iPhone backlight circuit operation question

[caphair]

Re: iPhone backlight circuit operation question

Why would Apple use an extra inductor and diode?
One inductor is 1uH/3.6A and the other 15uH/0.72A, so a high power and low power mode.

Boost converters have high peak input currents, even if the load is small.
I would say there's no room in an iPhone for a big capacitor to absorb those spikes and they switch over to the bigger inductor to lower electrical noise.
During a call, you want a low noise on power for cell comms. Also for lowering EMI with dual-frequency or multi-phase between the two boost converter outputs. But I don't see the one inductor capable of anything but low brightness.

It also looks like they are turning off the LED boost converter during a GSM TX (packet) burst, to save available power. Long range GSM transmit power can take almost 2A.

Is it in theory working like a power factor correction? Or simply to compensate for high current demand when needed?

[Stefan Payne]

Re: iPhone backlight circuit operation question

I've seen lots of videos on iPhone repairs. But I feel a lot of the people don't have clear understanding of electronics theory and circuit operation. They're mostly checking for shorts and replacing components. I'm sure they know basic stuff but I'm trying to understand circuit operation

I think I've heard about him. Don't know when or why, but that's a rather good thing.

But yeah, most people don't know what they're doing, maybe you'd like to watch something from "Grandma" as Louis Rossmann likes to call her:
https://www.youtube.com/channel/UCPj...e1o_lp1US9TpWA

[caphair]

Re: iPhone backlight circuit operation question

Why would Apple use an extra inductor and diode?
One inductor is 1uH/3.6A and the other 15uH/0.72A, so a high power and low power mode.

Boost converters have high peak input currents, even if the load is small.
I would say there's no room in an iPhone for a big capacitor to absorb those spikes and they switch over to the bigger inductor to lower electrical noise.
During a call, you want a low noise on power for cell comms. Also for lowering EMI with dual-frequency or multi-phase between the two boost converter outputs. But I don't see the one inductor capable of anything but low brightness.

It also looks like they are turning off the LED boost converter during a GSM TX (packet) burst, to save available power. Long range GSM transmit power can take almost 2A.

Is the 15uh inductor responsible for the high power?

When you say it's turning off boost during a gsm tx, does that only happen when the screen is off? If not how does the backlight stay lit if that's occurring

How does it know when to switch to the other inductor to lower noise?

[redwire]

Re: iPhone backlight circuit operation question

Why would Apple use an extra inductor and diode?
One inductor is 1uH/3.6A and the other 15uH/0.72A, so a high power and low power mode.

Boost converters have high peak input currents, even if the load is small.
I would say there's no room in an iPhone for a big capacitor to absorb those spikes and they switch over to the bigger inductor to lower electrical noise.
During a call, you want a low noise on power for cell comms. Also for lowering EMI with dual-frequency or multi-phase between the two boost converter outputs. But I don't see the one inductor capable of anything but low brightness.

It also looks like they are turning off the LED boost converter during a GSM TX (packet) burst, to save available power. Long range GSM transmit power can take almost 2A.

[caphair]

Re: iPhone backlight circuit operation question

It looks like both inductors are not driven at the same time, one is when high current demand is needed and one for lower current demanded.

That's my guess but the earlier iPhone 6 screen used same size screen so my curiosity is why add the dual coils. Unless the 6s is brighter.

[caphair]

Re: iPhone backlight circuit operation question

From what I've seen, there are dual circuits only in the Plus models, probably due to the larger screen.

Could this have anything to do with having two coils?:

If I'm understanding that circuit correctly, its to eliminate any start up voltage hiccup from boosting and produce smooth output?

[budm]

Re: iPhone backlight circuit operation question

You can see one of the signal line has the word 'BURST', may be it has something to do with that. You also check the 'SWx' outputs to see what is going on. One inductor is rated at 3.6A, another one is rated at 0.72 A.
No luck so far for the spec sheet of the IC.

[Hellfiya]

Re: iPhone backlight circuit operation question

Right but why are the coils two different kinds? At first I thought two of everything for one half of the screen for maybe two led strips, but when noticed coils are different specs it got me wondering

I must've been editing my previous comment whenever you responded, but check that link. It looks fairly similar to the iPhone's backlight circuit. Looks like it has something to do with Electromagnetic interference?

Oh, and this may be off topic but sometimes after I replace a screen on an iphone, the backlight flickers, even if I unplug it and plug it back in. But it always subsides over a few hours. Not sure if that has anything to do with EMI. It has happened maybe 5 or 6 times out of about 400 screens.

[caphair]

Re: iPhone backlight circuit operation question

From what I've seen, there are dual circuits only in the Plus models, probably due to the larger screen.

Right but why are the coils two different kinds? At first I thought two of everything for one half of the screen for maybe two led strips, but when noticed coils are different specs it got me wondering

[Hellfiya]

Re: iPhone backlight circuit operation question

This artical by jessa says The backlight circuit is split into two. There are two coils, two diodes, two filters that come together to feed one LCD connector.
http://mendonipadrehab.com/entries/g...-post-3?page=3

From what I've seen, there are dual circuits only in the Plus models, probably due to the larger screen.

Could this have anything to do with having two coils?:

[caphair]

Re: iPhone backlight circuit operation question

This artical by jessa says The backlight circuit is split into two. There are two coils, two diodes, two filters that come together to feed one LCD connector.
http://mendonipadrehab.com/entries/g...-post-3?page=3

That doesn't answer my curiosity as to why there's two of each and how they work together. In other iPhone models there's only one of each

[vinceroger69]

Re: iPhone backlight circuit operation question

This artical by jessa says The backlight circuit is split into two. There are two coils, two diodes, two filters that come together to feed one LCD connector.
http://mendonipadrehab.com/entries/g...-post-3?page=3

[caphair]

Re: iPhone backlight circuit operation question

I was thinking it might be multi-phase.

Multi-phased for different current demand or?

[PeteS in CA]

Re: iPhone backlight circuit operation question

It looks like both inductors are not driven at the same time, one is when high current demand is needed and one for lower current demanded.

I was thinking it might be multi-phase.

[budm]

Re: iPhone backlight circuit operation question

If you want to really understand the circuit operation then you need to start making actual measurement to see what is going on in the circuit such as this backlights controller IC

[caphair]

Re: iPhone backlight circuit operation question

Not sure if you watch chris long videos on youtube but maybe when hes doing a live stream ask him as hes really good at answering any questions people have regarding iphone repairs etc
https://www.youtube.com/channel/UCQV...ZSv5BgQ/videos

I've seen lots of videos on iPhone repairs. But I feel a lot of the people don't have clear understanding of electronics theory and circuit operation. They're mostly checking for shorts and replacing components. I'm sure they know basic stuff but I'm trying to understand circuit operation

[vinceroger69]

Re: iPhone backlight circuit operation question

Not sure if you watch chris long videos on youtube but maybe when hes doing a live stream ask him as hes really good at answering any questions people have regarding iphone repairs etc
https://www.youtube.com/channel/UCQV...ZSv5BgQ/videos

[budm]

Re: iPhone backlight circuit operation question

It looks like both inductors are not driven at the same time, one is when high current demand is needed and one for lower current demanded.

[caphair]

Re: iPhone backlight circuit operation question

Boost converter.
Just look up 'BOOST CONVERTER'
Since you have the P/N of the IC then just look up the spec sheet to see how it works, it is LED driver LED.

No data sheet available. I know it's a boost converter I'm wondering the purpose of two coils of different sizes as oppose to just one

[budm]

Re: iPhone backlight circuit operation question

Boost converter for driving the LED strings..
Just look up 'BOOST CONVERTER'
Since you have the P/N of the IC (LM3539) then just look up the spec sheet to see how it works, it is LED driver LED.