LP8866-Q1: LP8866-Q1 Backlight Driver Problems @-40

Hi,

Our expert is out of office and will give you feedback when comes back.

Hi,

Thank you for the information! Could you please share any details on when to expect the experts return ? This issue is sadly top priority in our current project . Thanks in advance.

Our expert will give you response after 5/5. Thanks for your patience.

BTW, could you please firstly send us your schematic and if you have any test waveforms, please also paste them. This will help us to better analyze this issue.

Hi, Juxhin

Could you pls kindly share your schematic and spec of LED?

Hello ,

Thank you for your Reply ! I sent the schematic to you via Private Message. Also on friday i noticed that after increasing the boost throught the changed value of R58 at the FB line, the display starts up but starts to flash.

After reading the registers, we get from the 0x10h (BOOST_STATUS) registers the decimal value of 34 which corresponds to BSTOCP_Status=1. In the datasheet it is mentioned that if BSTOCP_Status is 1 at the startup , the device will enter fault recovery and try to restart.

LEDs  have this part no:  NFSW157JT

 the readout of the register 0x10h

Also i would note that even under normal temperature conditions, the 0x10h register shows the decimal value of 2 although the Display turns ON normally. This means , if we refer to datasheet, that the BSTOVPL_STATUS bit is permanently set. 

Hi, Juxhin

Depending on your schematic and the LED information, this may related with the power limited.

And could you pls confirm that in normal temperature, all the input range like 10V can work normally?

Hello Zhiqian,

I can confirm that at normal temperature @10V it will work normally. Like i mentioned , the BSTOVPL_STATUS is always set. Also after discussing with the software team, we are trying now to do a softstart of the PWM, because our system now , starts the PWM at maximum below -10C.

Hi, Juxhin

If in normal temperature, the device worked normally. Which means that the power is enough. Pls check with the work condition in -40 degree. What is the maximum Vf and the typical Vf of the panel.

Hi Zhiqian,

Thank you for your reply ! According to the panel datasheet, the typical Vf is 33V and the maximum 35.2. It does not say anything regarding to the temperature as where these values come from.

• By checking the datasheet of the LEDs, 35.2V would be the calculated maximum Vf at -40C , where the forward voltage of 1 LED is 3.2V.

Hi, Juxhin

How about the maximum boost output you design?

Hi Zhiqian,

I apologise for the late reply due to national holidays. This are the design parameters we have for Sample C. Only the values of the resistors of the FB line are different from the schematic ( we have not yet officially updated the schematic).

We also have used the TI issued design worksheet as follows: 

Hi, Juxhin

I need to confirm that in your design the boost output is higher than the LED forward maximum voltage. Because in lower temperature, the Vf is higher.

Hello Zhiqian,

Thank you for your reply ! Should we try to set the V_boost_max  <  35.2V (maximum string voltage) ?

Hi Juxhin,

Zhiqian will give you feedback later.

Hi, Juxhin

The maximum boost voltage needs to higher than Max Vf.

Hello Zhiqian,

Sorry it took me so long to reply. I performed measurements at -40C with 24V,16V,15V,10V.

This measurements were done at the feedback line , on TP215 ( BLOVP) and on TP101( BLOU).

What we can see is that :

- At 15V and 16V, there is no PWM at all

- At 10V it looks like the PWM is trying to start.

10V

15V

16V

24V

I was thinking also lowering the BST_FSET and PWM_FSET. Do you think this would make any difference ?

Hi, Juxhin

You can change it to have a try.

From my side, after I checked with the waveform, the boost output(42V) is higher than your design(38V). So pls consider the resistor tolerance for -40 degree.

Hi Zhiqian,

- We set the BST_FSET to 500KHz, and now the Backlight is working at -40°C. We will now adjust the boost output accordingly.

- The resistors we are using are with 1% tolerance. Do you recommend a higher precision resistor or is the 1% tolerance enough?

- Also we noticed that during Radiated Emissions test, the driver is very noisy with the BST_FSET = 1818KHz. Do you expect these emissions to be lower if we reduce the boost frequency to 500KHz?

HI, Juxin

- We set the BST_FSET to 500KHz, and now the Backlight is working at -40°C. We will now adjust the boost output accordingly.

[ZW] So you mean when you reduce the switching frequency, it will work in -40 degree. May I ask do you check the waveform of GD in 1.8MHz? Is it correct?

- The resistors we are using are with 1% tolerance. Do you recommend a higher precision resistor or is the 1% tolerance enough?

[ZW]No, I just want to check why in your table the maximum value is 38V, but the waveform is 42V. Are there any changes in the hardware?

- Also we noticed that during Radiated Emissions test, the driver is very noisy with the BST_FSET = 1818KHz. Do you expect these emissions to be lower if we reduce the boost frequency to 500KHz?

[ZW]Could you pls kindly share the EMI result first?