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Extending LM2679 with wires

Osama Ennasr
Prodigy 10 points

I have a 20V source that I am regulating down to 12V using LM2679T-12. Initially, the regulator was soldered to the board and testing was carried out to drive 2 linear actuators. When the regulator was soldered to the board, the regulator was able to drive 2 linear actuators. However, for my project, the regulator needs to be mounted away from the board. Therefore, wires are soldered to the pins of the regulator and are connected to the board.

When the wires are short (less than 3"), the regulator still works fine. On the other hand, when we use longer wires (we need the wires lengths to be at least 6"), problems start appearing: the output of the regulator initially starts at 12V. However, when the actuator is activated the voltage drops below 4V and the actuator does not move any more.

I was wondering if anyone might know what I am missing. Do the lengths of the wires affect the output voltage because of the switching? Is this something that could be compensated for using different values for the LC filter?

I can supply more information if needed.

Thank you in advance for your help.

  • 0
    TI__Genius 16025 points
    If I understand correctly, the part is floating in mid air with 6" wires connecting the part to the board, correct? If so, this is absolutely not recommended for any power IC. Two main reasons:
    a) The thermal pad is no longer soldered down, this affects the long term reliability of the IC since the thermal extremes created in the absence of heatsinking will shorten the life span of the IC or cause it to fail destructively at any time.
    b) A more severe problem is the parasitics. The long wires will introduce a lot of inductance in the leads and push the operating point of the IC beyond it's designed target. For example, input voltage will see spikes that exceed the abs max of the IC and the compensation will be completely ineffective at stabilizing the IC during transients increasing the risk of failure manifold.

    Please design the IC on a 4 layer PCB with good thermal and electrical layout with thick traces for the Power pins (at least 15mils for each Ampere of power required) and place adequate amounts of input and output capacitance. Also, ensure you place a .1uF capacitor very close to the IC, between VIN and GND.

    Thanks,
    Anston