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LM1876: High VCC Current Draw On Power Up

Part Number: LM1876

I am seeing  very high current draw on Vcc upon power up. Vee is steady at approximately 50mA, but Vcc is initially current limited by my power supply (1A) and falls to approximately 50mA after a few seconds. My circuit is essentially identical to the Typical Application circuit shown in Figure 2 of the LM1876 datasheet. If I add a series capacitor with the load (8ohm speaker) the behavior is eliminated. I would prefer to avoid the use of these caps in the final design.

Thanks in advance

  • Hi Dan,

    I am wondering if there is a DC offset at your amplifier input that is being applied to the output and generating the current. Can you please check for a DC offset at the input and output to your amplifier by measuring the voltage with no music playing? If there is an offset what is the voltage of it? Also did you include the capacitor Ci from the typical application circuit in your circuit?

    Regards,

    Alex

  • Hi Alex,

    Thanks so much for the reply.

    Here are the DC voltage measurements at each of the input/output nodes of the circuit with no input signal present. These measurements are taken in the circuit's steady state:

    Input A +   -3.9mV

    Input A -   -2.9mV

    Output A   -56.4mV

    Input B +   -3.9mV

    Input B -   -3.4mV

    Output B   -67.4mV

    Ci is not present in the circuit; Ri is connected directly to ground.

    Thanks!

  • Hi Dan,

    Those values look good and should not cause this problem into an 8 ohm load. Reviewing your original post it also seems that this issue only occurs on startup after which the device functions properly, correct? The capacitor Ci can help with DC stability and forms a high pass filter, which may help if there is instability at power up. Would you be able to populate it with a value from the datasheet and see if that helps with your issue?

    Best,

    Alex
  • Alex,

    Yes, the high current draw is seen only at power up and becomes normal (approx. 50mA) after a few seconds.

    The layout is fairly tight at that point of the PCB. Would a 1uF Ci work as a test?

    Thanks,

    Dan

  • Hi Dan,

    Sorry for the delayed response. In the datasheet it says to set Ci so that the high pass filter is at the bottom of the audible range (e.g. around 4Hz). The only problem with 1uF is it will set your filter cutoff a bit high (160Hz if Ri is 1kOhm in your design) this will attenuate some of your lower frequency sound. However, you could try 1uF to see if it fixes the power up problem, but you will eventually want to use a higher cap value. For example Ci ≥ 1/(2π * 1kohm * 4 Hz) = 39.8 μF gives 39 μF assuming your Ri is set to 1kohm.

    Best,

    Alex

  • Alex,

    Sorry for the delay in my response. I was able to place a 33uF cap as Ci on both channels and am no longer seeing the start up issue.

    Thanks for the excellent support!

    Dan