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Low Dropout Adjustable Regulator Circuit

Low Dropout Adjustable Regulator Circuit
Low Dropout Adjustable Regulator Circuit

We frequently acquire requests for voltage regulators for systems for which the accepted LM317 is not suitable. Although the LM317 can acquire ascribe voltages up to 3-40 Volts, and achievement adapted voltages from 1.2-37 Volts, it can alone achievement a reliable anchored voltage if the ascribe voltage is at atomic 2-3 Volts college than the called achievement voltage - e.g. to get a reliable 12.0V output, the ascribe voltage would charge to be 15V+. This voltage aberration is the dropout, the bulk of voltage alone (dissipated/wasted as heat) in the regulator.

Low Dropout Anchored Voltage Regulators

Where a anchored voltage regulator is required, article like the LM2940 ambit can be used. These are low dropout voltage regulators about accessible with anchored achievement voltages of 5, 10, 12, and 15 Volts.

Such a regulator is complete for use for archetype area 12V rated LED spotlights are to be powered with a 12V battery. The array voltage can ambit from 12.5 to 14+ Volts, and we charge a anchored 12.0V to ability the LEDs, so although an LM317 would not be reliable, the LM2940 (which will bead no added than 0.5 Volts) would accord a anchored 12.0V achievement beyond the abounding voltage ambit of the array reliably.

Unfortunatately we cannot use an LM2940 area a non-standard voltage - e.g. 8.50 Volts - is required. Ideally we appetite article with the adaptability of the LM317, but the low dropout characteristics of the LM2950. Fortunately such a basic exists and it is the LM2941 adjustable low bead out voltage regulator (which is pictured above).

The LM2941 can achievement voltages from 5.0 to 15.0 Volts * with a archetypal dropout voltage of aloof 0.5 Volts (@ 1 Amp, beneath for lower currents), and an complete best dropout of 1.0 Volts beyond the abounding temperature range. The LM2941 can accumulation up to 1 Amp of current.

* Note that the best ascribe voltage is 26 Volts, so this basic is not acceptable for 24 Volt rated systems.

Pictured aloft is the archetypal appliance ambit application the LM2941 as an adjustable voltage regulator. The advertence voltage (measured beyond ADJ and GND) is 1.275V. With the resistor R1 about set to 1K, the amount of R2 for any accustomed adapted achievement voltage is affected application the afterward equation:

R2 = R1 (( Vout / Vref) - 1 ))

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

Turning the blueprint about we can additionally account the achievement voltage for any accustomed ethics of R1 and R2:

Vout = Vref * ((R1 + R2) / R1) if we accumulate R1 as 1K and set R1 as 5K1 (since the 5276 Ohm amount affected is acutely not a accepted resistor) we see that Vout = 1.275 ((1000 + 5100) / 1000) = 7.78 Volts. (Note that it is accessible to affix accepted resistors in alternation abacus up their ethics to get an exact amount if a actual authentic voltage regulator is appropriate - e.g. 4K7 + 560R + 10R + 5R6 = 5276 Ohms for an 8.00V regulator)

Click actuality to appearance the LM2941 Datasheet (PDF) with sample circuits, and abundant blueprint data.

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