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Low-dropout current source

Borge Strand-Bergesen
Intellectual 710 points

Other Parts Discussed in Thread: TPS717

I'm looking for an LDO based current source to bias a shunt regulator. There isn't much headroom in the application so I need to make a circuit which works as a high-impedance current generator with even small voltages across it.That means both low reference voltage and low dropout voltage for an LDO.

My goal is to power the current generator + shunt series circuit with down to 4.5VDC and have the shunt output close to 4V. The current will be in the 100-400mA range.

Thanks,

Borge

  • 0
    TI__Expert 3300 points

    Borge,

    Thanks for your post. As I understand, you need a 4V, 400mA LDO. The biasing resistor for your shunt regulator will control the amount of current you draw from this LDO. Please take a look at TPS73601 as it is low drop out LDO and may serve your purpose well. Please let me know if you have any further questions.

    Regards,

    Kartik

  • 0 in reply to Kartik Sinha
    Intellectual 710 points

    Thanks Kartik,

    Actually, what I believe I need for a low drop-out current generator is an LDO where the sum of dropout and reference voltage is at a minimum. The regulator you suggest looks good but with a 1.2V reference voltage, the series drop in the current output will become too large.

    Please let me know if you can suggest a similar device with a low reference voltage.

    Regards,

    Borge

  • 0 in reply to Borge Strand-Bergesen
    TI__Expert 3300 points

    Hi Borge,

    Can you please share a schematic of what you are trying to implement? The LDO will only source as much current as is pulled from it and this is based on the load. The value of the internal reference does not contribute to extra current. Maybe I am not understanding properly, sorry.

    Regards,

    Kartik

  • 0 in reply to Kartik Sinha
    Intellectual 710 points

    Here is a typical schematic of an LDO based current source. My goal is to bias a shunt regulator with a current source, not a pure resistor, and thereby increase PSRR. The input voltage is 4.75-5.25V VBUS. With, say, 4V out of the shunt, 0.75-1.25V will be across its bias, giving a large variation in current. That's where a low-dropout current generator will do a good job.

    http://i.stack.imgur.com/YvyY3.png

    Regards,

    Borge

  • 0 in reply to Borge Strand-Bergesen
    TI__Expert 3300 points

    Hi Borge,

    I understand now. Earlier I thought you were interfacing the Shunt regulator as a way of adjusting the LDO output more accurately. Please take a look at TPS717, the REF output is 0.8V and typical dropout is 170mV.

    Regards,

    Kartik