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LM3409 low frequency PWM

G. Taob
Prodigy 125 points
Other Parts Discussed in Thread: LM3409

We have a requirement to PWM the LM3409 < 38Hz.  Using the EVAL board, above 38Hz, everything looks good, sharp rise and fall times, very clean LED current .  Below 38Hz, ILED is very slow, probably because the support circuitry takes a few ms to start up.

Questions:
1. Is there anyway to force the support circuitry to stay on for very low PWM? (I am guessing no, but I have to ask)
2. Would PWMing the IADJ pin be a suitable alternative? possible issues?

I know a typical solution would be a FET on the LED string and PWMing that but I am hoping for a low part count solution since I have restricted solution space.

  • 0
    TI__Mastermind 25500 points
    The IADJ pin will have delays associated with it due to propagation delays and the internal error amplifier response time. It isn't really recommended for PWM dimming as it will have some delay and be slower.
    When EN pin dimming it needs to be within a certain time constant. As the datasheet states, if the EN pin is pulled low long enough for the VCC capacitor to discharge below its UVLO threshold the part will enter a low power shutdown mode. So when EN is pulled high the next time you will have all the typical delays of startup (mostly the charging time of the VCC cap past the UVLO threshold).
    In either case you have other options. The first is to increase the VCC cap value. That will get you to a lower frequency depending on how high a value you use. It will become impractical at some point.
    Another way is to use shunt FET dimming which will give you faster edges and will not restrict the frequency.
    The easiest way may be to put a 6.2V zener across the VCC cap and connect a resistor from VCC to ground to bias the zener. That will keep the VCC up and prevent entering VCC uvlo.
  • 0 in reply to Clinton Jensen
    Prodigy 125 points
    Given how slow I am pulsing, I don't think the delay from the IADJ pin will be an issue.

    Thanks
  • 0 in reply to G. Taob
    TI__Mastermind 25500 points

    Then using IADJ is perfectly fine, no issues doing it that way.