LM723 Circuit: A Detailed Information on the Voltage Regulator

Introduction to LM723 Voltage Regulator

The LM723 is a versatile and widely used voltage regulator integrated circuit (IC) that has been around since the 1970s. This IC is designed to provide a stable and adjustable output voltage for a wide range of applications, including power supplies, battery chargers, and voltage reference circuits. In this article, we will dive into the details of the LM723 voltage regulator, its features, applications, and how to use it in various circuit configurations.

Key Features of the LM723 Voltage Regulator

The LM723 voltage regulator offers several key features that make it a popular choice among engineers and hobbyists alike:

1. Adjustable output voltage: The LM723 allows users to set the output voltage between 2V and 37V by using external resistors.
2. Current limiting: The IC has built-in current limiting protection, which helps prevent damage to the load and the regulator itself.
3. Thermal shutdown: To protect the IC from overheating, the LM723 includes a thermal shutdown feature that automatically disables the output when the temperature exceeds a safe level.
4. Low dropout voltage: The LM723 can operate with a low dropout voltage, meaning it can maintain regulation even when the input voltage is close to the output voltage.
5. Wide operating temperature range: The IC can function in temperatures ranging from -55°C to 150°C, making it suitable for various environmental conditions.

Internal Architecture of the LM723

To understand how the LM723 works, it’s essential to have a basic understanding of its internal architecture. The IC consists of several key components:

1. Voltage reference: The LM723 has an internal voltage reference that provides a stable 7.15V output, which is used as a basis for the adjustable output voltage.
2. Error amplifier: This component compares the output voltage with the reference voltage and adjusts the output accordingly to maintain a stable voltage level.
3. Series pass transistor: The series pass transistor is responsible for controlling the current flow from the input to the output, based on the signals from the error amplifier.
4. Current limiting circuitry: This part of the IC monitors the output current and limits it to a safe level to protect the load and the regulator.
5. Thermal shutdown circuitry: When the temperature of the IC exceeds a certain threshold, the thermal shutdown circuitry disables the output to prevent damage.

Basic LM723 Circuit Configuration

To use the LM723 in a basic voltage regulator circuit, you will need the following components:

• LM723 IC
• Input capacitor (C_in)
• Output capacitor (C_out)
• Feedback resistors (R₁ and R₂)
• Load resistor (R_load)

The basic circuit configuration is as follows:

         +V<sub>in</sub>
          |
         | |
        C<sub>in</sub>
          |
     +----+----+
     |    |    |
     |   LM723 |
     |    |    |
     +----+----+
          |
          |
         | |
        C<sub>out</sub>
          |
         ___
        /   \ R<sub>load</sub>
         ___
          |
         GND

To set the output voltage, you need to calculate the values of R₁ and R₂ using the following formula:

V_out = V_ref * (1 + R₂ / R₁) + I_adj * R₂

Where:
– V_out is the desired output voltage
– V_ref is the internal reference voltage (7.15V)
– R₁ and R₂ are the feedback resistors
– I_adj is the adjustment pin current (typically 45μA)

LM723 Applications

The LM723 voltage regulator is used in a wide variety of applications, including:

1. Power supplies: The LM723 is often used in linear power supplies to provide a stable output voltage for electronic devices.
2. Battery chargers: The current limiting and adjustable voltage features of the LM723 make it suitable for use in battery charging circuits.
3. Voltage reference circuits: The stable internal reference voltage of the LM723 can be used as a basis for creating precise voltage reference circuits.
4. Overload protection: The current limiting feature of the LM723 can be used to protect sensitive electronic components from damage due to excessive current draw.

Advanced LM723 Circuit Configurations

In addition to the basic voltage regulator circuit, the LM723 can be used in several advanced configurations to suit specific application requirements:

Adjustable Current Limiting

To set a specific current limit for the LM723, you can use an external resistor (R_sc) connected between the current sense (CS) and voltage reference (VREF) pins. The value of R_sc can be calculated using the following formula:

R_sc = 0.65V / I_limit

Where I_limit is the desired current limit.

Remote Voltage Sensing

In applications where the load is located far from the voltage regulator, voltage drops across the connecting wires can cause the load voltage to be lower than the desired output voltage. To compensate for this, you can use the remote voltage sensing feature of the LM723.

To implement remote voltage sensing, connect the positive sense (SENS+) pin to the positive terminal of the load, and the negative sense (SENS-) pin to the negative terminal of the load. This allows the LM723 to monitor the voltage directly at the load and adjust the output accordingly.

Paralleling for Higher Current Output

If your application requires a higher current output than a single LM723 can provide, you can parallel multiple LM723 ICs to increase the current capacity. To do this, connect the output (VOUT) and adjustment (ADJ) pins of each IC together, and use a separate current sense resistor (R_sc) for each IC.

When paralleling LM723 ICs, it’s essential to ensure that the current sharing is balanced among the ICs to prevent overloading any single regulator. This can be achieved by using matched current sense resistors and proper PCB layout techniques.

FAQ

1. What is the maximum output current of the LM723 voltage regulator?

2. The maximum output current of the LM723 depends on the package type and the heatsinking used. Generally, the LM723 can provide up to 150mA of output current without additional heatsinking. With proper heatsinking, the output current can be increased to several amps.

3. Can the LM723 be used as a negative voltage regulator?

4. Yes, the LM723 can be configured as a negative voltage regulator by connecting the appropriate pins to a negative input voltage and ground. The output voltage can be adjusted using external resistors in the same way as with a positive voltage regulator.

5. How do I choose the input and output capacitor values for the LM723?

6. The input capacitor (C_in) should be chosen to provide sufficient filtering of the input voltage ripple. A value of 0.1μF to 1μF is typically used. The output capacitor (C_out) helps to improve transient response and reduce output voltage ripple. A value of 1μF to 10μF is commonly used, depending on the load requirements.

7. What is the minimum input-to-output voltage difference required for the LM723 to maintain regulation?

8. The LM723 requires a minimum input-to-output voltage difference (dropout voltage) of about 3V to maintain regulation. If the input voltage drops below this level, the output voltage will begin to decrease and regulation will be lost.

9. Can the LM723 be used in switching regulator applications?

10. While the LM723 is primarily designed for linear regulator applications, it can be used as a control element in switching regulator circuits. However, additional external components, such as a switching transistor and an inductor, are required to implement a switching regulator topology.

Conclusion

The LM723 voltage regulator is a versatile and reliable IC that has stood the test of time. Its adjustable output voltage, current limiting, and thermal protection features make it suitable for a wide range of applications, from simple power supplies to more advanced circuit configurations.

By understanding the internal architecture, basic circuit configuration, and advanced application techniques, engineers and hobbyists can effectively utilize the LM723 to create stable and efficient voltage regulation solutions. Whether you are designing a power supply for a new project or looking to improve an existing design, the LM723 voltage regulator is a valuable tool to have in your arsenal.