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LT1123CST#PBF

IC LNR REG CTRLR 1OUT SOT223

Part Number: LT1123CST#PBF

Manufacturer: Analog Devices Inc.

Category: Voltage Regulators - Linear Regulator Controllers

Package: TO-261-4, TO-261AA

Datasheet:: LT1123CST#PBF.pdf

Description: IC LNR REG CTRLR 1OUT SOT223

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PRODUCT INFORMATION

Product Details

PRODUCT TECHNICAL REFERENCE

LT1123CST#PBF Low Dropout Regulator Driver

A comprehensive technical overview of the LT1123CST#PBF, a 3-pin low dropout regulator driver from Analog Devices designed to be used with an external PNP pass transistor for low-cost, high-current linear regulation. Covers device function, core features, electrical specifications, 3-lead SOT-223 package, pin configuration, typical applications, and replacement selection guidance.

Analog Devices / Linear Technology 3-Lead SOT-223 (ST) Up to 30V Input

Device Classification Low Dropout Regulator Driver

Part Number: LT1123CST#PBF

Mounting Type: Surface Mount

Operating Temperature Range: 0°C to +125°C

PRODUCT OVERVIEW

What Is the LT1123CST#PBF?

LT1123CST#PBF is a 3-pin bipolar device from Analog Devices (formerly Linear Technology) designed to be used in conjunction with a discrete PNP power transistor to form an inexpensive low dropout regulator.[reference:0] The device consists of a trimmed bandgap reference, error amplifier, and a driver circuit capable of sinking up to 125mA from the base of the external PNP pass transistor.[reference:1][reference:2]

The LT1123CST#PBF is designed to provide a fixed output voltage of 5V, trimmed to ±1% accuracy.[reference:3][reference:4] The drive pin of the device can pull down to 2V at 125mA (1.4V at 10mA), allowing a resistor to be used to reduce the base drive available to the PNP and minimize the power dissipation in the LT1123.[reference:5] Total quiescent current for the LT1123 is only 700µA.[reference:6] The drive current is folded back as the feedback pin approaches ground to further limit the available drive current under short-circuit conditions.[reference:7]

With an external PNP transistor such as the MJE1123, the LT1123CST#PBF can deliver output currents up to 4A at a dropout voltage of less than 0.75V.[reference:8][reference:9] The device is available in a 3-lead SOT-223 (ST) package and is rated for operation from 0°C to +125°C.[reference:10][reference:11]

Low Dropout Regulator Driver Designed with external PNP pass transistor for inexpensive, high-current linear regulation up to 4A.

Fixed 5V Output Trimmed bandgap reference provides ±1% accurate 5V output voltage.

Low Quiescent Current Only 700µA quiescent current with 125mA base drive capability.

DEVICE HIGHLIGHTS

Key Features and Benefits of LT1123CST#PBF

Extremely Low Dropout

With an external PNP pass transistor, the LT1123CST#PBF can achieve dropout voltages of less than 0.75V at 4A output current.[reference:12]

Fixed 5V Output with ±1% Accuracy

Trimmed bandgap reference provides a fixed 5V output with ±1% accuracy over temperature and load conditions.[reference:13]

Low Quiescent Current

Total quiescent current is only 700µA, making the device suitable for low-power applications.[reference:14]

125mA Base Drive Capability

The driver circuit can sink up to 125mA from the base of the external PNP pass transistor.[reference:15]

Excellent Line & Load Regulation

1mV typical line regulation and 5mV typical load regulation provide stable output voltage.[reference:16]

Compact SOT-223 Package

Available in a 3-lead SOT-223 surface mount package with a low profile for space-constrained designs.[reference:17]

REFERENCE DATA

Technical Specifications for LT1123CST#PBF

Part Number	LT1123CST#PBF	Manufacturer	Analog Devices (Linear Technology)
Device Type	Low Dropout Regulator Driver	Output Voltage	5V Fixed
Output Voltage Accuracy	±1%	Output Current (with External PNP)	Up to 4A
Base Drive Current	125mA (Max)	Quiescent Current	700µA
Drive Pin Voltage (Max)	30V	Feedback Pin Voltage (Max)	30V
Feedback Voltage (TJ=25°C)	4.90V to 5.10V	Feedback Voltage (Full Range)	4.80V to 5.20V
Feedback Pin Bias Current	300µA to 500µA	Drive Pin Saturation Voltage @ 125mA	2.0V
Line Regulation	1mV Typ, 20mV Max	Load Regulation	–5mV Typ, –50mV Max
Temperature Coefficient	0.2 mV/°C	Package	3-Lead SOT-223 (ST)
Mounting Type	Surface Mount	Operating Temperature	0°C to +125°C
Storage Temperature	-65°C to 150°C	Junction Temperature (Max)	125°C
Lead Temperature (Soldering)	300°C	RoHS	Yes (#PBF Suffix)

Electrical values shown above are reference values. Test conditions, limits, and guaranteed performance should always be confirmed from the current official manufacturer datasheet before design or production use.

DESIGN REFERENCE

Circuit Considerations for LT1123CST#PBF

External Pass Transistor Configuration

The LT1123CST#PBF is designed to work with an external PNP power transistor:

External PNP Transistor: A discrete PNP power transistor such as the MJE1123 is used as the pass element[reference:18]
Base Drive: The DRIVE pin sinks up to 125mA from the base of the external PNP transistor[reference:19]
Feedback: The FB pin senses the output voltage and provides regulation feedback[reference:20]
Drive Resistor: A resistor can be used between DRIVE and the PNP base to reduce power dissipation in the LT1123[reference:21]
Output Capacitor: A 10µF capacitor is required for stability[reference:22]

PCB and Power-Supply Guidance

Regulator driver performance depends on layout, bypassing, and component selection:

Input Voltage: The drive pin (DRIVE) can handle up to 30V; the feedback pin (FB) can also handle up to 30V[reference:23]
Output Capacitor: A 10µF output capacitor is required for stability; larger values increase stability[reference:24]
Input Capacitor: A 10µF capacitor is required if the device is more than 6" from the main filter capacitor[reference:25]
Thermal Management: θJA for the SOT-223 package is approximately 20°C/W with the tab soldered[reference:26]
External PNP Selection: Choose a PNP transistor with adequate current and power ratings for the application
Verify the recommended PCB footprint from the official datasheet before production

3-LEAD SOT-223 (ST)

Pin Configuration Reference for LT1123CST#PBF

LT1123

SOT-223 (ST) TOP VIEW

Confirm package orientation and pin numbering against the official package drawing before PCB layout. The ST package has 3 pins with the tab connected to GND. Package marking for the LT1123CST#PBF is "ST".[reference:27]

1 FB Feedback Input - Output voltage sense; connects to output

2 GND Ground - Tab is connected to GND

3 DRIVE Drive Output - Sinks base current from external PNP transistor

APPLICATION AREAS

Typical Applications for LT1123CST#PBF

5V/4A Low Dropout Regulator

Using an external PNP transistor such as the MJE1123 to form a 5V, 4A low dropout regulator with dropout voltage less than 0.75V.[reference:28][reference:29]

Sealed Lead Acid Battery Charger

5.4V to 7.2V input to 5V/4A output for battery charging applications.[reference:30]

Post-Regulator for Switching Supplies

Clean up noisy switching regulator outputs with low dropout linear regulation.

Industrial Power Supplies

Cost-effective linear regulation for industrial equipment requiring 5V at up to 4A.

Test and Measurement Equipment

Low noise power for precision test and measurement instruments.

General Purpose 5V Power Supplies

Inexpensive 5V linear regulator solution for a wide range of applications.

REPLACEMENT GUIDANCE

Selecting an Alternative for LT1123CST#PBF

A replacement for the LT1123CST#PBF should not be selected only by product category. The electrical characteristics, package, and pin configuration must be evaluated against the actual circuit requirements.

Output voltage (5V fixed)

Output current capability (up to 4A with external PNP)

Base drive capability (125mA)

Quiescent current (700µA)

Package (3-lead SOT-223)

Operating temperature (0°C to 125°C)

Dropout voltage (with external PNP)

RoHS compliance (#PBF suffix)

Family alternatives: LT1123CZ#PBF (TO-92 through-hole package, same electrical specs), LT1123CST#TRPBF (SOT-223, tape and reel packaging), LT1123CZ#TRPBF (TO-92, tape and reel). The LT1123 is a unique low dropout regulator driver; for integrated LDO solutions with similar performance, consider the LT1086 or LT1587 series. Select based on package, mounting type, and packaging needs.

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MANUFACTURER INFORMATION

Analog Devices / Linear Technology

The LT1123 (LT1123CST#PBF) belongs to the low dropout regulator driver portfolio of Linear Technology, now part of Analog Devices. The device is designed to be used with an external PNP pass transistor to create an inexpensive, high-current low dropout regulator.[reference:31]

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Technical Information Disclaimer

This technical guide is provided for product identification and preliminary reference only. Electrical limits, package dimensions, pin configuration, product status, and application suitability must be verified using current official manufacturer documentation before design, procurement, or production.

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Specifications

Product Attribute: Attribute Value

Part Number: LT1123CST#PBF

Manufacturer: Analog Devices Inc.

Package: TO-261-4, TO-261AA

Series: -

Package/Case: TO-261-4, TO-261AA

Packaging: Tube

Product Status: Active

Type: Positive Fixed

Number of Outputs: 1

Current - Supply: 700µA

Voltage - Input: -

Operating Temperature: 0°C ~ 125°C

Mounting Type: Surface Mount

Supplier Device Package: SOT-223-3

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Part Number: LT1123CST#PBF

Manufacturer: Analog Devices Inc.

Package: TO-261-4, TO-261AA

Category: Voltage Regulators - Linear Regulator Controllers

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