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Current regulation accuracy (?) problem with LM3409

Wolfgang Wegner
Prodigy 20 points
Other Parts Discussed in Thread: LM3409

Hi all,

I am having some problems getting my LM3409 design to work properly.

Spec is:
12V in (11V to 14.5V specified)
8V-10V out (issue with 11V in, I know)
120mA - 2.3A range for the current, adjusted via IADJ pin, to adapt different LED rails
PWM 100 Hz - 1kHz for brightness control

My problem is that current regulation is not stable over the input voltage range of 11V - 14.5V. The output current is ok at 12V nominal voltage, but varies in a non-linear way with input voltage.
I am right now testing with a 2.8 Ohms resistive load and 1.2A current (V_IADJ 620mV).
When increasing Vin to reach the limit of 14.5V, at some point the current increases very fast and then further runs off the scale (due to some thermal effects?), only to be stopped by me at around 2.5 A by turning down the input voltage.

Strangely, this does not happen when setting the current to 2.3A, at this operating point I can increase the input voltage up to 16V with the current increasing somewhat linear with the voltage, but not seeing a trip point like in the above case.

Did anybody else experience such behaviour? What might be the cause, in which direction should I investigate?

V_IADJ is constant, measured directly at the IC pins.

Any help is appreciated, I am out of ideas!

Best regards,
Wolfgang

 

powerstage.pdf
  • 0
    Prodigy 20 points

    *sigh*

    It seems I can answer myself now - a colleague led me to the solution.
    I had used an X7R capacitor for Coff (C27 in my schematic), which was
    placed close to the power stage. Thus, temperature rise led to shortened
    off-time and thus further increasing current, and so on.

    Now I am using 150pF + 330pF - both COG - in parallel (no 470pF in 0402
    at hand), and the effect seems to be gone. Overall current regulation
    depending on the input voltage also seems to be improved now. I have
    to run some more thorough tests, but regulation is stable so far over the
    whole input voltage range at 1.2, 1.8, 2.1 and 2.3 Amps.

    I hope this can help others experiencing strange regulation effects.

    Best regards,
    Wolfgang