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LM3697 / PWM sequence

Guru 20090 points
Other Parts Discussed in Thread: LM3697

Hello,

Can we use LM3697 with the following sequence?

<Sequence>
1. PWM signal input to LM3697 from LCD panel
2. LM3697 power on
3. HWEN high 
4. Initialize the register (PWM Enable)

Is the initial LED current setting PWM input setting?
If the Dpwm is 50%, the initial LED current will be the Iled which is calculated by the Dpwm=50%?


Best Regards,
Ryuji Asaka


  • Hello Ryuji

    There are several registers that require initialization following power on and HWEN high.  There is an example for PWM operation in Table 26 on page 31 of the datasheet.  The initial led current will be the 11-bit brightness code (Reg 0x20, 0x21) * DPWM.

  • Hello,

    Sorry for the poor explanation.
    The initialization of the registers are the following.

    Addr: 0x10, Data:0x00
    Addr: 0x16, Data:0x01
    Addr: 0x19, Data:0x07
    Addr: 0x1A, Data:0x04
    Addr: 0x1C, Data:0x0D
    Addr: 0x24, Data:0x01
    Addr: 0x20 and 0x21 is set the led current value

    <Issue>
    From the datasheet P.16, I found that the following description.

    "(1) The PWM input should always be in the inactive state when setting the Control bank PWM Enable bit. The PWM configuration bits should only be changed when the PWM is disabled for both Control Banks."

    Thus, I think that we can't apply the PWM input before initialization.
    However, we expect that the initial LED current will be ILED calculation result from equation (1) or (2) from the datasheet P.12.

    <Question>
    Can we apply the PWM input before power on of the LM3697?
    Can we apply the PWM input before HWEN of the LM3697?
    If yes, the initial LED current will be the ILED calculation result from equation (1) or (2) ?
    If no, please let me know the PWM input timing for the initial ILED current with equation (1) or (2)?

    From your first reply, I think that we can apply the PWM input before Power on/HWEN/Initialization the registers.
    Also, the initial LED current will be the equation(1) or (2) current.


    Best Regards,
    Ryuji Asaka




  • Hello Ryuji,


    It is not the preferred operating mode to have PWM active with Power off but it should not be an issue.

    The LED current will be equation 3 or 4 since the device is configured for Linear Mapping.

    Let me know if you have any other questions.

    Best regards,

    Randy

  • Hello Randy san,

    Thank you for the reply.
    I would like to know the following.
    The customer expected operating is the led current is fixed at the

    (1) Recommended sequence for the PWM dimming
    Could you please let me know the recommended sequence with the PWM dimming ?


    (2) about " It is not the preferred operating mode to have PWM active with Power off "
    Could you please let me know the reason of the not preferred operating mode to have PWM active with power off?
    You said that "it should not be an issue." however, I think that there is any problem , since this operation is not preferred.

    I would like to avoid the flicker of the LED or other unexpected operation of LED when we input the PWM signal before HWEN.


    Best Regards,
    Ryuji Asaka


  • Hello Ryuji-san,

    The PWM LPF (figure 9) is disabled until the PWM input in enabled and the Control Bank is enabled and the Brightness code is a non-zero value.  The PWM LPF will add some delay (~1.2 msec) to the startup time.  PWM signal active before device configuration does not cause any flicker of the LED.  The recommend sequence is Power On, HWEN High, I2C Initialization, then PWM active to minimize the startup time. 

    Best regards,

    Randy

  • Hello Randy san,

    Thank you for the reply.
    And sorry for my late reply. We took a golden week holiday in Japan.
    I understood that the recommended sequence. And there is no flicker of the LED if we apply the PWM signal before device configuration.

    Could you please let me know the negative effect when we don't follow the recommend sequene? (i.e, we apply the PWM signal before decvice congiguration, device power on)

    Best Regards,
    Ryuji Asaka

  • Hello Ryuji san,

    There is a power up delay of ~1.2 msec associated with the PWM LPF during configuration.

    Best regards,
    Randy
  • Hello Randy san,

    Thank you. I understood.

    Best Regards,
    Ryuji Asaka