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LM3405A: Different Brightness with use of 2 LM3405

Peter Reichart
Prodigy 20 points
Part Number: LM3405A
Other Parts Discussed in Thread: LM3405, , TPS92515, TPS92518, LM3409

Hello,

we use in our application 2 LM3405 devices.

Each of the LM3405A drives a Group of 4 LED´s in series.

Both of the LM3405A get the same PWM-Signal at the EN Input.

When the On time of the PWM is very low you see big differences in the brightness

of the two LED Groups.

But we need the same Brightness of both Groups.

We use the application shown in figure 14 of the datasheet.

Can you explain why we have a different brightness of the LED´s

Best regards

Peter Reichart

  • 0
    TI__Mastermind 25500 points

    Hello Peter,

    What kind of pulse width are you using here? Are the two circuits identical? A lot of things can come into play regarding dimming, particularly if the circuits are at all different. But even if they are not you will have differences in delays on the DIM pin as well as tolerances of capacitors and what not. If you are using a high frequency with very small pulse widths it is possible to have some variation circuit to circuit, particularly with a current mode control device such as the LM3405 (current mode control requires output capacitance generally).

    If you can decrease the PWM frequency you should see improvement in matching. If that isn't a possibility you may want to consider a much faster hysteretic device such as the TPS92515 or LM3409. Actually, in your case since you are using 2 channels, the TPS92518 may also be a good option. Let me know how fast you are trying to PWM dim and what kind of dimming ratio you need and I could make some suggestions. Thanks.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 20 points

    Hello Clint,

    we use a pulse width up to 90us and a PWM frequency of 3kHz.

    Both circuits are absolutly identical.

    When I measure the pulse width of both LED currents I see differences up to 8us.

    A bad board shows the following pulse widths:

    Pulse width (LED-current 1) = 82us

    Pulse width (LED-current 1) = 90us

    The greatest difference in Brightness is visible when the pulse widths is very short (about 5us) and

    we want to adjust a low Brightness.

    Best regards

    Peter

  • 0 in reply to Peter Reichart
    TI__Mastermind 25500 points

    Hello Peter,

    Given all of the tolerances involved a 8us difference at a 90us pulse width is easily plausible, even likely. Not only that you are getting into the range where a single device's own dimming is non-linear. For example, look at the graph for PWM dimming and see the 5kHz dimming curve. You can see it becomes very non-linear below a certain duty cycle/pulse width. With this device if you need wide range dimming and good matching it is really intended to be operated in the 100Hz to 200Hz dimming range.

    If you need good range and good matching at higher frequencies you really need to go with a hysteretic type device. Current mode devices vary too much with tolerances at such low pulse widths.

    Regards,

    Clint