Understandings LM350: A Comprehensive Guide to Adjustable Voltage Regulators

A voltage regulator maintains a consistent output voltage, ensuring stability in the face of fluctuations in input, load conditions, or changes. The LM350 positive voltage regulator stands out as a user-friendly yet highly efficient option. Its versatility allows it to function as either a programmable voltage output regulator or a precision current regulator, depending on the specific needs of your circuit. This article delves into the ultimate LM350 IC pin configuration, workings, and datasheet guide. Everything you need to know about this adjustable voltage regulator.

What is LM350?

The LM350 is an adjustable three-terminal positive voltage regulator with the capability to deliver more than 3.0 A within an output voltage range of 1.2 V to 33 V. Its operation is exceptionally straightforward, requiring only two external resistors to set the output voltage. This IC is user-friendly and necessitates just two external resistors for configuring the output voltage. Additionally, it incorporates internal features such as current limiting, compensation of the safe area, thermal shutdown, and more. The regulator finds applications in various scenarios, including local and on-card regulation. It can be employed to design a programmable output regulator, an adjustable switching regulator, or utilized as a precision current regulator by incorporating a fixed resistor between the adjustment and output pins.

LM350 Pinout

LM350 Pin Configuration

Pin No.	Pin Name	Description
1	Adjus	This pin adjusts the output voltage
2	Output Voltage (Vout)	The regulated output voltage set by the adjust pin can be obtained from this pin
3	Input Voltage (Vin)	The input voltage which has to be regulated is given to this pin

LM350 Features

• Here are the features of the LM350 regulator:

• The regulator boasts an output current of 3 Amps and a high dissipation of nearly 85 Watts.

• It supports floating operations in high-voltage applications.

• It offers a line regulation of 0.005%/V and ensures a load regulation of 0.1%.

• The chip can withstand a peak input to an output voltage range of 35V, with the recommended operational range being close to 15V.

• It incorporates overload thermal protection, safeguarding circuit components from the effects of overloads. This protection ensures the chip remains in optimal condition even when the adjust pin is not connected.

• The output voltage can be precisely adjusted from 1.25V to 33V with a 1% tolerance.

• It features a built-in safety mechanism that triggers when the device exceeds the chip's rated current.

LM350 CAD Model

How Does the LM350 Regulator Work?

In the diagram below, the circuit closely resembles my initial variable DC power supply.

Upon pressing S1 to activate this power supply, AC220V or AC110V (for the USA) is applied. The ACV flows through F1 for protection in case of overload or excessive voltage input.

Subsequently, the ACV passes through a transformer capable of reducing high AC voltage to lower levels of AC-18V. It then reaches the BD1-bridge diode to perform the conversion (rectification) of AC to DC.

Following this, the current travels through the C1-4700uF electrolytic capacitor to effectively smooth (filter) the pulsating voltage from the transformer, transforming it into a stable direct current (DC).

At this stage, the voltage measures between 22V to 25V.

The current then proceeds to the input pin of IC1-LM350, as mentioned earlier.

This IC is a 3A adjustable regulator. By rotating VR1, the regulated output voltage can be adjusted within the range of 1.2V to 22V.

Where & How to Use LM350?

For variable voltage regulation requirements, the LM350 regulator stands out as the top choice. With a capacity of up to 3A, it becomes the preferred IC when a supply exceeding 1.5A is needed. If the goal is to set the voltage within the range of 1.25V to 33V and provide a current of up to 3A, the variable voltage regulator utilizing the LM350 IC becomes indispensable. This regulator proves to be an excellent option for various applications. Additionally, it incorporates a current regulator that finds utility in battery charging applications.

The majority of voltage regulators, including the LM350, incorporate both input and output capacitors. The operational principle of the LM350 aligns with that of other voltage regulators. By modifying the values of resistors linked between the adjustable and output pins, the output voltage can be varied from 1.25V to 33V. The presence of an output capacitor enhances regulation and transient response. The LM350 demonstrates excellent load regulation. For optimal performance, it is advisable to directly connect resistor R1 to the output to ensure effective regulation.

LM350 IC Circuit Diagram

Below is the circuit diagram of the LM350 IC, a versatile three-terminal integrated circuit used in various applications, especially as a variable voltage regulator.

As previously explained, the IC features three pins, with the input voltage supplied to the Vin pin. A potential divider, consisting of a pair of resistors, is employed to set a voltage at the adjust pin. This voltage can be determined at the Vout pin, where the pair of resistors in the circuit is connected.

To transform this IC into a variable voltage regulator, a potentiometer is employed to establish variable voltages at pin-1 within the potential divider. The potential difference is created by combining resistor R1 and the potentiometer at the adjusting pin, thereby controlling the Vout pin. The output voltage can be calculated using the following formula, depending on the resistor values:

VOUT = 1.25 × (1 + (R2/R1)) + Iadj(R2)

Example Problem:

Given:

• Resistor R1 value: 240 ohms

• Potentiometer R2 value: 5000 ohms (10k potentiometer set at 50%)

• Ladj value: 50μA

• Reference voltage: 1.205V

Calculating:

Vout = 1.25 * (1 + (5000/240) + (50*10-6)(5000) = 29.9V

The load regulation of the LM350 provides high-quality regulation, but certain precautions, such as connecting the current set resistor between the Adj and output terminals, are necessary for optimal performance.

LM350 Regulator using Protection Diodes

Whenever external capacitors are employed with a regulator, it becomes imperative to incorporate protection diodes to prevent the discharge of capacitors during low current conditions into the regulator.

Many 10 μF capacitors possess low internal series resistance, resulting in the delivery of 20A spikes when shorted. Despite the short duration of the flow, there is enough energy to potentially harm the IC components.

When an output capacitor is connected to a regulator and the input is shorted, the output capacitor discharges into the regulator's output. This depends on factors such as the capacitor's value, the regulator's output voltage, and the discharge rate of Vin.

The bypass capacitor may discharge over the Adj terminal during a low current junction. This discharge occurs when the input or output is shorted. The internal resistor in the IC is 50Ω, limiting the maximum discharge current.

The formula for Vout is given by Vout = 1.25(1+R2/R1) + IAdjR2

LM350 Equivalents/Alternatives

• Equivalent Voltage Regulators to LM350: LM317, LT1086, LM1117 (SMD), PB137, LM337 (Negative Variable Voltage Regulator).

• Alternative Voltage Regulators to LM350: LM7805, LM7806, LM7809, LM7812, LM7905, LM7912, LM117V33, XC6206P332MR.

LM350 Applications

The LM350 finds frequent use in the following applications:

• Current-limited chargers

• Light controllers

• Temperature controllers

• Switching regulators

• Tracking pre-regulator

• Battery chargers

• Variable power supply systems

• Motor control circuits

• Positive voltage regulations

LM350 Datasheet

Download LM350 Datasheet PDF.

LM350 vs. LM338

When comparing the LM350 and LM338 voltage regulators, it's essential to understand the nuanced differences that can influence your circuit design. Both ICs belong to the LM3xx series and share similarities, such as adjustable output voltage and thermal regulation. However, the primary distinction lies in their current handling capabilities. The LM350 is generally preferred for higher current applications, offering a maximum output current of 3A, while the LM338 surpasses it with a 5A current rating. Designers must carefully assess their project requirements, considering factors like load demand and power consumption, to choose between the LM350 and LM338 effectively. Additionally, the voltage reference and thermal regulation mechanisms in both ICs contribute to their overall performance, making a comparative analysis crucial for achieving optimal results in specific electronic circuits.

Conclusion

In conclusion, the LM350 adjustable voltage regulator is a versatile component widely used in electronic circuits demanding stable and adjustable power supplies. Understanding its features, pinout, working principle, and best practices is essential for successful integration into various applications. Whether you're designing a power supply or working on a hobbyist project, the LM350 provides reliable voltage regulation with flexibility.

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