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LM3404EVAL: Flickering of the LEDs using an LM3404 especially at low intensity.

Part Number: LM3404EVAL
Other Parts Discussed in Thread: LM3402

Hello,

  Working with the following schematic I have several questions.  Thank you for your help.  The below circuit was designed using TI LM3404xx-1-A data sheets SNVS465G and the AN-1839 LM3402/3404 evaluation board users guideSNVA342E.

There are 11 of these driver circuits on each circuit card with additional circuitry.  Some boards are set to drive up to 200ma through the LEDs, other boards 800ma.  Each of the above driver circuits drive a string of 6 LEDs, either red, PC amber, or green.  The LEDs are Cree and the green P/N is XPEBGR-LL1-0000-00F02.  Our goal is a contrast ratio of 25000 to 1 or better.  The PWM signal is applied to the external MOSFET at LAMP DIM on the right end of the above circuit.  We are using a PWM base frequency of 200 to 250 Hz.  Our lowest pulse width is about 150 nano sec with the 250 Hz base rate.  There is constant random flickering especially noticeable at the low intensities.  We have 15 levels of intensity.  The lowest 4 intensity settings are generated by 2 CD 54HCT123s and intensity levels 5 to 15 are generated by 555 timers.  

  The intensity signal is sent to a differential receiver AM26LS32 on the board.  This receiver drives all 11 external MOSFETs on the board.  Besides the flickering the LEDs do not light a intensity levels 1 and 2.  Level 3 lights some of the 6 LED strings.  I tried adding pictures to this thread of the intensity signals but haven't figure out how to add them.  Cut and paste doesn't seem to work for pictures.  Let me know how to do this please.

However I will describe the signals.  The intensity signal going to the AM26LS32 looks fine.  Level 2 signal is a 5 volt square wave about 250 nano sec wide.  Coming out of the receiver the signal is a triangle wave only 2 volts high.  Level 3 is a nearly a triangle but 4.5 votls and about 750 nano sec wide.  At level 4 the wave is 5v and the triangle wave point is flattened.  The wider part of the wave is about 1.5 micro sec.  Level 5 is a 5 v square wave about 4 micro sec wide.  

Do you have any information on why the LEDs are flickering?

I'm wondering if the triangle waves are due to loading as only one receiver is driving all the driver circuits.  Or is it propagation delays?

Thank you for your help.  I will wait until I figure out how to add pictures to the posts before I ask additional questions.