Thursday, December 18, 2014

Low-dropout Voltage Regulator

Good afternoon everyone, hope you guys are having a great time. This post will be about a regulator, a linear one, although I have posted about linear voltage regulators earlier, this one is a bit different, it is a low-dropout voltage regulator. Let's dive in and see what it is. In this post I will not be explaining how it works, will talk about that in a later post.


What is a Low-dropout Voltage Regulator?

A low-dropout voltage regulator is such a regulator that can work at a very small input-output differential voltage, that means this voltage regulators can regulate the output voltage even when the output voltage is very close to the input voltage. If you use a standard linear regulator(for example a 78XX), you need a minimum difference between input and output voltage of about 2V, so if you want 5V, you have to provide minimum 7V as its input. If input goes below that voltage the regulation will no longer work. This 2V margin is called dropout voltage, you can find it in the datasheet of respective element. Low-dropout voltage regulators have much lower dropout voltage about 0.5V-1.5V.

So why do we need this?

As for the linear voltage regulators, the lower the difference between input and output the higher the efficiency it will have because the power dissipation of the device is usually the difference between input and output voltage multiplied by the current its supplying, so low-dropout regulators allows you to input a much lower voltage than that of a normal regulator allowing much higher efficiency, lower heat generation and of-course lower minimum operating voltage.

What do we need?

In this post I will be writing about three different low-dropout voltage regulators. First one is a fixed 5V output regulator which has a 0.5V dropout and can provide 1A of current. This is ideal for many small projects that does not require much current. This integrated circuit can provide other voltage levels too but I will write about the 5V one right now.

Components needed for this:

1. Integrated Circuit : LM2940
2. Capacitor : 0.47uF at the input to suppress noise, 100uF electrolytic at the output to maintain stability, low ESR(Equivalent Series Resistance) capacitor preferred, 0.1ohm-1ohm capacitors.
3, Heat sink and thermal interface material for LM2940 if necessary.


Low dropout voltage regulator using LM2940

Pretty simple diagram, not much to talk about. Just for the capacitors 0.47uF one is needed if this circuit is located far from the power supply unit itself and you can put a lower rated capacitor at the output(although minimum 22uF is needed) but a 100uF will just work fine. I have used this 100uF one for better transient response.

Great thing about this integrated circuit is it has built in short circuit current protection and over-voltage and over-temperature shutdown feature.

Next I want to write about a variable voltage one which can provide an output of 5V-20V with a 1A of output current and has the similar 0.5V dropout like the LM2940.

Components needed for this:

1. Integrated circuit: LM2941.
2. SPST switch(single pole single throw).
3. Capacitor: 0.47uF(ceramic type), 100uF low ESR electrolytic(Same as above)
4. Resistor: Depending on voltage. It is good to use 1Kohm resistor as R1 to ensure bias current error from adjust pin will be negligible and none of these resistors should be shorted.
An 18Kohm resistor is needed for the ON/OFF pin.
5. Heat-sink and thermal interface material of LM2941 if necessary.


Variable Low dropout voltage regulator with LM2941

This one is also another simple diagram which can be used to build a variable voltage output. Here the R1 and R2 sets up the output voltage as shown by the formula. Here 1.275V is the typical reference voltage of this integrated circuit. For example a resistor value of R1=1kohm and R2=8.6Kohm will provide an output of 12.24V.

For the capacitor, it is same as above. 

For the On/Off switch, as this integrated circuit does not have internal pull up resistors, an external pull up resistor is needed for proper shut down. If you want to omit this function you can just connect the ON/OFF pin directly to the ground so that the circuit will always run.

Next and the last one is another very simple diagram that can provide a 5V output and very popular as SMD(Surface mount device-SOT223 package). In this diagram I will using a LM1117-5.0 which can provide 5V, although it is available at other voltages and also variable type and about packaging it is also available in TO-220 package. It can provide around 1A and has a dropout voltage of 1.2V.

Components needed: 

1. Integrated Circuit: LM1117-5.0
2. Capacitor: Two 22uF electrolytic with low equivalent series resistance. Higher capacitance will provide better transient response.
3. Diode: IN4007 as a protection diode.
4. Heat-sink and thermal interface material if needed.


Diagram Using LM1117 5.0

This one is a very simple diagram too. About the capacitors it is same as above and capacitor with higher capacitance can be used. 

About the protection diode, it is optional, in normal condition it is not needed however with large capacitor and the input and ground shorted that capacitor will be discharged through the output pin that might damage the integrated circuit, in that case a protection diode is needed.

This integrated circuit is widely used in Arduino Uno or similar development boards, In this picture of this arduino board, you can see that the 1117-5.0 (marked in red square), the input and output 47uF low ESR solid capacitor(marked in black square) and the protective diode M7(marked in white square, It is similar to 4007).

Arduino Uno

So that is pretty much about Linear Low-dropout voltage regulators, hope you have enjoyed reading.

More Resource:

1. Understand low-dropout voltage regulator.
2. LM2940 datasheet.
3. Wikipedia entry about low-dropout voltage regulator.
4. Linear voltage regulator
5. LM2941 datasheet.
6. LM1117 datasheet.
7. What is ESR.
8. Solid Capacitor.
9. Learn about Arduino.

You can read my other posts here!
Happy Experimenting!

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