# Converting a 5 Volt controlled current source to 3.3V compatible

I want to create a relatively simple voltage controlled current source with a 3.3V micro controller. I have found THIS circuit and it seems to be what I need. The problem is it is designed for 0 to 5V.

So here are my questions and my research so far:

Question 1) Do you think a simpler solution exists (fewer parts, updated parts, etc.)?

Research 1) I can go with a single chip solution, something like LT3092 But, is that chip can be controlled by a microcontroller?

Question 2) What should I change in the schematic to make it 3.3V?

Research 2) In original circuit, 0V gives 0A and 5V give 100mA, so if I do not change anything in this schematic, 3.3V will give around 66mA. I guess it has to do with the pulldown resistor at the first input which forms a voltage divider?

Question 3) What should be changed if required maximum current is 200mA?

Research 3) It will be probably the internal resistance of the PFET at the output that limits the current, right? then, will I need more voltage at its input or I should change it with another PFET?

I am asking these questions to learn how the original circuit works, e.g.:

• What is the role of those 100pF capacitors?
• Why are the feedback resistors 10k?
• 1. yes; 2. use the solution from 1; 3. nothing, 1. already solves that. Commented Aug 20, 2017 at 13:17
• But seriously, if an input is labeled "0–5V", then quite obviously, "0–3.3V", being a sub-interval of that, works, too. Commented Aug 20, 2017 at 13:19
• I personally also refuse to comment on schematics that you've found somewhere – if you have a concrete question about the workings, then ask that, based on your understanding of the circuit. But I don't think it's fair to expect us to explain someone else's (rather complex) circuit AND improve it. Your third question is pretty close to being a good question, though! I think if you asked something like "is the output MOSFET the only thing limiting the output current", then we'd have a good question, but like this, I'm really not sure where to start :) Commented Aug 20, 2017 at 13:21
• @MarcusMüller lol your comments made my day. I updated my question with some more details...is it worth answering now? Commented Aug 20, 2017 at 15:28
• Can you add designators to the schematic? The ratios of the values of R1, R2, and R5 determine the output current range of this circuit. Now you just have to figure out which resistors I chose to label as R1, R2, and R5. If you had provided a schematic with designators, I would have used your choice. Commented Aug 20, 2017 at 16:00

What should I change in the schematic to make it 3.3V?

What should be changed if required maximum current is 200mA?

The ratio of output current to input voltage is R4 / (R5 * R8).

Aside: Why are R5 and R8 specified as 1% types, but R4 isn't? It looks like an oversight by the original designer (or maybe he figured 4.99 kohms is not available in lower tolerances).

So you can adjust any of these resistors to change the scaling to achieve 200 mA output with 3.3 V input. It's probably easiest to reduce R5, since that's least likely to cause saturation of the circuit where it didn't happen before. But you might find that the available standard resistor values require you to also slightly adjust the other resistors to get exactly the range you want.

You should also verify that the output MOSFET and its heatsink are able to handle the higher thermal demand if driving 200 mA into the minimum possible output load (a short circuit). A larger heatsink or a beefier FET may be required.

Edit: A very brief read of the IRFD9024 datasheet says that it is usable up to 1 W with adequate heat-sinking. In this circuit, 200 mA into a short circuit load would require the output FET to burn 1.6 W. Increasing R8 could slightly reduce this at the expense of lowering the output compliance voltage, but you'd still want to have some margin in the thermal capability of the output device. So you will want a new FET for the higher-current design.

Research 1) I can go with a single chip solution, something like LT3092 But, is that chip can be controlled by a microcontroller? <<<

Why not give a try to LTC3623 ?

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Or, considering that you'll use a microcontroller, maybe it would make sense to step back and rethink the concept. You could get a high side current sense amplifier like TSC101. Get it to the microcontroller's A/D converter.

Now what you need is just a high-side power thing fed by your microcontroller's PWM.

• Thats too complex for my design I need it to be DIL or QFP so I can solder it Commented Aug 20, 2017 at 18:12