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LM3409 - About PWM dimming using the EN Pin

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Guru 21045 points
Other Parts Discussed in Thread: LM3409

Hi Team,

 

Our customer are evaluating LM3409 at the following conditions.

However, LED current is stable at condition1 and unstable at conditon2.

Therefore, we would like to know whether the frequency and duty cycle of PWM dimming using the EN Pin has a limit.

Could you please let us know if you have any information?

 

----------------

[Condition1]

-Vin=12V

-Vout=2V

-ILED=1.5A

-PWM dimming is VPWM=5V , Frequency=62.5Hz, Duty=0.25

----------------

 

[Condition2]

-Vin=12V

-Vout=2V

-ILED=1.5A

-PWM dimming is VPWM=5V , Frequency=62.5Hz, Duty=0.1875

----------------

 

[Condition1 & 2 image]

 

 

If you contact to below my e-mail address or let me know your e-mail address,

I can send the waveform and the circuit.

I’m looking forward to hearing from you.

 

[my e-mail address]

Kanemaru-h@clv.macnica.co.jp

 

Regards,

Kanemaru

  • 0
    TI__Mastermind 25500 points
    There is a limit to the off time when using the EN pin. When it is pulled low VCC begins to discharge. When VCC discharges below its UVLO threshold the part goes into low power shutdown mode. When you pull EN high again there will be a delay and then a normal startup sequence. So low frequency dimming will have a limited range with the EN pin. This is easy to get around though. Connect a small schottky diode's anode to the UVLO pin and drive the cathode with the dimming signal. The UVLO pin will act just like the EN pin for PWM dimming but VCC will not discharge and the part will not go into low power shutdown mode.
  • 0 in reply to Clinton Jensen
    Guru 21045 points

    Hi Clinton-san,

     

    Thank you for your kind support!

    My understanding is as follows.

    If there are any mistakes in my understanding please let me know.

     

    [Circuit]


     

    [Advantage]

    We can use the low frequency PMW for the dimming.

     

    [Disadvantage]

    We can’t adjust UVLO using Ruv1 and Ruv2.

     

    Regards,

    Kanemaru

  • 0 in reply to Hide
    TI__Mastermind 25500 points

    Actually I suggested the diode with the assumption you were using Ruv1 and Ruv2. The diode is there so you can use the UVLO function still. If you do not use Ruv1 and Ruv2 you could just drive the UVLO pin directly with a micro. So you can still use the UVLO function if you want.

  • 0 in reply to Clinton Jensen
    Guru 21045 points

    Hi Clinton-san,

    Thank you very much for your help.

    I understand. Thank you very much.

     

    Regards,

    Kanemaru