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DAC8750: The value of Rload

Y.Ottey
Mastermind 6145 points
Part Number: DAC8750

Dear all,

I would like to ask about Rload in the following figure.

I have been asked by our customers how much Rload is acceptable.

The data sheet does not specifically describe the range of Rload, and only Rload = 300Ω is shown in the reference data.

What is the maximum allowable Rload?

Best Regards,

Y.Ottey

  • 0
    TI__Mastermind 46290 points

    Hello,

    Current sources express maximum load restrictions via a specification called compliance voltage, which for the DAC8750 is 2V. This means that the maximum voltage developed at the load when full-scale current is flowing through the load must be less than AVDD-2V.

    So, for example, with a 24V AVDD and 20mA full-scale current, the maximum load impedance would be 1100 ohms (22V / 20mA).

  • 0 in reply to Kevin Duke
    Mastermind 6145 points

    Dear Kevin

    Thank you for your reply.

    Let me check two points.
    1. Is it possible to recognize that the "load impedance" you wrote is only Rload in the following diagram?

    2. Our customers are considering AVDD above 17V. If the maximum output current is 20mA at 17V, is the maximum allowable value of Rload as follows?

    Maximum output voltage: AVDD-2 (v) = 17-2 = 15V
    Maximum allowable value of Rload: 15 (V) / 20mA = 750Ω

    3. According to the calculation above, Rload thinks that 600Ω can be used. Is it correct?

    Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Mastermind 46290 points

    Hello,

    The specification in the datasheet speaks only to the characteristics of the internal components where the value of the gain setting resistors in the high-side current source current mirror are known and the characteristics of the output PMOS are known. When using external components, this will depend on the selected BJT and the value of the 20 ohm resistor (still internally you will pass through the gain setting resistor as the BOOST pin is basically in between this resistor and the PMOS). Most likely, though, there will be very little impact vs the datasheet specification.

    I think you have plenty of margin for 600 ohms.

  • 0 in reply to Kevin Duke
    Mastermind 6145 points

    Dear Duke

    Thank you for your reply.

    ”I think you have plenty of margin for 600 ohms."

    Is it OK to recognize that 60Ω mentioned in the above sentence is the value of Rload?

    Also, does "margin" mentioned in the above sentence mean that there is a margin compared to the maximum value of

    Rload (750Ω obtained from our customers' conditions)?

    Best Regards,

    Y.Ottey

  • 0 in reply to Y.Ottey
    TI__Mastermind 46290 points

    Hello,

    I guess the mention of "60Ω" was a typo, meant to be 600Ω. In that case, yes I am referring to a 600Ω load that does not include any of the optional boost circuit.

    When I said there is margin for 600Ω, I meant that between guard-bands in the specification and the small size of the resistor in series with the boost circuit that you would have no problem driving 600Ω, not necessarily that there is so much margin that you could extend the load by 150Ω. Most likely it would work on most units, but I would suggest adding some additional headroom to AVDD if possible. Even 1V would be helpful.