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I am designing simple logic analyzer with SN74LVC16T245DGGR bus transceiver which could be used for 1.2V to 5V logic levels I would like to control supply voltage of input port with DAC, currently TLV5624, but this DAC could deliver only 1mA current and I doubt that it's enough for transceiver so I put LM321 OPAMP but this OPAMP has 1.5V voltage drop so with 5V supply I can get only to 3.5V which doesn't enable me to interface 5V logic.

Is there any single supply OPAMP with lower voltage drop? I would need to get to at least 4.5V. And using some charge pump to get higher supply voltage is not the way for me, because it should be USB powered and I don't want to have high power consumption.

Or would it be better to control feedback of some buck voltage regulator with DAC? It seemed to me as overkill to use voltage regulator for this with all inductors and capacitors and I think that these regulators have high voltage drop as well.

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  • \$\begingroup\$ I'm curious. Why do you want to generate the supply of the logic circuit? You're going to need ground from the target circuit anyway, why not also pull in logic supply? \$\endgroup\$
    – Chintalagiri Shashank
    Jan 29, 2013 at 7:56
  • \$\begingroup\$ That might be valid point, but I didn't want to resort on supply of target device and I wanted to use it as signal generator also, that's why I chose generating my own supply. \$\endgroup\$
    – Bruno Kremel
    Jan 29, 2013 at 11:18
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    \$\begingroup\$ Fair enough. You should be aware of how you're dealing with referencing the signals, though. It's generally not a good idea to use logic signals generated by one power supply to interface with another, especially when they are separate supplies entirely. If the target is powered off but your device is on, you risk turning on the target through the IO lines your device is connected to, which can cause unwanted behavior and also damage over the long term. \$\endgroup\$
    – Chintalagiri Shashank
    Jan 29, 2013 at 11:35
  • \$\begingroup\$ This is after all good idea, I might be as well add some jumper that would make me able to chose from internal or external supply.. \$\endgroup\$
    – Bruno Kremel
    Jan 29, 2013 at 11:41

2 Answers 2

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Maybe you are looking for a rail to rail opamp. These are opamps like any other, but are designed to allow the outputs to get very close to the supply rails. I'd give you examples, but there are thousands. Whatever electronics distributor you prefer should have an entire category for them.

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  • \$\begingroup\$ Thank you. This is exactly what I needed, do you know some pin compatible drop-in replacements for LM321 and LM324? Or where I could find some? \$\endgroup\$
    – Bruno Kremel
    Jan 28, 2013 at 21:43
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    \$\begingroup\$ It's probably worth adding the caution that most rail-to-rail op-amps lose drive capability when the output is near the rail. \$\endgroup\$
    – The Photon
    Jan 28, 2013 at 21:46
  • \$\begingroup\$ @BrunoKremel just searching op-amps for "rail to rail" at mouser turns up 6229 results. Most op-amps have the same pin configuration, so should be compatible with LM321. \$\endgroup\$
    – Phil Frost
    Jan 28, 2013 at 21:47
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    \$\begingroup\$ Have you considered using an LM317 adjustable voltage regulator, with a digital pot to set the output voltage? \$\endgroup\$
    – Scott Seidman
    Jan 28, 2013 at 22:13
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    \$\begingroup\$ TPS77001, for example, is adjustable and supplies up to 50 mA with an advertised 0.035 V drop-out (I haven't tried the part myself). The other thing to check is how much power your regulator will burn when the output voltage is at minimum, but this will also be an issue for an op-amp solution, and the regulator package is more likely to be optimized for thermal dissipation. \$\endgroup\$
    – The Photon
    Jan 28, 2013 at 23:34
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I have ended up with something like this with possibility to bypass internal logic supply.. Using LDO regulator from TI and digital pot: enter image description here

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