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LM3409 Demo Board at low frequencies

Stephen Brown
Prodigy 30 points
Other Parts Discussed in Thread: LM3409

Hello,

I'm using the LM3409 Demonstration Board.  I'm having a problem pulsing LEDs at low frequencies.  I'm eventually trying to enable the LEDs with a 60 Hz PWM at 2% duty cycle.  

I've tried various low frequencies and whenever the off-time (low part of duty cycle, not switching frequency) is greater than ~11 ms, there is a delay and no constant current to the LEDs.  I've pored over the datasheet, but I can only find information related to higher PWM frequencies.  

What would possibly cause this problem?  The only information I see is related to the off-time of the switching frequency for the peak current detection / regulation.

Thanks

  • 0
    Prodigy 30 points

    I just wanted to update that the problem has been resolved.  The capacitor between VIN and VCC had to be increased to 10uF.  When the difference between VIN and VCC is less than ~3.2V, the device won't become enabled.  VCC was approaching VIN too quickly for the low frequency / low duty cycle application.  Now the off-time can't exceed ~135ms which is plenty adequate for my purposes.