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REF6250: Output Current Limit

Mehmet Karaaslan
Prodigy 40 points
Part Number: REF6250
Other Parts Discussed in Thread: TL431LI, LM334

Hi,

I work on a current sensitive circuit and I'd like use the current limiting feature of REF6250. It's stated as +/- 3mA. On page 22 of D/S, two equations given without any units:
Isc=80e-9*Rss+3e-3              - eqn 4
IL=23.75e-9*Rss+0.15e-3     - eqn 8

I understand IL is the recommended current the device will operate safely, and Isc is the current level the device will "hard-limit" the output to. I presume the hard limit can't be changed.

Will you please clarify the units in the equations? The aim is to operate the device at 2mA. What it the correct value of Rss?

I intend to use this REF as the current source of an NPN, not as an ADC reference.

Thank you.
Mehmet

  • 0
    TI__Mastermind 19370 points

    Hi Mehmet,

    The equations are below.

    Isc = 80e-9V * Rss + 3e-3A             - eqn 4
    IL = 23.75e-9V * Rss + 0.15e-3A     - eqn 8

    i would recommend to use a Rss that is large enough to make IL > 2mA. This means Rss > 78k.

  • 0 in reply to Marcoo Z
    Prodigy 40 points

    Thanks Marco.

    So when Rss=78K, IL=2mA and also Isc=9.24mA. It's way above the maximum current the load (a sensitive diode) can handle. 

    What if I short SS pin to ground? In this case, Isc=3mA, IL=0.15mA. I think I'll achieve the hard limit but I'm not so sure about IL. Can I not just draw 2mA from this device anymore?

    The intention is to draw 2mA in operation but no more than 3mA (or 4-5mA tops but not 9mA).

    Hope you can further clarify what exactly Iss and IL do.

    Thank you.

  • 0 in reply to Mehmet Karaaslan
    TI__Mastermind 19370 points

    Hi Mehmet,

    I believe we have a much simpler solution to this problem. If all you need is a way to limit the current to 2mA, we have other solutions. You basically need a current source/sink.

    Will the TL431LI fix and solve this issue?

    Please refer to figure 36 of the datasheet.

    We also have the LM334 family of adjustable current sources/sinks that can do this.