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The LM334 is a small, three-terminal adjustable current-source IC. In its basic circuit, one external resistor sets the current; the important catch is that the current rises as the part warms. It is useful for simple biasing, LED drive, and other circuits that need a controlled current, provided its temperature behavior, voltage limits, and exact package pinout suit the job.
What does “constant current” mean?
A current source is a circuit element that tries to keep current flowing at a chosen level even when the voltage across the load changes. That differs from a resistor, whose current varies with voltage according to Ohm’s law. “Constant” here does not mean perfectly unchanging under every condition: the LM334’s current also depends on temperature, and real parts have specified error limits.
The LM334 has three terminals, but in a simple arrangement it can function as a floating two-terminal current source. Its current is established with an external resistor; TI’s datasheet notes that “Current is established with one external resistor and no other parts are required.” The exact circuit still needs to meet the device’s voltage, current, and temperature conditions.
How does the resistor set the current?
The resistor connects to the LM334’s set terminals. The device regulates a small voltage at that connection, so changing the resistor changes the operating current. Texas Instruments gives this nominal relationship at 25°C:
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ISET = 67.7 mV / RSET
For example, a 6.77 kΩ set resistor corresponds to about 10 μA at 25°C using the nominal formula. A 677 Ω resistor corresponds to about 100 μA. These are calculated starting points, not guaranteed measured outputs; resistor tolerance, device error, operating voltage, and temperature all affect the result.
TI’s 2013 Rev. E datasheet lists a programmable range of 1 μA to 10 mA. Its set-current error table at 2.5 V specifies maximum errors of 6% for 10 μA to 1 mA, 8% above 1 mA through 5 mA, and 12% from 2 μA to under 10 μA. Those figures apply to the stated test conditions and ranges, not as a universal accuracy guarantee.
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Why does the current rise with temperature?
The LM334 is not temperature-invariant: its set current is proportional to absolute temperature. TI specifies a current temperature dependence of 0.336% per °C at 25°C. In practical terms, a warmer LM334 produces more current for the same set resistor. For calculations across temperatures, the datasheet describes current at another temperature as following the ratio of absolute temperatures, rather than a simple fixed-current assumption.
If an application needs a roughly zero-temperature-coefficient current, TI shows a compensation arrangement that adds a resistor and a diode. That is a different circuit from the basic one-resistor setup, so use the datasheet’s diagram and conditions rather than assuming the simple formula cancels temperature drift.
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What voltage and temperature limits matter?
TI lists a 1 V to 40 V operating range for the LM334 family in its 2013 Rev. E datasheet, but the minimum operating voltage depends on the set-current band. Do not assume the part will regulate at 1 V for every chosen current; consult the electrical-characteristics table for the intended current and circuit conditions.
The LM334’s specified operating temperature is 0°C to +70°C. Related family members have different ranges, and those ratings do not extend the LM334’s own limits:
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| Variant | Specified temperature range |
|---|---|
| LM134 | −55°C to +125°C |
| LM234 | −25°C to +100°C |
| LM334 | 0°C to +70°C |
TI also cites LM234-3 and LM234-6 sensor versions with separate initial-accuracy specifications of ±3°C and ±6°C. These are distinct variants, not an accuracy rating for a standard LM334. Check the specific part’s datasheet before substituting one family member for another.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which package and pins should you check?
TI lists TO-92 and SOIC-8 package offerings. Pin locations and diagrams are package-specific, so identify the complete ordering code and match its datasheet diagram before wiring. Do not rely on a pinout image for a different package or a related LM134/LM234 part.
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Where is the LM334 useful?
TI lists applications including bias networks, surge protection, low-power references, ramp generation, LED drive, and temperature sensing. Its datasheet also describes two-wire remote sensing, where series resistance in long leads does not affect accuracy in the stated arrangement. Analog Devices lists further examples such as current-mode temperature sensing, current sources for shunt references, cold-junction compensation, constant-gain bias, micropower bias networks, photoconductive-cell buffering, and current limiting.
These are application examples, not proof that the LM334 is suitable for a particular design. For LED drive, for instance, the chosen current, available voltage, thermal conditions, and allowed LED current variation must all fit the circuit. Temperature sensing likewise requires accounting for the device’s operating range and current accuracy rather than treating its temperature response as a precision sensor by default.
Quick Recap
What should you verify before building?
- Current target: Use the 25°C formula to choose a starting set resistor, then account for resistor tolerance and the datasheet’s current-error conditions.
- Temperature: Estimate current over the full expected temperature range; the basic connection’s current rises with temperature.
- Voltage headroom: Check the datasheet’s minimum operating-voltage table for the selected current band and ensure the circuit stays within the stated operating range.
- Package and pinout: Confirm the exact ordering code and its package-specific pin diagram before connecting power.
- Application fit: Confirm that the required current variation, temperature range, and circuit behavior fit the device; do not assume an application-list example is a design guarantee.
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