Voltage Regulator Calculation

Description: Before designing an adjustable voltage regulator into a circuit or performing a redesign, it is essential to calculate the values for two resistors. While this calculation is straightforward, locating the appropriate resistors may present challenges. Fortunately, a technique exists to simplify the process. For most adjustable voltage regulators, such as the LM317 and LM337, the input voltage must be 1.2 to 1.25 volts higher than the desired output voltage. This requirement arises because the voltage at the ADJ (adjust) input is compared internally to a reference voltage of that value. The reference voltage exists across resistor R1, and together with preset resistor R2, it determines the current flowing through the ADJ pin. The output voltage is given by the equation: Vout = VREF [1 + (R2/R1)] + I_ADJ * R2. If I_ADJ is ignored for simplicity, and the reference voltage (1.2 V) is used with R1 set to a value of 1.2 kΩ, the equation simplifies to: R2 = 1000 * (Vout - 1.2). In practice, determining the voltage drop across R2 (output voltage minus reference voltage) directly provides the resistance value in kilo-ohms. For instance, for an output voltage of 5 V, R2 becomes 5 - 1.2 = 3.8 kΩ, which can be achieved by connecting a 3.3 kΩ and a 470 Ω resistor in series. For lower output voltages, smaller resistor values are recommended to ensure sufficient current flow for the voltage regulator to function effectively. A practical approach is to select a value of 120 Ω for R1, leading to R2 being calculated as: R2 = 100 * (Vout - 1.2).

In designing an adjustable voltage regulator circuit, the selection of resistors R1 and R2 is crucial for achieving the desired output voltage. The LM317 and LM337 are common choices for adjustable voltage regulation, with the output voltage dependent on the ratio of these resistors. The reference voltage of 1.2 V ensures that the output voltage can be precisely controlled.

When calculating resistor values, it is important to consider the power ratings of the resistors, especially when dealing with higher output currents. Standard 1/4 W resistors are often sufficient, but for applications requiring higher currents, 1/2 W or 1 W resistors may be necessary to avoid overheating.

Additionally, layout considerations should be taken into account. Keeping R1 and R2 close to the ADJ pin minimizes parasitic resistance and capacitance, which can affect the regulator's performance. Bypass capacitors should also be placed near the input and output terminals of the regulator to filter out noise and stabilize the output voltage.

In summary, the design of an adjustable voltage regulator involves careful selection and calculation of resistor values, consideration of power ratings, and attention to layout to ensure optimal performance.Before you can design an adjustable voltage regulator into your circuit, or do a redesign, you need to calculate the values for two resistors. This is not difficult in itself, but actually finding the right resistors may pose problems. Fortunately a trick is available to make it all much easier. With most adjustable voltage regulators like the LM3 17 and LM337, the input voltage has to be 1. 2 to 1. 25 volts above the desired output voltage. This is because the voltage at the ADJ (adjust) input is internally compared to a reference voltage with that value. The reference voltage always exists across R1. Together with preset R2 it determines the current flowing through the ADJ pin, as follows: Vout = VREF [1+(R2/R1)]+I ADJ R2 If for the sake of convenience we ignore I ADJ, enter the reference voltage (1.

2 V) and for R1 select a value of one thousand times that voltage (i. e. , 1. 2 k ) then the equation is simplified to: R2 = 1000 (Vout 1. 2) In practice, simply determine the voltage drop across R2 (output voltage minus reference voltage) and you get your resistance value directly in kilo-ohms. For example, for 5 V R2 becomes 5 1. 2 = 3. 8 k which is easiest made by connecting 3. 3k and 470R resistors in series. In the case of relatively low voltages, smaller resistor values are recommended. This is because sufficient current needs to flow to enable the voltage regulator to do its job. A simple solution is to choose, say, 120 for R1. R2 then becomes: R2 = 100 (Vout 1. 2)