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TPA3121D2: AVCC Current when VCC = 24V

Marti Castany
Prodigy 40 points
Other Parts Discussed in Thread: TPA3121D2

Hi everyone,

I'm developing an audio amplifier system using the TPA3121D2, and we need to implement the AVCC filter.

For choosing the right wattage for the 220 ohm resistor, which is the AVCC current when VCC is 24V ?

Thank you very much.

Martí Castany

  • 0
    Genius 3295 points
    Hi Marti,

    In looking at the block diagram, AVCC "becomes" AVDD (being pedantic, how "Collector-Collector" becomes "Drain-Drain" is beyond me...). If we were to assume that AVDD is "only" powering the internal integrators, and assuming a "good" opamp can draw anywhere between 2-10mA each, we can go through the math - this brings us (in round numbers) between 15-190mW (assuming anything else "in there" isn't drawing gobs of current). A 1/4W resistor should do it. In-circuit testing should be done to verify that this is in fact the case.

    Mike Tripoli
  • 0 in reply to Mike Tripoli
    Prodigy 40 points
    Hi Mike,

    Thank you for your answer. It will be fantastic if someone from TI could confirm your theory. But assuming 10mA, we have 240mW (24V*0.01A) right?

    Regards,
    Marti Castany
  • 0 in reply to Marti Castany
    Genius 3295 points

    Hi Marti,

    It depends on the internal "AVDD" voltage, as well as the "load", neither of which we know. There is mention in the datasheet that AVCC is divided by 8 for internal biasing of the opamps, but I see nothing about the internal regulator voltage. The internal "regulator" is a bit specious as drawn; I suspect there is more to it than a simple linear regulator. The recommended power supply voltage is 10-26V. 26/8=3.25V; not reasonable for a Vin range of -0.3 to 7V (peak, peak-to-peak?).

    Let's make an assumption that the "AVDD" power supply is 24VDC (no regulator). With just a series resistor and cap to ground, there is no DC current flow (ignoring cap leakages, etc.). For arguments sake, we'll treat the load as purely resistive (it is not, it is dynamic, though each each sub-circuit i.e. "opamps" will have a quiescent current, which is what we're talking about here). 

    If each "opamp" as shown in the block diagram draws 2mA, that's a total of 8mA (again, we don't know for sure how much circuitry is being powered from the AVDD regulator internal to the device, but we're ignoring this). 8mA translates to a 3000Ω resistive load. Current through the 220Ω is the same as through the load giving us ~7.5mA with ~12.5mW dissipation in the 220Ω and ~167mW through the load. Bumping the load to 40mA (600Ω) we get ~29.5mA; ~189mW dissipation in the 220Ω resistor and ~513mW in the device. As such, a 1206 resistor (in my opinion, the minimum size one should use for the signal path -one more for power supply filtering isn't going to hurt anything) is good for 1/4W.

    Now, all of this is academic; we have no idea what the AVDD voltage is - without that we can't "calculate" anything. I'm assuming you already have a working circuit, EVM, etc. If it were me, I'd put a resistor in, say 1/2W, and cap, and take some measurements. Start with the system muted, some number of arbitrary points at different volume levels and "flat out" maxed. Know that your "static frequency" test results are going to be wildly different than playing music (playing "music" through a system uses far less power than people realize). The resulting graph will tell you far more than a single data point in a datasheet. 

    Mike Tripoli