We frequently receive requests for **voltage regulators** for systems for which the common **LM317** is not suitable. Although the LM317 can accept input voltages up to 3-40 Volts, and output **regulated** voltages from 1.2-37 Volts, it can only output a __reliable__ fixed voltage if the input voltage is at least 2-3 Volts higher than the chosen output voltage – e.g. to get a reliable 12.0V output, the input voltage would need to be 15V+. This voltage difference is the *dropout*, the amount of voltage *dropped* (dissipated/wasted as **heat**) in the regulator.

### Low Dropout Fixed Voltage Regulators

Where a fixed **voltage regulator** is required, something like the **LM2940** range can be used. These are ** low dropout voltage regulators** typically available with fixed output voltages of 5, 10, 12, and 15 Volts.

Such a regulator is perfect for use for example where 12V rated LED spotlights are to be powered with a 12V battery. The battery voltage can range from 12.5 to 14+ Volts, and we need a fixed 12.0V to power the LEDs, so although an LM317 would not be reliable, the **LM2940** (which will *drop* no more than 0.5 Volts) would give a **fixed 12.0V** output across the full **voltage range** of the battery reliably.

Unfortunatately we __cannot__ use an **LM2940** where a non-standard voltage – e.g. 8.50 Volts – is required.

### LM2941 Adjustable Low Dropout Voltage Regulators

LM2941C – Top View |

Ideally we want something with the adjustability of the **LM317**, but the low dropout characteristics of the **LM2950**. Fortunately such a component exists and it is the **LM2941 adjustable low drop out voltage regulator** (which is pictured above).

The **LM2941** can output voltages from 5.0 to 15.0 Volts * with a typical dropout voltage of just **0.5 Volts** (@ 1 Amp, less for lower currents), and an absolute maximum dropout of **1.0 Volts** across the full temperature range. The LM2941 can supply up to **1 Amp** of current.

* Note that the maximum input voltage is **26 Volts**, so this component is not suitable for 24 Volt rated systems.

Pictured above is the *typical application* circuit using the **LM2941** as an adjustable **voltage regulator**. The *reference voltage* (measured across ADJ and GND) is **1.275V**. With the resistor *R1* typically set to**1K**, the value of R2 for any given desired **output voltage** is calculated using the following equation:

**R2 = R1 (( Vout / Vref) – 1 ))**

For example, if an 8.00V low voltage drop out regulator is required, R1 = 1K, Vref = 1.275, and so R2 = 1000((8.00/1.275) – 1) = 5276 Ohms.

Turning the equation around we can also calculate the **output voltage** for any given values of R1 and R2:

**Vout = Vref * ((R1 + R2) / R1)**

…so if we keep R1 as 1K and set R1 as 5K1 (since the 5276 Ohm value calculated is obviously not a**standard resistor**) we see that Vout = 1.275 ((1000 + 5100) / 1000) = 7.78 Volts. (Note that it is possible to connect standard resistors in **series** adding up their values to get an exact value if a **very accurate voltage regulator** is required – e.g. 4K7 + 560R + 10R + 5R6 = 5276 Ohms for an 8.00V regulator)

Click here to view the **LM2941 Datasheet** (PDF) with **sample circuits**, and detailed specification data.

### Making a User-Adjustable Voltage Regulator Circuit

Sometimes it is desirable to make a **voltage regulator** which can have its output voltage **adjusted** __after __it has been put together – e.g. if the device to be powered has to be changed for a unit with a different specification. In order to avoid having to put together an all new voltage regulator, the resistor *R2* can instead be replaced with a **variable resistor** (aka *potentiometer*) such as the one pictured below.

The **LM2941** can be used reliably for output voltages from **5 to 15 Volts**. Setting *Vout* as 5V and then as 15V, we can calculate that (if *R1* is kept at 1K) *R2* must be set as 2,900 Ohms for a 5V output, and 10,700 Ohms for a 15V output. Therefore, if *R2* is replaced with a 3K resistor in series with a 10K **potentiometer**, the user can easily change the output voltage through the full range available (since *R2 *can be set to any value between 3K and 13K).

### Buy LM2941 Voltage Regulator ICs

The **LM2941** is available in *TO-263* and *TO-220-4* packages from Rapid. Click here now to view and buy these **LM2941 Adjustable Low Dropout Voltage Regulators**.

Pictured above is an 8.3 Volt **low dropout voltage regulator** built using an **LM2941**. With R1 set at 1K and R2 set as 5K6, the calculation suggested that the output voltage would be 8.4V, but the resistors used are only **accurate** to +/- 5% leading to the difference. Had we required an exact 8.40V regulator we would have selected higher specification resistors, or measured a selection standard resistors to find those that best matched our requirements.

With a load of 200mA the output voltage of this circuit was stable at 8.30V as long as the input voltage was above 8.45V. When the input voltage fell below 8.45V, the output voltage was equal to input voltage minus 0.15V.

If you need a similar **low voltage dropout regulator circuit** (or an **adjustable** low dropout voltage regulator) then please email neil@reuk.co.uk with details of your exact requirements.