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Problem with LM4780

Suneetha Vadlapatla
Intellectual 260 points


Hello Sir,

I used LM4780 application circuit Fig.39 for our Audio Power amplifier Project.I am testing with  ±35V DC Supply.In Idle Condition,i.e with out applying load at the output and at input,it is drawing a current of 120mA on each rail.

I connected to  a heat sink through a heat sink compound.The LM4780 gets heated up without applying input signal.For each VCC and VEE Pin I connected a decoupling capacitor of 0.1uF.Will the IC will gets heated up normally or is there is any problem? If heat sink is the problem,recommend a heat sink size for that circuit.

What could be the possible problem?What is the tempertaure on the IC on idle condition with 2 stages of op-amps at a supply of ±35V?

With Regards,

Suneetha.

  • 0
    TI__Guru 52115 points

    Hi Suneetha,

    I will take a look at this issue and get back to you soon.

    Andy

  • 0
    TI__Guru 52115 points

    Hi Suneetha,

    Here are some comments I got from my colleague:

    "1) 120mA idle current on the both rail is normal value, which means that power dissipation in idle mode is about 8.4W. This power consumption causes heat sink to get warm.

    2) This pkg is not fully molded and the die attach pad is exposed on the back side. When customer mounts the pkg to heat sink, customer needs to use isolator and silicon compound.

    3) Please check customer uses snubber network on the outputs. Make sure snubber GND point. Please refer PCB layout.

    4) Please check oscillation on the outputs. If the device is oscillating, the device will have excess power consumption.

    5) If OSC is observed, make sure power supply decoupling cap and GND routing. If GND line is making loop between input signal GND line and output GND line, it needs to be cut the loop and make 1 point GND.

    6) PCB pattern layout is very important for stable operation. So please refer from fig40 to fig43 as PCB pattern layout.

    7) In idle condition, about 8.4W of power dissipation, how much the heat sink will get warm is depending on what kind of heat sink thermal resistance is. Please refer heat sink consideration on datasheet pg17 and 18."

    Andy