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.

LM4766: BTL Connection Issue

Part Number: LM4766

Hi

I am currently trying to build an amplifier circuit in a BTL configuration to drive a stepper motor. Before I am connecting it to an inductive load I have been trying to test the circuit with a normal resistive load to ensure it is working as expected. My problem at the moment is that the output on amplifier A is basically dead (following the input), but the output on amplifier B is working as expected. I have used the bridge example circuit in the LM4766 datasheet and I have tried to look on the forums to see if people might have struggled with a similar problem, but could not find any.

My first question: Does the mute time constant have to be larger than a certain amount to ensure the mute is turned off? My time constant is 1.65ms at the moment (using a 5k ohm resistor and a 330nF cap)? Is that too low? I am definitely pulling more than 1mA through those pins and have tested both outputs in a non-inverting manner to ensure that the output/chip is still working. I am just not sure if I am missing something when I connect one side to make an inverting amplifier and if that would somehow affect something in that circuit that might cause the output to be responding incorrectly.

The output of amplifier A looks like it is clipping to my negative supply voltage Vee. (I am using a +- 15V supply voltage for testing purpose at the moment). I tried to see if I have not shorted anything, but I am fairly positive that the connections are correct.

Do you maybe have anything else I can look at or a different design circuit I could try or am I missing something? I could get the amplifiers to work when I connect them separately in a non-inverting connection but as soon as I combine them in BTL connection I am having problems with the one output (the one connected as an inverting op-amp connection).

Thanks in advance for the help.

Kind Regards

Nardus

  • Hi Nardus,

    Welcome to e2e.
    Could you share your schematic? Have you tried to use separate resistors for pins 6 and 11?
    The capacitor on these pins is used to create a charging ramp on mute/unmute in order to avoid pops.
    Are you able to use one of the amplifiers alone in an inverting configuration?

    Best regards,
    -Ivan Salazar
    Texas Instruments
  • Is there by any chance you can share a picture of your connections and their corresponding values ? Did you design it to look exactly like the BTL configuration (Figure 5) ? And as for output A, you say it is "basically dead" but you also say it's clipping to the negative rail ? So we are getting a signal on output A ? Or is it a constant DC at the negative rail ?

    Clipping typically refers to a signal not capable of exceeding a voltage level due to supply rail-rail limitations, diode clamping/clipping circuitry, etc.
  • Hi Ivan and Nabil

    Thank you very much for your response. I am currently using the schematic used in the data sheet with the following changes.

    Rf = 22k instead of 21k (Only output A)

    Ci = 47uF instead of 22uF

    Rm = 5k

    Cm = 330nF (Should this be bigger and an electrolytic cap? currently using ceramic one)

    Vcc = 15V (pins 2 and 15)

    Vee = -15V (pin 4)

    GND = pins 5 and 10

    NC = pins 9 and 14

    Rl = 33ohm (for testing purpose only)

    Figure 1: Schematic of circuit.

    I have separated the connections on pins 6 and 11 in my current application and have given them each their own capacitor and resistor.

    This is the following results that I have obtained from my various measurements.

    Figure 2: No load at the output. 

    Blue is the output at B, Yellow is the output at A and purple is the Input Signal.

    Input = 0.5Vpeak sinusoidal

    Output A = +-  -13V DC

    Output B = 10Vpeak 

    If I connect the outputs in a BTL configuration over the load the following is obtained.

    Figure 3: BTL Connection.



    Green is the current through the load (250mA peak more or less). Nothing much that changed.

    I have also measured what you suggested Ivan. I connected amplifier B in an inverting connection by taking the input at 13 and connecting it to 12 and also disconnecting Ri from 12. I connected 13 to ground.

    This was the results open with no load. I connected each one separately to the load which didn't change the results significantly. (Other end of the load was grounded)

    Figure 4: Separate measurement with both amplifiers being in an inverting connection.


    I am going to swap the chip with one of my spares to see if there might be something wrong with the chip and get back to you as well. 

    Thanks again for the response and your help.

    Kind Regards

    Nardus

  • Hi Ivan and Nabil

    I have solved my problem. I am receiving a 0.5Vp signal from a DAC that has a 2V DC offset. I didn't AC couple this signal at the input of my amplifier which explains the clamping I get when I built it in an inverting configuration. I have managed to get the circuit working and will now be playing with the amplifier circuit in order to use it with a stepper motor.

    Thank you for your quick response and your time to help me out. If I struggle in the future I will definitely make use of this forum again.

    Kind Regards
    Nardus