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I have an opamp configured as a constant current sink like this:

enter image description here

As you can see, the reference voltage for the SENSE pin is set by the voltage divider on the left. This requires a stable voltage on VCC - if it varies, so will the set voltage and thus the current passing through Q1.

I'd like to be able to run the opamp off a supercap or unregulated battery as a power supply, which means VCC won't be entirely stable. Is there an alternative topology for the opamp that achieves the same result without requiring a stable voltage source, or the introduction of an LDO or voltage reference?

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No. There has to be some reference. IF you find a standalone current sink part, it has the reference built in.

The LM10 is a good choice for your circuit: it has a low-voltage reference built-in, and it will work over a wide supply range.

A modern alternative to LM10 is TL103 or TSM102.

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  • \$\begingroup\$ The LM10 looks brilliant - I hadn't figured on finding an opamp with built in voltage reference! It's expensive, though. \$\endgroup\$ – Nick Johnson Feb 21 '13 at 17:26
  • \$\begingroup\$ Correct me if I'm mistaken, though, but I'll need to use both opamps in the LM10 to implement the circuit above - the reference amp alone doesn't appear to be enough. \$\endgroup\$ – Nick Johnson Feb 21 '13 at 17:30
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    \$\begingroup\$ @NickJohnson You don't have to use a LM10.. There is always TL431! I love the modesty of TL431 and it amazes me how much you can do with it :) \$\endgroup\$ – abdullah kahraman Feb 21 '13 at 17:33
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    \$\begingroup\$ @abdullahkahraman I was considering the TL431, but it draws more current than I'd like. Given I need stability more than voltage precision, I think I'd be better with a low quiescent current regulator in that case instead. \$\endgroup\$ – Nick Johnson Feb 21 '13 at 17:35
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    \$\begingroup\$ TLV431 is the low-power variant of TL431. \$\endgroup\$ – markrages Feb 21 '13 at 18:30
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I understand your concern with current draw since you are planning to supply power from supercap, BUT if you just bridge 10K pot with schottky diode (you work with pretty low voltage there), that will greatly increase current stability of your sink without (with minimal) adding to the current consumption. Even better solution from stability point of view is with low power version of TL431 (tlv431 as markrages pointed out) that one needs 50uA to lock in stable region (ok add a bit to get away from boundary condition) but considering your use of MCP6001/2 that draws 100uA, that should be of no concern, you should source opamp with even lower quiescent current (and you can easily go down to 1/10 of existing MCP600x draw down even to sub uA if money is of n concern). Also you might consider going to larger sense resistor or extra devider in case of voltage reference since one recomended has Vref1.2V whish i 10x with what you work with. In the case you use lower quiescent current opamp, use diode solution.

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