Formula Used
The basic LM723 current limit estimate uses the sense threshold and sense resistor.
Current limit: ILIMIT = VCS / RS
Sense resistor: RS = VCS / ILIMIT
Sense resistor power: PRS = ILIMIT² × RS
Worst current range: ILOW = VCS min / RS max, and IHIGH = VCS max / RS min
Pass power near regulation: PPASS ≈ (VIN − VOUT − VCS) × ILIMIT
Short circuit foldback: ISHORT = ILIMIT × foldback percent / 100
How to Use This Calculator
- Select whether you know the sense resistor or target current.
- Enter the LM723 sense threshold voltage.
- Add resistor tolerance, threshold tolerance, and temperature data.
- Enter input voltage, output voltage, and pass headroom.
- Set resistor count, wattage, derating, and foldback percentage.
- Press calculate to view current, power, and warning results.
- Use CSV or PDF export for design records.
Example Data Table
| VCS |
RS |
Nominal Limit |
VIN |
VOUT |
Sense Power |
Typical Use |
| 0.65 V |
1 Ω |
0.65 A |
15 V |
12 V |
0.42 W |
Small bench output |
| 0.65 V |
0.47 Ω |
1.38 A |
18 V |
12 V |
0.90 W |
Medium supply rail |
| 0.65 V |
0.33 Ω |
1.97 A |
20 V |
13.8 V |
1.28 W |
Radio supply |
| 0.65 V |
0.22 Ω |
2.95 A |
24 V |
15 V |
1.92 W |
Higher current pass stage |
LM723 Current Limit Design Guide
The LM723 is a regulator controller. It can drive pass devices. It also supports simple current limiting. The limit depends on a small sense resistor. The controller watches voltage across that resistor. When the sense voltage reaches the selected threshold, drive is reduced. Output current then stops rising.
Why Sense Resistor Choice Matters
A low value resistor gives a high limit. A high value resistor gives a lower limit. The resistor must also handle heat. Power rises with the square of current. Small mistakes can create large heating changes. Tolerance adds another problem. The real resistor may be lower than its label. The internal sense threshold may also move. This calculator includes both effects.
Worst Case Thinking
A safe design does not use only nominal values. It checks the highest possible current. It also checks the lowest possible limit. The high current case stresses pass transistors and wiring. The low current case may trip too early. Temperature drift should be checked when the supply runs warm. Metal film or current sense resistors often improve stability.
Pass Device Heating
Current limiting protects the load first. It does not always protect the pass transistor. In a short circuit, the pass device may carry high current while dropping most input voltage. That creates heavy heat. Foldback limiting reduces short circuit current. It can greatly lower pass transistor dissipation. However, foldback can also cause startup issues with large capacitors.
Practical Design Notes
Use short, heavy traces around the sense resistor. Keep the sensing leads close to the resistor terminals. Avoid sharing those tracks with load current paths. Use a resistor with enough wattage margin. Many designers select at least twice the calculated power. Use more margin for enclosed supplies. Check the pass transistor safe operating area. Add a heat sink when needed. Test the final circuit with a current limited source first. Increase load slowly. Confirm trip current with a meter. Watch resistor temperature during testing.
Using the Calculator Results
The nominal value is a starting point. The worst case range shows expected spread. The power section shows resistor stress. The pass section estimates heat risk. Use the warnings before choosing parts. Recheck values after selecting real resistor sizes.
FAQs
What is LM723 current limiting?
It is a protection method that reduces regulator drive when sense resistor voltage reaches the selected threshold. This helps stop output current from rising beyond the planned limit.
What sense voltage should I use?
Many practical LM723 designs estimate the current sense threshold near 0.6 V to 0.65 V. Use the value from your circuit notes or device data.
How do I choose the sense resistor?
Divide the sense threshold voltage by the desired current limit. Then select a real resistor value. Check tolerance, heat, and available wattage before final use.
Why is resistor power important?
The sense resistor carries load current. Its heat equals current squared times resistance. Underrated parts can drift, fail, or damage nearby components.
What is foldback limiting?
Foldback limiting lowers current during a short circuit. It reduces pass transistor heating. It must be tested because some loads need high startup current.
Does this replace lab testing?
No. It gives design estimates. Always test the final circuit with meters, a dummy load, and safe current limits before connecting valuable loads.
Why include tolerance values?
Tolerance shows real part spread. The highest current case may stress components. The lowest current case may cause early limiting during normal use.
Can I use parallel sense resistors?
Yes. Parallel resistors can share heat. Use matched values when possible. Confirm total resistance and check power sharing during real testing.