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AMC7908: AMC7904 || DAC output current mode select

Muhammed Mufeed
Prodigy 20 points

Part Number: AMC7908
Other Parts Discussed in Thread: AMC7904

Hi,

We are using the AMC7904 to bias a GaN PA device, with the VSS supply set to -9 V.
We observe different values in Page 1, Configuration 1 Register (offset: 0x08) depending on the DAC output load condition:

High impedance (Normal condition):

Register value = 0x01
Normal current mode (70 mA)

Low impedance / short to GND (fault condition, e.g., damaged PA):

Register value = 0x19
Indicates High current mode (130 mA) and Wide VSS configuration

Observed Issue:
When a short occurs at the DAC output, the AMC7904 automatically switches to high-current mode, increasing the current drawn from the VSS regulator.
Our goal is to limit the output current during a short condition to reduce stress on the VSS supply and improve system robustness.

Regards,

Mufeed

  • 0
    TI__Genius 17120 points

    Hi Mufeed,

    I'll take a look at this today. It should not be able to swap ranges like this without software interference. 

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    TI__Genius 17120 points

    Hi Mufeed,

    I performed a test on bench shorting the DAC1 to ground when it was set to -5V. I saw around 50mA, and the Configuration 1 register did not change.

    All Configuration 1 data is stored in EEPROM. Is it possible the default you have programmed is 0x19? Please also check the current limitations of your VSS supply. 

    Thanks,
    Erin

  • 0
    Prodigy 20 points

    Hi Erin,

    Could you please clarify when the DAC1 pin is being shorted to ground—before powering up the device or after power is applied? Based on our observations, this condition appears during power-on, likely because the power amplifier is already in a faulty state.

    Additionally, Configuration Register 1 is not being programmed to 0x19. When the short condition is removed, the register value correctly reads back as 0x01.

    Regarding your comment on the VSS supply current limitation, could you help elaborate on your concern? Currently, the VSS supply is limited to a maximum of 200 mA and is shared across four negative DAC devices.

    As a test, could you try reducing the current limit to below 50 mA and repeat the experiment? This may help us better understand the impact of the VSS supply behaviour under these conditions.


    Regards,

    Mufeed

  • 0 in reply to Muhammed Mufeed
    TI__Genius 17120 points

    Hi Mufeed,

    My apologies for the long wait time.

    I'm currently at a conference so I won't be able to test this configuration until next week. The idea that this could be caused by the short appearing at startup is an interesting one - In general, the device defaults to "Startup current mode" which is around 15mA, so ideally you would see very little current during startup, and then the registers will be populated with your programmed values (0x01 in this case). 

    Another thing to confirm, do you know if you are reading from the correct page? If the device is set, say, to page 2, you'd be reading back from a different register. 

    Thank you for the additional information though, I'll look into this more when I get back (or get a different engineer to do so while I'm gone.)

    Thanks,
    Erin

  • 0 in reply to Erin Bowrie
    TI__Genius 17120 points

    Hi Mufeed,

    Back in office so I can support this more easily. I tried adding the short on the EVM and then powering up the device, but I don't see the same result. When powering on the device, my VSS supply shorts to around 50mA, which is expected with the default 40mA condition.

    Something important to note. If you are shorting the gate to GROUND, the device is in Sink current mode (DACs more negative than gate short). This is limited to 40mA across both current output ranges. This means that if you are seeing an increase in current, something else is going on (such as, your gate is shorting to VSS somehow and pulling the DACs lower). 

    Thanks,
    Erin

  • 0
    Prodigy 20 points

    Hi Erin,

    In our design, the same LDO supplies VSS to four AM7904 devices, and the LDO Power good is connected to the reset pins of all four AM7904 devices.

    The LDO output is -9.09V, and the maximum output current is 200mA.

    A faulty PA is showing gate impedance as a few Ohms instead of Mega Ohms. We are seeing variation in the VSS supply, but the Power Good is stable.





    Regards,

    Mufeed

  • 0 in reply to Muhammed Mufeed
    TI__Genius 17120 points

    Hi Mufeed,

    Unfortunately, your images didn't seem to save. 

    If your PA is showing low impedance, it would make sense that the gate is almost shorted to ground. In this case, you should only see ~40mA of current from this gate.

    Thanks,
    Erin