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LM2755: Non-monotonic increasing of brightness

Johannes Brecher
Prodigy 110 points
Part Number: LM2755
Other Parts Discussed in Thread: LP5521, LP5569, LP55231

Hello,

I am using the LM2755 for generating a waveform by changing the content of the register nID1H (D1 High Level). The LED is enabled in the General Purpose register (Bit 0).
The supply voltage is 3V.

I observed that the brightness is not monotonically increasing.
Two steps can be observed, where the brightness is decreasing instead of increasing:
- 0x0D -> 0x0E : The brightness decreases when switching from 0x0D to 0x0E
- 0x1C -> 0x1D: The brightness decreases when switching from 0x1C to 0x1D

I also did a current measurement of my system and also this was not monotonically increasing - it confirmed my visual observation.

Has this behavior already been observed?
Is this normal due to some tolerances within the LM2755 or should this never happen?

Best regards,

Johannes

  • 0
    TI__Expert 4445 points

    Hi Johannes,

        Would you mind to send me all the register setting then I can try it in the EVM since it is lagecy device?

        Thanks!

    Summer

  • 0 in reply to Summer SU
    Prodigy 110 points

    Hi Summer,

    what do you mean with legacy device? How long will this product be available?
    Can you recommend a similar product that replaces this device?

    The register settings are basically those from Table 1 of the datasheet "LM2755 Charge Pump LED Controller with I2C-Compatible Interface in DSBGA Package" (SNVS433D –OCTOBER 2007–REVISED APRIL 2013). I.e. they contain the Power On Value.

    The only registers that are changed are:
    - General Purpose (address 0x10) that I set to 0x01
    - D1 High Level: nID1H (address 0xA9) which I am changing in the range 0x00 to 0x1f with a step size of 0x01

    Thank you,

    Johannes 

  • 0 in reply to Johannes Brecher
    TI__Expert 4445 points
    Hi Johannes,
    This device was released more than 12 year, I just the request the EVM to verify your question, it will take more than 1 or 2 weeks.
    May I know your requirement of device than I can recommend the new one for you?
    Thanks!
    Summer
  • 0 in reply to Summer SU
    Prodigy 110 points
    Hi Summer,
    are we talking about the same product?
    - The datasheet states "October 2007".
    - Also when I go to www.ti.com/.../samplebuy
    there is written "LM2755 (Active)". So I can not see why this should be a legacy device. Will the production be discontinued soon?

    My requirement would be a Charge Pump LED Controller that is pin compatible with the LM2755 and shows a monotonous increasing behavior of the brightness of the LED with regard to the brightness setting in the register.

    Thanks and best regards,
    Johannes
  • 0 in reply to Johannes Brecher
    TI__Expert 4445 points

    Hi Johannes,

    The datasheet just inform the before version not the first version time.

    This device is still available but we didn't know when is the last buy since it depends on the customer order. But we would like recommend the customer to use the other device if current waveform modification is not necessary.

    I am so sorry  that we have no device pin compatible with LM2755.

    If your requirements are charge pump and monotonous,

    the below candidates will be better solution.

    LP5521,LP5569 and LP55231.

    Could you check the datasheets and let me know the feedback?

    Thanks!

    Summer

  • 0 in reply to Summer SU
    Prodigy 110 points

    Hi Summer,

    thank you for your detailed reply.
    We had a discussion on this issue and the decision has been made that the LM2755 shall be used.

    Would it therefore be possible that our observation with those mentioned steps (where the brightness is decreasing instead of increasing) is checked?

    It would not be a problem for us, if this is a systematic problem in this device...  but it would be a problem, if different batches of this product would show different behavior.

    Best regards, Johannes

  • 0 in reply to Johannes Brecher
    TI__Expert 4445 points
    Hi Johannes,
    We checked with the original team, please refer the below answer:
    There can be some non-monotonic behavior where the DC current changes and the duty-cycle changes. The value of the RSET could influence this some as well but we don’t have much data on this anymore.
    So it is the systematics problem in thi device.
    Any more question please let me know, thanks!
    Summer