# Can I use the VIN line of a switching LED driver after its sense resistor for powering an MCU?

I have a circuit where I use a PT4115 LED driver for lighting a 3W LED at 500mA.

The datasheet for the PT4115 LED driver is here.

The normal configuration for the PT4115 driving an LED is below:

I need to put the LED and an MCU away from the battery and LED driver, and I need to use a 4 pin 1 meter long USB cable to connect the LED and MCU to the battery and driver. I need to use this four wire cable. There´s no other option of using a cable with more wires inside. It has to be a four wire cable for this design.

I need a VIN line (for the MCU and LED+,) GND line, an output line from the MCU (to turn on, off and PWM the LED) and an LED- line, like this schematic below:

I want to know if I use the VIN line from the LED+, after it passes through the sense resistor (Rs), which limits the led current, I could supply an MCU and the LED+ altogether. The MCU would use the VIN and GND, and the LED would use VIN and LED-.

I know the sense resistor (Rs) would limit the LED (and then the MCU current) so I would need to calculate the current needed for the LED and for the MCU altogether and calculate a proper sense resistor to keep the LED at 500mA, because the MCU would draw some more milliamps. Does this make sense? Would it be possible to be done?

I made a small quick protoboard test and it seems to work, but I want to know if it´s safe to supply both the LED and the MCU using the same VIN branch (after the sense resistor Rs.)

Would my idea work?

Yes, this will work if you take one more detail into account.

Your microcontroller board has smoothing capacitors which will mess with the LED driver's current regulation scheme if they're not properly isolated from the LED driver's current sense pin. To do this, you can add a 2.2 Ohm resistor in series, as well as a 10µF capacitor to ground, like so:

simulate this circuit – Schematic created using CircuitLab

The resistor has to be about an order of magnitude bigger than the LED driver's current sense resistor (so it doesn't influence the current sensing) and the capacitor has to be sized so that it forms a low-pass with a corner frequency of a couple kHz to prevent excessive ripple from reaching the MCU board.

It'd likely be easier to just place the LED driver next to the LED and MCU though - that way you'd just need two wires: power and ground.

Note also that routing the LED driver's SW node through a cable is going to be an electromagnetic interference nightmare. Don't do that in a product that you actually want to sell. It's likely fine for a hobby project, though.

• Hi! The R1 (and C1) would be put after the sense resistor and the led +, or between the sense resistor and the led +? In my older prototype I'm using for some months the SW node through a cable, with no problems.. What kind of problems related to EMI could arise? Dec 8, 2022 at 11:12
• The 2.2 Ohm resistor should not be in series with the LED. I've updated the schematic to make it clearer. The EMI problems I'm talking about can arise when other electronic devices are put near your cable. The switching transients on the SW node cause interference that might affect other devices. As an example, a phone touchscreen will likely malfunction when it gets near the SW node wire. Dec 8, 2022 at 14:25