Switch with no backlight, no enable signal at the IC

Hi, I have a switch which has a working display (no backlight) but can be seen with a torch and that works fine when docked. It came to me water damaged and not working so have replaced quite a bit now and this appears to be my last stumbling block.

On a working switch I have 0.3v at the enable pin but this one barely has 0.1v at enable for the LCD. I cant for the life of me work out what is generating the enable signal for the backlight. Can anybody help?

I used the data sheet tps61163a.pdf and Ive swapped the IC, swapped the connector and am testing on a known good LCD. I’m guessing the issue is likely whatever is generating the enable signal.

Any help would be appreciated ive been working on this damn thing on and off all day.

OK think ive been reading the thing wrong and ive got the pinout wrong. I think I might need to look at it again and get my pins right, either way im not getting the 0.3v on pin 2 at the LCD. Ill update once I have had a look again tomorrow but if anybody has any tips on working this one out.

Verify the 8316 IC first, search the forum as I’ve covered this quite a few times before.

Nice one I will have a look - Found a few posts on it so will look them over and report back. Thanks for taking the time dude.

So I did some measuring and I can confirm I get +5v and also -5v - I can also measure a 3.3v here. The LCD itself I can make out a picture but that backlight is still alluding me. I tried to measure it during boot and it looks like the backlight circuit tries to boost for a split second and then drops back off. I am fairly sure that the 8316 is doing what it should be based on what im reading in other posts, unless of course I am missing something which is very possible.

Should probably also say that I can measure 1.8v at the enable and 1v which i think is vref

Yeah seems fine.

I would check the backlight ribbon itself, it’s not uncommon in liquid damage cases for the ribbon contacts to break (at a fold) or corrode away. If it’s hard to tell you can sometimes see the break by shining a torch through the ribbon, or you can scrape the trace further down and check continuity to the main contacts on the ribbon. Of course the other way is just to try another board in the assembly and see if the backlight works to confirm.

As for the backlight driver IC, was this a new preballed IC or from a donor?

Failing the above, then looking at the backlight IC datasheet, this paragraph is where I’d be drawing my attention

In order to enable the device from shutdown mode, three conditions have to be met:

  1. POR (Power On Reset, that is, VIN voltage is higher than UVLO threshold);
  2. Logic high on EN pin; and
  3. PWM signal (logic high or PWM pulses) on PWM pin.
    When these conditions are all met, an internal LDO linear regulator is enabled to provide supply to internal
    circuits and the device can start up

I would imagine 0.3V is too low to constitute a logical high (though I can’t seem to find this info specifically in the datasheet) but my guess is it would be somewhere in the region of 0.8 to 1.8V - It’s been a while since I worked on a switch backlight related issue so I don’t remember tbh off the top of my head, but I imagine that enable line is pulled up at boot for a few hundred MS, so you may have to put your meter in it’s min/max mode to catch it while prompting the console to boot - so these would be the thing’s I check next :slight_smile:

Im using a brand new LCD - Tested working on another switch. In terms of the chip ive tried perhaps 4 or 5 that I reballed from donors. Then I removed one from a known working board, slapped it on this board, it didnt work. Put it back on the known good switch and it works a treat. So I know its not the IC. It’s something else. I am going to have a really close look at those “working conditions” and see if I can make anything out. I have put it to one side for now because im fairly sure it will be this that genuinely sends me mad.

For what its worth the 0.3v isn’t the enable its some sort of drain so far as I can tell from the pinout. I have checked and you are right 1.8 v is enable on backlight IC on switch. I have check continuity between the backlight connector to the IC and can pick up all 3 pins that are active back at the IC and of course the 4th is ground. Everything looks good… its something else but those power on conditions may very well be the answer and lead me to something.

I see, rules that out then :frowning:

Haha no worries I understand

That’s good, With the LCD connected and following prompting the console to boot, what voltage do you measure directly at the backlight connector pins? (be very careful with your probes in this area) I’m guessing you get the approx 4V but not boosted (I forget what it is but somewhere in the region of >15V if i remember right)

As LCD ribbon needs to be connected for backlight boost to be enabled (if I’m remembering right) then I suppose it’s possible you have an open line at the LCD connector somewhere, though this is somewhat unlikely given that your getting image produced (which implies most if not all pins are good) but you never know. I don’t know what line/s is specifically required at the LCD connector to enable backlight boost unfortunately (sorry, haven’t got round to tracing out the backlight circuit) so can’t let you know which pin specifically to check so might have to be the lot, might also be worth checking the filters (if they’re present on your PCB version) by the LCD connector

That’s good, With the LCD connected and following prompting the console to boot, what voltage do you measure directly at the backlight connector pins? (be very careful with your probes in this area) I’m guessing you get the approx 4V but not boosted (I forget what it is but somewhere in the region of >15V if i remember right)

So on a good working board ive got left to right on connector, gnd, 0.3v, backlight voltage (think it was 17v or 19v), 0v on the last pin.

On this board I get gnd, 0v, 4.2/battery voltage, 0v but it boots fine.

As LCD ribbon needs to be connected for backlight boost to be enabled (if I’m remembering right) then I suppose it’s possible you have an open line at the LCD connector somewhere, though this is somewhat unlikely given that your getting image produced (which implies most if not all pins are good) but you never know.

Ill check the LCD connector or even swap it for one of my good donors just to check. This is a cpu-20 board which I had to replace the whole 77621? PMIC circuit to the left of the CPU as that was all rusted out. I replaced the whole backlight circuit just in case. I’ve even had to remove the shield from around the wifi because it was literally rusted out and the one around the cpu is also rusted out but still just to get it this far is a personal achievement if nothing else. So far its only got one silly jumper :rofl:

I don’t know what line/s is specifically required at the LCD connector to enable backlight boost unfortunately (sorry, haven’t got round to tracing out the backlight circuit) so can’t let you know which pin specifically to check so might have to be the lot, might also be worth checking the filters (if they’re present on your PCB version) by the LCD connector

No filters present on this one but ill get onto the LCD fpc connector, if it’s that after this mission ill be a happy chap. Thanks for keeping me sane dude appreciate it.

Also one day ill try and work out properly how to use the quotes in here.

1 Like