LM334Z Three-Terminal Adjustable Current Source

Overview

The LM334Z is a 3-terminal adjustable current source belonging to the LM134/LM234/LM334 family of devices. It is designed to provide a highly stable and adjustable current over a wide operating range. With a dynamic voltage range of 1V to 40V and an operating current range spanning 10,000:1, the LM334Z is ideal for precision current regulation in a variety of applications. The device requires only a single external resistor to set the current, simplifying circuit design while maintaining high accuracy (±3% initial current accuracy). Its floating current source architecture eliminates the need for separate power supply connections, making it versatile for both DC and AC applications.

Features

• Wide Operating Voltage Range: Operates from 1V to 40V, making it suitable for low and high-voltage applications.
• Adjustable Current Source: Current can be set with a single external resistor, with no additional components required.
• High Current Range: Supports a 10,000:1 operating current range for flexible design options.
• Excellent Current Regulation: Maintains stable current output across varying load conditions.
• Temperature Proportional Operation: Sense voltage is directly proportional to absolute temperature (64mV at 25°C), enabling temperature-dependent current control.
• Reverse Voltage Tolerance: Can withstand reverse voltages up to 20V with minimal leakage current (a few dozen microamperes), allowing use in AC rectification applications.
• Low Drift Operation: Can achieve zero-drift operation with additional compensation techniques.
• True Floating Current Source: No separate power supply connections needed, simplifying integration.

Applications

The LM334Z is widely used in precision current regulation and temperature sensing applications. Key applications include:

• Current Sources for Sensors: Provides stable bias currents for thermocouples, RTDs, and other sensitive analog sensors.
• Temperature Compensation Circuits: Leverages its proportional-to-absolute-temperature (PTAT) characteristic for thermal compensation in amplifiers and voltage references.
• LED Drivers: Acts as a constant current source for driving LEDs with minimal variation.
• Battery Charging: Used in low-power charging circuits where precise current control is necessary.
• AC Signal Rectification: Functions as both a rectifier and current source in AC-coupled applications due to its reverse voltage tolerance.
• Current Limiting Circuits: Protects sensitive components by providing a fixed current limit.
• Oscillator and Timer Circuits: Provides stable timing currents in conjunction with capacitors.

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (V+ to V-): 40V
• Reverse Voltage (V- to V+): 20V
• Operating Temperature Range (LM334Z): 0°C to +70°C
• Storage Temperature Range: -65°C to +150°C

Key Electrical Parameters (Typical @ 25°C)

• Sense Voltage (V_SENSE): 64mV
• Temperature Coefficient: +0.33%/°C (proportional to absolute temperature)
• Initial Current Accuracy: ±3%
• Minimum Operating Voltage: 1V
• Reverse Leakage Current (@ 20V reverse bias): <50µA
• Dynamic Output Impedance: >1MΩ (ensures stable current under varying load conditions)

Package Information

The LM334Z is available in a TO-92 package, which is a through-hole, 3-pin plastic casing suitable for prototyping and general-purpose applications. Key package details include:

• Package Type: TO-92 (3-Pin)
• Pin Configuration:
  • Pin 1 (R): Current Set Resistor Connection
  • Pin 2 (V+): Positive Supply/Output Terminal
  • Pin 3 (V-): Negative Supply/Ground Terminal
• Dimensions: 4.95mm (L) x 3.94mm (W) x 4.95mm (H)
• Weight: Approx. 0.35g
• Mounting Style: Through-Hole
• Operating Temperature Range: 0°C to +70°C (Commercial Grade)

Design Considerations

When implementing the LM334Z in a circuit, consider the following:

• Resistor Selection: The current setting resistor (R_SET) determines the output current according to the formula: I_OUT ≈ V_SENSE / R_SET, where V_SENSE is 64mV at 25°C.
• Temperature Effects: Since the sense voltage is PTAT, applications requiring temperature-independent current should include compensation techniques.
• Power Dissipation: Ensure the device does not exceed its thermal limits, especially at higher currents and voltages.
• Reverse Voltage Protection: Although tolerant to reverse bias, prolonged exposure to reverse voltages near 20V should be avoided.

Conclusion

The LM334Z is a versatile and reliable adjustable current source, offering precision, simplicity, and robustness for a wide range of applications. Its ability to operate with minimal external components, combined with excellent regulation and temperature-dependent characteristics, makes it a preferred choice for designers needing stable current sources. Whether used in sensor biasing, temperature compensation, or AC signal conditioning, the LM334Z delivers consistent performance across diverse operating conditions.