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I am trying to build a voltage modulation circuit using an adjustable LDO and a uC switchable resistor network (to provide different Vout levels). I need the circuit to provide 4.9, 5.1 and 5.3V at the output depending on the resistor network set up by the uC.

I started the design using the NCP694 LDO; however I am noticing that the output capacitor in the circuit (2.2uF) is affecting my transient response and as such I can not modulate the voltage at the desired frequency (2kHz).

I have looked around, and all the LDO's I found needed at least a 1uF output cap, which I am thinking might be too high for my application. Do you happen to know of an LDO with a lower output cap requirement? or alternatively, is there a better solution for achieving the voltage modulation scheme that I need?

Thank you in advance;

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    \$\begingroup\$ The point of an LDO is to produce a constant, unchanging output voltage. If you want to make a voltage signal that changes at 2 kHz you should not use an LDO. It would be a good question for the site how to do that. When you ask, be sure to specify just what kind of signals you want to produce, and what kind of load you need to drive. \$\endgroup\$
    – The Photon
    Commented Nov 28, 2012 at 17:20
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    \$\begingroup\$ Any design needs to define input voltage and current or impedance with same for output. The Function being 2KHz modulation without a load impedance is not a spec. \$\endgroup\$
    – D.A.S.
    Commented Nov 28, 2012 at 18:37
  • \$\begingroup\$ I think that in DIY "exploratory electronics" you don't always have a precise spec. The circuit is already built and the load is what it is. Basically I think the next steps would be to try lower capacitor values until it either works or breaks due to unwanted oscillation. The data sheet calls for 4.7 uF, so it's already violated anyway. Since the aim of the device is to produce a ruler-flat voltage, all recommendations in the data sheet are toward that aim. So, the data sheet has to go out the window if that part is to be used. \$\endgroup\$
    – Kaz
    Commented Nov 28, 2012 at 18:55
  • \$\begingroup\$ The input voltage into the LDO is 6.5V, and the nominal load impedance is 780 ohms. I am not restricted to using the NCP694, it just happened to be the part I had on hand when I started the design. A part of my question is whether there's a more suitable LDO for my need \$\endgroup\$
    – EagerToLearn
    Commented Nov 28, 2012 at 19:00
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    \$\begingroup\$ At your max output voltage of 5.3V, you're less than 7mA into the load. There are lots of op amps out there than can push 7mA. Add a zener voltage reference and some controllable gain resistors and you've got your high bandwidth supply. \$\endgroup\$
    – HikeOnPast
    Commented Nov 28, 2012 at 19:54

3 Answers 3

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I recently built a device that included a precision controllable voltage supply. It used the op amp technique suggested by DeanB above.

In our configuration, the op amp gain was set at unity, driven from a DAC. The feedback was taken from a separate wire run to the target circuit; this lets the amplifier correct for the voltage drop in the power output wire.

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  • \$\begingroup\$ Thanks for your insight. I attempted building a precision controllable voltage supply and managed to get a constant output voltage of 5V; however the transient response characteristics of the precision controllable supply did not match my requirements (switching between 4V9 5V1 and 5V3 at 2kHz). \$\endgroup\$
    – EagerToLearn
    Commented Dec 11, 2012 at 19:10
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Checkout Class AB amplifier (or any other audio amp topologies):

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page3.html

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The requirements for my design changed to needing to switch between 5V0 and 5V2 and 2kHz (instead of 4V9, 5V1 and 5V3). Given the new spec, modulation can be achieved at 2kHz with the NCP694 and a 2.2uF output cap. I understand that an LDO is not meant to be used for modulating a voltage signal, but my design somewhat forced me to use an LDO in this non-conventional way. Thanks for all of you who replied to my question.

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