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LM3404EVAL: ???

Gregory Dyczko
Prodigy 30 points
Part Number: LM3404EVAL
Other Parts Discussed in Thread: LM3402, LM3404, LM3404HV

We are powering a string of 6 LEDs with the AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board.  SNVA342E-July 2008-Revised April 2013.  The strings of 6 LEDs are either yellow, red ,or green and driven from 200ma to 700ma.  We need a minimum dimming ratio of 25000 but would prefer 30,000.  We are having noise issues and flickering at our lowest intensity settings.  The power supply is 28VDC for the Eval board/LEDs.  5VDC for our logic circuits.  Jumper 1 on the eval board is closed.  Our 4 lowest intensity setting pules are generated by 74HCT123.  The PWM frequency is 250Hz generated by a 555 timer.  Our lowest pulse widths are 150 nano sec, 330 ns, 660 ns and 1.3 micro sec.  These pulse width signals and all the higher intensity signal are sent to a 74HCT4067 and the selected intensity signal is sent to the LM3404 Eval board CONN-1.  The 555 timer/hct123 circuit board is hand built with dip chips.  The HCT4067 is a surface mount circuit board. The lowest intensity, 150 ns, causes random flickering around 1 or 2 per second and then may be steady for a second or two.  Occasionally the 2nd intensity level will cause some flickering.

My questions are;

Can we operate the LM3404 or LM3404HV using MOSFET shunting with this short of a pulse, 150ns, or are we exceeding the specs of the device. Looks to me like it should work.  We would like to go down to 100ns.

Should the LM3404 Eval board work with 150 ns pulse(PWM 250Hz).  

Are there any circuit changes that you can suggest that would help eliminate the noise.  The 150ns pulse looks good (and the other low intensity pulses) until the power is turned on for the LM3404 Eval board.

Thank you for your help

  • 0
    TI__Genius 15776 points

    Hi robin. Could you help on this case

  • 0
    TI__Genius 13001 points

    Hi,

    Pls provide the schematic or simple block diagram to us, so we can get a better understanding about your issue description.

    BR,

    Robin

  • 0
    Prodigy 30 points

    Hello, I am having trouble figuring out how to cut and paste schematics to this spot.  I will hopfully figure it out before you write me back.

  • 0 in reply to Gregory Dyczko
    TI__Genius 13001 points

    Hi Gregory,

    You can save the picture of BD and click the "insert" button to upload it. Thanks!

    BR,

    Robin

  • 0
    Prodigy 30 points

    Robin, I can paste text to this box, but when I try to copy pictures to the box nothing happens.  When I go to insert, as you suggested, I do not see a way of of copying an image.  It asks for an URL and I do not know how utilizes that.  

    Anyway the pictures where of a 160 nano second 5V logic pulse being fed into your eval board (AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board) into the BNC connector on the eval board.   With the eval board on the same ground as the signal the input pulse degrades about 0.5V when it is connected to the input BNC connector.   Mostly reduces in amplitude.  Next when the eval board is powered up the LEDs turn on dim and the input signal degrades into several pulses (3 main pulses)  that have an amplitude from 2 to 4V and go minus 1V.  The problem is the the signal jitters and as it jitters, the intensity varies.  This jitter stops when the pulse width gets to about 800 nano seconds.  Also the pulse is a nearly clean 800 nano sec pulse with some trailing noise.

    I hooked up an HP pulse generator which had a little cleaner pulse and set it to the same PWM frequency of 250 Hz.  Still had the same issue; jitter with intensity variation up until 800 nano seconds.  

    Any ideas.  Please further explain to this dummy how to add pictures to this forum.

    Thank you.

  • 0 in reply to Gregory Dyczko
    TI__Genius 15776 points

    Robin, please keep following this case