Find Resistors for an LM317
Choose Two Resistors and Find the Output Voltage
- VOUT may be from 1.2V to 37V at 1.5 A
- VREF is a typical value of around 1.25 V
- IADJ is a typical value of around 50 µA
VOUT = VREF(1 + R2 / R1) + IADJ * R2
Choose an Output Voltage and R1, and Find R2
Similarly, the equation (1) in the LM317 Datasheet can be solved for R2, allowing us to select an output value and a sample R1 and solve for R2. For this, we use a slightly less precise version of the equation, expressing only the radio of R2 to R1 and the Reference Voltage (see: Answers/FAQs).
VOUT = VREF(1 + R2 / R1), Or solving for R2 = R1 * (VOUT / VREF - 1)The 50 µA biasing current is small compared to 5 mA and causes only a small error in actual output voltages.
Heat Dissipation Calculation for LM317
Equation (3) in the LM317 Datasheet can estimate the heat dissipation. IL is the load current. IG is the current consumed by the LM317, measured at the Adjustment Pin. The datasheet shows 'Adjustment Pin Current' to be a typical value of 50µA.
PD = ((VIN − VOUT) × IL) + (VIN × IG)
For more technical data, see: National Semiconductor, LM317 - 3-Terminal Adjustable Regulator