Mixed voltages best practices? - Page 1
Forum section: electronics / general

author: Chris3D
author_rank: Newbie
timestamp: September 28, 2019, 04:22:11 pm
content: Hi everyone, have a question on the best approach (if one exists) for mixed voltages. I'm starting on a project that needs 5 volts (for an addressable RGB LED), 3.3 volts for a sensor, and uses an MCU that can take either. I'm currently thinking the MCU and sensor will be 3.3 volts to remove the need, and components, for i2C level shifting, and a small charge pump to provide the LED with 5 volts. Any reason to do it the other way around? 5 volts for the LED and MCU, and 3.3 for the sensor? ?On the same topic, does anyone know of a single 2 output regulator that can step up/down to these voltages from a single lipo cell? I've searched Digikey and can't find anything. It seems like these would be common enough voltages for a dedicated regulator.Thanks in advance.

author: fourfathom
author_rank: Super Contributor
timestamp: September 28, 2019, 05:29:01 pm
content: If the sensor is low-power, it would probably be simpler to run the MCU (and LED) on 5V and use a small linear regulator to drop the 5V down to 3.3V. This assumes:* The MCU can accept 3.3V logic levels on the I2C pins, and has open-drain output stage. In this case, use pull-up resistors to 3.3V This arrangement is pretty common.* It's easy to get the 5V supply.

author: jealcuna
author_rank: Contributor
timestamp: September 28, 2019, 05:37:05 pm
content: Pretty simple,Use 3.3v for mcu and sensor, 5v for rgb leds. In order to drive rgb leds, you can use transistor that will take 5v to supply leds and you probably can play with resistor to achieve the brightness desired.It is better if you supply the circuit with 5v and then reduce to 3.3v with DC-DC buck converter to get the best efficiency. If you look to reduce cost, and current consumption is not a big problem, you can use LDO regulator instead of DC-DC buck since the current consumption expected from mcu and sensor should be less than 100mA in total as worst case, therefore the thermal loss in LDO is not significant.I f you decide to use dc dc regulator and your sensor is very sensitive to ripple voltage, probably is a good idea to use ferrite in combination with capacitors and inductance few millimeters to pin supply of sensor. Other idea is taking from 5v supply voltage and reduce to 3.3v using ldo with ultra low noise rejection only for the sensor supply voltage, and still using DC-DC buck for mcu, this could cost more.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 28, 2019, 06:07:05 pm
content: How much current does the sensor draw? If the current is low enough, it might be possible to feed it with a simple voltage divider composed of a resistor connected to the 5V power rail, joined to a 3.3V zener connected to ground. This is cheap and simple, but it's not capable of delivering much current, nor is it terribly efficient. But it might be enough...There are multiple ways of handling this, and choosing the "right" one is an exercise in deciding what you want to optimize. Power efficiency? Parts count? Parts cost? Board area? Ripple and noise on the voltage rails?

author: Chris3D
author_rank: Newbie
timestamp: September 28, 2019, 06:32:58 pm
content: Thanks for the responses!The sensor is pretty low current, it's a small baro, one of the LPSxx from ST, so current consumption is in the micro amps range.On a similar project, I've powered the MCU @ 5v and dropped it to 3.3 just for the sensor, but I had multiple devices that needed 5 volts. For this project, the LED is the only 5v device and I'm trying to keep the PCB as small as possible, ideally ~ 10mm x 20mm, so the driver is lowest part count/size as opposed to cost. I'll have to experiment to see how reliable a WS2812 type LED is with 3.3v logic... if it's not, then I'm down to saving just one i2C line's worth of level shifting vs. just going with 5v for everything but the sensor and shifting those 2 lines.I also want to read the battery voltage, and having the MCU powered by 5 volts simplifies that as well, so maybe that's the most straight forward way to go. I can reduce part count with transistor/resistor arrays for the sensor's i2C level shifting as opposed to my previously using individual BSS138's and separate pull ups.Any ideas on regulators that take a single lipo cell and boost/reduce to both 3.3 and 5v?Thanks again.