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LM1084: LM1084

Part Number: LM1084

Using a LM1084IS-ADJ as a load regulator in a circuit. 14vdc in, 0.200 ohm resistor between output and load, load sense line back to the adj pin. Load is only applied for 100 msec. Everything works fine until load gets above 6.2 amps. At 6.2 amps output starts to oscillate, by 6.5 amps fully oscillating. Should device be doing this? is it going into a short circuit/overload mode? 

  • Hi Tom,

    Can you share a scope shot of this happening? Also, what output voltage is set? 

    I wonder if the device is cycling in and out of thermal shutdown. at 6A and, for example, 12V output, you would be looking at 12W being dissipated and the junction temperature rise would be about 40.4C/W * 12W = 485C, which is clearly too much. So if the output voltage is set any lower than 12V with 14V at the input then it only gets worse.

    Best regards,

    Nick

  •  Picture of scope, 6.1 amp load Picture of scope 6.2 amp load.

    There is a relay after the regulator that I am energizing with a J-tag cluster test, then triggering my electronic load at the currents indicated by the pictures for 100 milli seconds for each test.. I am performing the test with a differentail 0'scope probe to make sure I am not effecting the circuit with my o'scope.

  • Hi Tom,

    This looks to me like the LDO is cycling in and out of thermal shutdown while the load is applied. During thermal shutdown the LDO output will go to 0V until its junction temperature has fallen enough and then try rising back up to its nominal Vout and Iout, and if the junction temperature again triggers the thermal shutdown circuit the cycle will repeat. 

    If you want to get more current out of this device without triggering thermal shutdown you must reduce the headroom.

    Best regards,

    Nick

  • Thanks Nick,

    This is what i suspected all along, but the people that designed the circuit do not believe me, I have since confirmed that this is the problem by freezing the regulator with chill spray and running the test, the regulator does not oscillate when it is chilled.