How can a microcontroller (such as an Arduino Uno) that requires 7-21V input voltage be powered via USB-B which can only run 5V?

Question

I am messing around with battery supplies and feel kind of dumb for this question. The specification sheets for a lot of these microcontrollers say they require at least 7V to power via the input voltage. If I were running one of these devices on my laptop via USB then the microcontroller would only be getting 5V (unless that is also incorrect.) I'm assuming there must be some sort of boost converter that amplifies the 5V from the USB. Or is it a stepdown converter from Vin. Just curious.

Answer 1

An arduino uno is not a microcontroller; it's a dev board that has a microcontroller on it, along with a bunch of supporting components. One of those supporting components is a device called a voltage regulator, which can take a wide range of input voltages and output a steady 5 V (which is the voltage required by the ATmega328p, the microcontroller on the uno). The type of regulator on the uno, which I believe is either a *7805 or a *1117 type, is a relatively old variety that requires an input voltage several volts higher than its output in order to work.

When powered by USB, however, a nicely regulated 5 V supply is already present (as required by the USB standard), so the regulator is bypassed and the microcontroller is powered directly by bus power.

Answer 2

You should study the Arduino Uno reference design:

The MCU chips themselves run from 5.0V. That can be supplied either from PWRIN (which requires a bit more than 5V because of the drop of the regulator and of the reverse voltage protection diode D1), or directly from the USBVCC line.

Switching takes place via T1 (a P-channel MOSFET) and U1A (an inexpensive op-amp used as a comparator). There's also a 3.3V regulator, which is provided as an output and only used on the board to compare Vin/2 so if Vin > about 6.6V then Vin is used..

Answer 3

The voltage on the supply rail that supplies the microcontroller that is the processor on the specific model of Arduino board that you have is 5V (thank you Mr gerber) but this voltage can come either from the USB input, which is tightly regulated to within a quarter-volt of 5V or from the onboard linear regulator. The Uno uses an AMS1117 (but I'm also seeing the ONSemi logo on some pictures so probably they are going with whatever is available and cheap) regulator capable of supporting an output of up to an amp.

The regulator also supplies 5V to the supply rail but its dropout voltage - the minimum difference between the input and output - requires that the regulator is fed by at least 7V. The spec for the AMS1117 is 1.0-1.3V so you could probably go a bit lower than 7V but not by much. On the upper end, the maximum input voltage could be set by the limitations of the regulator itself; different models of the 1117 max out at 18 or 20V so I'm not sure where the 21V number came from but it's kind of a moot point since you will probably run into a limit set by the amount of heat generated.

Linear regulators act as self-adjusting resistors to dissipate energy and drop the input voltage to the output voltage. This means that they are notoriously inefficient and create large amounts of heat that must be dissipated in order not to burn up the chip. With a 21V input, the regulator is less than 24% efficient (5/21); the remaining 76% of the input power is turned into heat. Switching regulators are much more efficient (>90%) but they are more expensive.

Answer 4

Your board has multiple input supply options. There is no one thing that you call an "Arduino" - depending on who made the clone or compatible board it it might work at 2.5V or any other voltage.

So you have an "Arduino" board, not an "Arduino" microcontroller. The board does contain a microcontroller of some sort, which eventually gets powered with some voltage made onboard - and usually it's not the voltage you feed in but less.

It has an onboard voltage regulator that makes 5V from the 7V or more you feed into it. Usually such Arduinos have a linear regulator so they don't even work well with 5V from the external input.

Then it will also take in 5V directly from the USB 5V. This is generally ORed with a diode or FET to the onboard MCU supply.

Answer 5

This complements other answers.

The Arduino "family" input voltage range is determined by the internal voltage regulator. This provides 5V to the processor in most systems and 3V3 in some systems.

When operated from a USB power source the internal regulator is bypassed and the processor is operate directly from the USB "5V" line or via a Schottky diode. The diode further decreases the voltage but ensures tat current is not back-fed to the USB power line when an eternal power supply is simultaneously connected to the system. The end result is a processor voltage of around 4.5V

The ATMEGA328P, used in many Arduinos, operates from2.7V to 5.5V. See eg page 268 of this datasheet.

So the about 4.5V is adequate to operate the processor.
Some earlier AVR processors would operate at 16 MHz (and maybe 20 MHz) at Vcc=5V but only at 8 MHz at Vcc = 3.3 V. At Vcc ~= 4.5V they would probably function at 16 mHz "in many cases".

SOME USB interface ICs provide regulated voltages above and below their supply voltage, but the currents available are usually not adequate to operate the whole system.

The LGT8F328 processor is an AVT ARMega328P close-clone/superset processor and will operate at 20 MHz at Vcc=1.8V. (I use it with no problems).

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This diagram is a subset of Spehro's diagram (which I copied it from) with the two portions moved together for compactness.
The following says about the same as Spehro said but put somewhat differently. (Vote his answer up if this helps).

• Vin from a power supply is applied at A and E.

• When Vin is present at A it is used to turn the FET OFF at B to disable the USB power input (at C).

• When USB voltage is present at C AND Vin is not present the FET at B is turned on and Vusb is connected via the FET to supply 5V to the system. . I mentioned a Schottky diode being used in this role - that does not require U1A or FET T1 or several resistors BUT drops maybe 0.2 - 0.3 V, whereas the FET drops well under 0.V when on.

• If Vusb is not present and Vin is applied at E (and at A) then requlators IC1 and IC2 are supplied via diode D1, producing 5V at F (and at D). The diode drop is unimportant here as, unlike the USB supply, excess voltage is available.

• The 5V at D (whether from USB or Vin + regulator) is applied to regulator U2 to produce 3V3.