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AMC7834: Biasing GaN Power Amplifier

Manik Singh
Prodigy 10 points
Part Number: AMC7834
Other Parts Discussed in Thread: AFE20408, AFE10004

Tool/software:

Hi, I plan to use AMC7834 to bias a GaN Power Amplifier. I had some queries regarding that:


  1. Here, they have mentioned that the settling time to go from 400h to C00h is 10us. I want to know how long it will take to go from -5V to -2.5V with a step size of 0.5V? (Given, 000h is -5V and FFFh is 0V)

  2. If Avss = -5V, does the thermal pad also need to be connected to -5V?
  3. What is the frequency of the clock used for DAC? Can we control the clock rate using any software code?

Thanks,
Manik 

  • 0
    TI__Genius 10480 points

    Hi Manik,

    1) The voltage change from -5V to -2.5V is also 10us.

    2) Yes, you should tie the thermal pad to the AVSS supply.

    3) The DAC update speed is dependent on the SPI writing speed. The SCLK has a speed up to 10Mhz - 15MHz depending on your IOVDD voltage.

    Let me know if you have any other questions.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Prodigy 10 points

    1. Does 10us value depends on the step size we take, or is it irrespective of the step size , and depends only the initial and final voltage value?
    2. OK
    3. Can i change the DAC clock(multiple of SCLK) to change the 10us value?

  • 0 in reply to Manik Singh
    TI__Genius 10480 points

    Hi Manik,

    1. The slew rate is static no matter the step size. In this case, a 0.5V step size would take ~2us, for a total of 10us for a 2.5V change.

    3. Unfortunately, this is not possible. The output slew rate is determined by the output buffer (unchangeable), max output current (unchangeable), and the output capacitance. For a faster slew rate, you can lower the capacitance value. The 10us value is for a 200pF load. An unloaded DAC will change output faster.

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Prodigy 10 points

    Hi Erin,

    1. According to your statement, the slew rate will be 0.25 V/us. But, the slew rate in the data sheet is 1.25V/us. Please clarify this.

    3. Ok, thanks.

  • 0 in reply to Manik Singh
    TI__Genius 10480 points

    Hi Manik,

    You're correct. Unfortunately, the datasheet is unclear about where these values come from, it only has a rather unclear settling plot. I'm out of office this week, but next week I can take the EVM and test the settling time.

    You could also take a look at our newer PA controllers, the AFE10004 or AFE20408 - these devices have internal switches that do fast output switching, within 400ns. 

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    Prodigy 10 points

    Hi Erin,
    Waiting for the test results from your end. And, thanks for the new suggestions. We already went through the datasheet of AFE20408, it doesn't matches our requirements. But, will go through the datasheet of AFE10004 for further study.

    Thanks,
    Manik

  • 0 in reply to Manik Singh
    TI__Genius 10480 points

    Hi Manik,

    Long time no talk - thanks for your patience. I took a scope screenshot of DAC1 going from -5V to -2.5V (Blue line). Orange line is CS. As you can see, this takes aprox 2us, so it seems our initial estimation was correct. Best I can say here: the DAC slews to -2.5V within 2us. It may take an additional few microseconds to fully settle to the correct output (based on capacitor vs time). Let me know if you have any other questions!

    Thanks,
    Erin