnt work after I reassembled the computer, so I bought a replacement. The replacement didnt work either, so I started to check the board. I was unable to find any kind of schematic, but luckily I was able to find documentation of the screen, which told me what was the pinout for the screen connector.

Screen is receiving Vled in normal range, but it doesnt get the 3,3v for the controller, neither it gets the Venable signal. Both come from the same 3,3v line which is created by what I guess its a PMIC, the ThinkEngine.

That line has its 3,3v present, but then it arrives to a 8 pin chip, 6 of the pins are connected to the same 3,3v line. Of the last 2 pins, one is connected to a resistor, which is in series with a cap and another resistor, those 2 parts are in parallel and connected to ground (it can be seen in the image I`ll attach). The last pin is connected to the fuse which goes to the Ven and Vctr pins of the connector.

I assume that last pin should have 3,3v present, but it doesnt. As the chip has 8 pins, I cant tell what it is, if its a fuse of some kind, or what it does. The chip has F2D written on it, cant find anything about it on the internet, and its identified as U2901 on the board. The board number is 48.4EO31.011, with lenovo FRU 00JT676.

I hope someone can help me, I was thinking on shorting the 3,3v line and the fuse, but as I have no clue about what this chip does I prefer not to risk releasing the magic smoke, not yet  .

Question is: what is this chip? what does it do? can I buy it online?

Thanks for the help, and sorry for any mistakes ive done while writting, english is not my main language.

[TurboTom]

The "chip" is a Toshiba TPCF8004 MOSFET, used to enable power to the display. Check that gate voltage is at least about 3V higher than drain voltage when the display is supposed to be turned on. The "big" white (1206?) component next to this transistor is probably a fuse. You may also want to check that this is not interrupted.

Good luck,
Thomas

[IRmska]

Hi, thanks for the quick reply.

In the datasheet you provided (which was very helpful, thank you ) ive seen that pins 1, 2, 3, 6, 7, 8 are drain pins, and all of them are connected to the 3,3v line that the PMIC outputs. The 4th pin (gate) is the one connected to the resistor which is in series with a parallel cap and resistor (both seem connected to ground).

As for the source pin, that is the one that should be the output, but there is no output. As for actual voltages, drain pins are all at 3,3v, source pin is at 0v.
The fuse is fine.

Could the mosfet be faulty?

[TheSteve]

what is the voltage on the gate pin?

[mariush]

the mosfet turns on when there's a voltage on the gate
then you should get same or almost same voltage on drain and source .. there's a tiny drop due to rds(on) resistance.


The signal probably comes through a via to the right of that ceramic capacitor above that mosfet, goes through the tiny black part which is probably some kind of inductor or something, and turns on the gate of the mosfet as needed.
The cream / light brown parts are capacitors, you should check them with meter in continuity mode and you should not get continuity between their terminals.
The white/ceramic like surface mount parts are fuses, you have two there, and you can tell they're fuses also by the fact they're soldered to wide thick traces which carry power, not signals.
There's a fuse to the left of the connector and that one to the left of mosfet - those should read continuity when tested with a multimeter, unless they're blown.
Also check the manual to see if there was a component installed to the right of that fuse on the left side of the connector - there's solder but that could have been pre-applied, or a component broke off the board.

[TurboTom]

Gate drive of this Mosfet is somewhat tricky. Since there is always a gate threshold voltage in "enhancement" type mosfets, an N-channel device like this require a gate-source voltage of round about 3V more positive than the source terminal. Hence, the gate cannot be driven from a 3.3V logic output if the mosfet is used in common-drain configuration. Here the designer utilized a charge pump: In order to lift the gate high, the drive pin has to be toggled at a certain frequency. This AC signal is fed through the slightly darker brown capacitor to the cathode of a diode. The anode of this diode is connected to source of the mosfet. During the high-level intervals of the drive signal, this rises the cathode of the diode approximately 3V above source potential. Now, all that takes place is some smoothing via an RC-low-pass with one of the small resistors and the larger, slightly lighter brown cap. The other resistor is probably meant to discharge the RC filter so the mosfet turns off when the AC drive is disabled. There seems to be a tiny test point at the "driven end" of the charge pump cap. You should be able to observe a (squar"ish") frequency signal there when the display is supposed to be enabled. If that's not there, then the problem has to be searched elsewhere.

Just to test the tablet, you can also temporarily bridge drain and source of the mosfet to permanently enable the power supply to the display. This will allow you to find out if the tablet is working otherwise.

[IRmska]

Ive got sad news.. Something else broke while I was trying to probe the mosfet, the screen lit up for a short period when I turned on the tablet, in which I could measure 13vdc at the gate pin, this reading could be wrong, as TurboTom says (if I understood correctly) it should be an AC signal, I was measuring DC, so the reading may be wrong.

After this event, the tablet wouldnt do the beeping on post, which it did prior to the event. The beeps were my way of knowing that the laptop/tablet was still alive, it just didnt feed the screen controller so the screen didnt lit up. As I was saying, now there are no beeps, and the gpu gets over 70ºC instantly when I press the power button. The whole board heats up, but thats the component that heats up the most.

At the moment I dont really know what happened, no magic smoke was released so I lack a main culprit. My guess is that the gpu is fried, but it could be anything actually. I have basic knowledge about electronics (im studying a degree about it, so no deep understanding for some time to come), and for now I think there is no solution for this board. The shitty part is that I guess it was my fault, but I just dont know where did I make the mistake.


Anyways, I wanted to thank you for the help you provided me, especially TurboTom, I really didnt expect somebody to reply with the actual datasheet of the part ive been looking for a week in just 15 minutes, that was amazing. I will stay around here, and maybe I can help someone someday

Also, thanks for the explanations, I didnt expect a mosfet to be driven with an AC signal, but it makes sense, besides, you really helped me understand how the circuit works.

[fzabkar]

This is one of my go-to part marking search sites:

http://www.smdmark.com/en-US/search/code?id=f2d