HP Pavilion All in one power converter led backlight : how Mobo dim lights?

[1fg52]

Doing reverse engeneering to a IK-CVB20 Pegatron "inverter" board from an HP Pavilion 20f295la I found there is no real dim signal from Mobo.

Pin 1= GND
Pin 2= GND
Pin 3= Enable (works with 5 volts)
Pin 4= Pseudo Dim pin (i will explaint below)
Pin 5= Panel type selector (options are Samsung, LG and CMI, selected by jumpers in the board)
Pin 6= Same as Pin 5.



Link to the image https://imgur.com/gcTkPuK

That board works with a MPS MP3389 controller and needs a dim signal (pin 6) it works with a PWM signal or a DC voltage (0 to 1.2).

In that Pegatron board is configured to work with a PWM and 3389 dim pin is connected to a PWM generator ( Richtek r7731a ).

The problem is: the supposed dim pin in board connector (pin 4) go directly to r7731a 's VDD pin not to any PWM control pin as expected.

r7731a's Vdd have a very narrow range of operational voltages from 10 to 14v
changing that voltage will not work as a dim signal: In r7731a datasheet is no mention of behavior changes vs VDD value.

***r7731a Datasheet

Then, how HP mobos do a dim function with these Pegatron boards? Someone have an idea?

[dicky96]

Re: HP Pavilion All in one power converter led backlight : how Mobo dim lights?

You would expect the DIM pin to conenct to COMP input on the R7731A. COMP is Pin 2. You say it goes to VDD - that is pin 7. Which is the exact opposite side pin of the chip for the 8 pin SOIC package. Like this:

1--=--8
2--=--7
3--=--6
4--=--5

So are you sure you are not just getting Pin 2 and 7 mixed up (for example when the board is upside down)?

[1fg52]

Re: HP Pavilion All in one power converter led backlight : how Mobo dim lights?

Thanks for reply:
It's a SOT-23 six pins version. It's possible to mix up both of these pins in that package but i see again and my measurements were correct. I will try to put less voltage (maybe 11 volts) to see if something changes.
I connected the panel backlight and is working at full power, it's too bright. I used a damaged LG 20M35a-b monitor main board to drive the panel but that monitor uses a backlight led strip with higher voltage 40volts vs 27 volts in the HP All in one Samsung LCD panel.

Image link

[1fg52]

Re: HP Pavilion All in one power converter led backlight : how Mobo dim lights?

Well i made some experiments and clearly i misidentified that little chip: well under 10 volts control voltage created by that chip continues to be 1.2 v as required by MP3389 controller.
Maybe is a transistor or something but clearly is a signal conditioning for an external PWM not a PWM itself. I didn't try but an external PWM connected to pin 4 IK-CVB20 Pegatron will dim backlights for sure.