Linux/LP8556: LP8556 power consumption of maximum sink current is Not as expected.

Hi Joe,

     I don't understand why you target that value of 1016mA? How do you get that value? I don't think that 1037mA is a very strange value, is that right?

Regards

Sean

Hi Sean

   Thanks for the replay.

The 1016 mA is our customer target, I think the value is from panel spec, we will try to ask them how to get this value

And we use following formula to get measurement result 1037mA :

    Vout = 22.18V,  Iout = 159mA

    Calculate back to Vin (VPH_PWR=4V)

    4V * I input * 0.85 = 22.18V* 159mA

    I input = 1037mA

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However, Can you please help to answer below 3 questions ?

1. Is our formula for measurement result reasonable ?

2. For reduce power consumption, we set  register "0xa7>"  to "<0xf5>  for boost inductor current limit. And this modification really can improve  power consumption. Can you please help to check the setting is correct ?

3. We also found it can improve power consumption by settings  "VBOOST_RANGE = 0"  in register "A9h".  but in the document, they said the preferred value is  "VBOOST_RANGE=1". Can you please check is there any side effect if we set "VBOOST_RANGE=0".

Thanks

Joe

Hi Sean,

  Thanks for the great support.

About "VBOOST_RANGE" ; we try to measure the boost voltage and led current with different brightness,

the result is as below :

   Boost voltage / Leds current (brightness)

   18.69V        /1.19mA      (0%)

   20.41V       /80.5mA     (50%)

   22.18V     /159.1mA   (100%)

So do you think is it ok to set "VBOOST_RANGE=0" for our case ?

Thanks

Joe

Hi Joe,

    I think it is ok, since this regs just control the voltage loop and your maximum voltage has not exceeded this range.

Regards

Sean

Hi Sean,

    Thanks for the suggestion,  we will try to set "VBOOST_RANGE=0 and  IBOOST_LIM=11(1.8A) ".

By the way, at Page 35 of LP8556 document (SNVS871J –JULY 2012–REVISED JANUARY 2018", there is a comment :

" In applications with an output voltage higher than 16 V, VBOOST_RANGE = 1 is preferred."

Can I know the reason ?  We just want to know there is any risk, if we set "VBOOST_RANGE=0"

Thanks

Joe

Ji Joe,

    Oh, that is the new datasheet and they have updated, I will check with our enginner and reply to you later.

Regards

Sean

Hi Sean,

  Do we have any update ?

Thanks

Joe

Hi Joe,

    Yes, I'm sorry for late response,  For loop stability, you'f better refer to " In applications with an output voltage higher than 16 V, VBOOST_RANGE = 1 is preferred."

     So for your application, you'd better set the VBOOST_RANGE=1.

Regards

Sean

Hi Sean

    Thanks for the reply.

As your information, actually we found some displays have flicker problem with  "IBOOST_LIM=1.2A(01)"

So I think we can conclude the correct settings are "VBOOST_RANGE=1" and "IBOOST_LIM=1.8A(11)".(it's default setting)

But with this setting, the power consumption become higher than before.

Can I know are there other registers we can try to reduce power consumption ?

Thanks

Joe

Hi Joe,

    I'm sorry that I don't have more commons on improving the efficiency performance. But I suggest you tring with different switching frequency.

Regards

Sean

Hi Sean

   Thanks. What is "different switching frequency" ? do you mean "pwm frequency " ?

We only use i2c to control brightness, not use pwm frequency.

Please check

Thanks

Joe

Hi Joe,

    No, I mean the boost switching frequency, you can try to change the reg A6 bit[7][6].

Regards

Sean

Hi Sean,

This is power team Chanah.

SW Joe had tried to change the reg A6 bit[7][6], but it will cause flicker and brightness has been reduced.

If we keep default setting, VBOOST_RANGE = 1 and IBOOST_LIM = 11 (1.8A)

Our max. backlight would be 1448mA, min. backlight would be 234mA. 

It's mean backlight max. current consumption = delta 1214mA. 

But from our display datasheet, the max. current consumption = 3.998W.

4V * Iin * 0.85(eff) = 3.998W

Iin should be under 1176mA.

Is our efficiency set too ideal ? or is there any register need to adjust ?

We hope can reduce the consumption but cannot cause flicker..

Thanks.

Hi Chanah,

    The power consumption is not only for our device, it also include the power loss on the inductor and diode and capacitors and so on. It is an system level question. I don't have more suggestions to improve the efficiency, but I can only advise to adjust the swicthing frequency may help. 

Regards

Sean