• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 996 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3429 analog dimming current mirror issues

Craig Easdale19
Expert 1425 points
Other Parts Discussed in Thread: LM3429

I've been on and off working on trying to get analog dimming operational with an LM3429 driver PCB I designed.

The driver seems to operate fine on it's own (using nothing but Rcsh to set Icsh), with a few minor hiccups here and there which can be ironed out. However, as soon as I implement analog dimming, it all goes wrong. I'm using a digital potentiometer, which makes things a little more awkward, but I don't understand exactly why I'm getting the result that I am.

When I connect the analog dimming portion of the circuit Q2 decides it's time to have a barbecue and lets out all it's magical transistor smoke. Consequently, It also seems to ruin the cirtcuit after removal too (I assume it fried the driver since I'm sure the VCC pin is not rated to churn out a current of cooking magnitude). I thought it initially was because I had mistakenly connected the emitters of Q2 to Vin instead of VCC from the LM3429, but same thing happened again after fixing that.

Any idea what's going on here? I've fried it twice now and I have no idea what's going on. I've attached the schematic I'm using below:

Q2: BC856BDW1T1G (the one that cooks)
Q3: MMBT3904LT1G

I'm also pretty certain that it is physically impossible to accidentally connect Q2 the wrong way round since it's rotationally symmetrical.

  • 0
    TI__Mastermind 28655 points
    Hello,

    Seems something is connected incorrect. There is no path for Q2 to burn up if all those resistors exist, even if it's hooked up incorrect the current will be limited by the resistors, can't get even 1 ma with those three resistors tied from VCC to GND and 7 mW isn't going to smoke the transistor.

    I would change the CSH connection by adding another resistor since the ABSmax on CSH is six volts and VCC can be higher (just as a precaution) even though with those resistors it should be fine.

    What I would also consider trying is just removing the digital pot and use two resistors to set the current mirror to get that working first. Then add the digital once everything else is working. May even want to remove the CSH connection and measure external until it's working so the damage, if it happens, is minimal.