LM317 Adjustable Positive Linear Voltage Regulator

U1 regulates the OUT-to-ADJ differential to nominal VREF=1.25V. TI §6.6 specifies 1.20–1.30V under its stated differential, current and power conditions: 3V≤VIN−VOUT≤40V, 10mA≤IOUT≤1.5A and PD≤20W. These IC conditions do not establish a tested rating for this assembly.

R1=240Ω carries a calculated nominal current I1=1.25V/240Ω≈5.21mA. TI lists minimum load as 3.5mA typical and 10mA maximum at VIN−VOUT=40V across its operating temperature range. The divider therefore relies on typical behavior without an external load. Even a nominal 120Ω resistor with +1% tolerance gives only 1.20V/121.2Ω≈9.90mA at the minimum reference voltage. Check the complete tolerance and load budget rather than assuming worst-case regulation.

The nominal divider equation is:

For R1=240Ω, R2=0–5kΩ, VREF=1.25V and IADJ=50µA, the calculated setpoint is 1.25–27.54V. That upper setpoint cannot be reached from the shown nominal 12V supply. TI §6.3 and §7.3.1/§7.4.2 recommend 3V headroom. A nominal setpoint up to 9V respects that margin; it is not a physical output clamp or an assembly qualification. A setting near R2=1.47kΩ gives 9V in the nominal equation, including IADJ. Higher settings leave the recommended envelope; actual loss of regulation depends on load and temperature. No numerical dropout threshold is specified here.

Total output current includes the external load and divider: IOUT=ILOAD+I1. Input current is IIN=IOUT+IADJ. Exact terminal power balance gives:

For an illustrative VIN=12V, VOUT=5V, total IOUT=1A, VADJ=3.75V and assumed IADJ=50µA, PD=7.0004125W, approximately 7W. The omitted ADJ term must be evaluated at the actual operating point; no universal sub-milliwatt or percentage-error guarantee applies.

For a heatsink model, the calculated upper bound is θSA≤(TJ(limit)−TA)/PD−θJC(bot)−θCS. Using an operating limit of 125°C, assumed TA=25°C, PD≈7W and θCS=0.5°C/W gives θSA≤13.7°C/W. This example is not a guaranteed assembly rating; verify the actual thermal path. The tab is electrically connected to VOUT.

C1=100nF connects from IN to GND. Keep its connections short; TI recommends local input bypass when the regulator is far from the source filter. C2=10µF/35V connects positive to VOUT and negative to GND and can improve transient response. It is not required for stability, and its exact response depends on the selected part and load.

D1 has cathode VIN and anode VOUT. TI §8.2.1 recommends a bypass diode when output capacitance is used. Evaluate the returned current, pulse energy and impedance of the input path; the diode alone does not limit the capacitor surge. Vishay 88503 specifies IF(AV)=1A at TA=75°C with 9.5mm leads and IFSM=30A for a nonrepetitive 8.3ms single half-sine superimposed on rated load at TA=25°C. These are diode test conditions, not a demonstrated discharge rating for this circuit. D1 does not protect against reverse input polarity.

In the CAD symbol, R2 Pin 3 is the wiper, tied to end terminal Pin 2 and GND; end terminal Pin 1 connects to ADJ. Verify the physical pinout of the selected potentiometer. If the wiper loses contact, the divider uses the full 5kΩ track and requests the high setpoint. This prevents an open divider but does not protect a downstream load from overvoltage. Actual fault output depends on supply and load; provide independently designed overvoltage protection where needed.