Analog Devices Inc. LT1123CST#PBF

Overview

LT1123CST#PBF from Analog Devices (acquired Linear Technology) is a 3-pin bipolar regulator driver IC designed to interface with an external PNP pass transistor-such as the MJE1123-to form a low-dropout 5V linear regulator delivering up to 4A output current. It integrates a trimmed 5V bandgap reference, error amplifier, and Darlington driver capable of sinking 125mA base current, with 700µA quiescent current and 1.4V drive pin saturation at 10mA. It is used in battery backup systems, sealed lead-acid chargers, and industrial 5V power rails where dropout <0.75V at 4A is required.

For engineers reviewing the LT1123CST#PBF datasheet, LT1123CST#PBF pinout, LT1123CST#PBF application, or LT1123CST#PBF equivalent, key selection criteria include guaranteed 4A output capability with external PNP, fixed 5V feedback voltage (±1% initial accuracy), thermal limiting, foldback current control under short-circuit, and SOT-223 package thermal performance (θ_JA ≈ 50°C/W with PCB copper).

Technical Context

The LT1123CST#PBF operates as a feedback-controlled current sink: its FB pin senses output voltage via resistive divider or direct connection, while its DRIVE pin sinks base current from an external PNP transistor to regulate V_OUT. Internal circuitry-including reference, error amp, and Darlington driver-is biased from the DRIVE pin, which must be supplied ≥3V above ground and ≤30V.

Its drive current folds back when FB approaches ground (e.g., during startup or short-circuit), limiting available base drive to ~25–150mA depending on V_FB and temperature. The device features internal thermal shutdown and relies on external RD resistor to set drive current, enabling precise control of dropout voltage and output current limit in conjunction with the selected PNP transistor.

Key Specifications

Pinout & Package

LT1123CST#PBF is housed in a 3-lead plastic SOT-223 package with exposed tab electrically connected to GND. Thermal resistance is ~50°C/W with adequate PCB copper area (≥1000 mm² per side), enabling high-power dissipation without external heatsink in space-constrained designs.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar regulator driver applications.

Compared with LT1123CZ#PBF, the LT1123CST#PBF delivers 4× better thermal performance and supports higher continuous output current; compared with LT3080IMSE#PBF, it enables scalable 4A solutions with discrete PNP optimization but requires additional external components and layout attention.

Availability

LT1123CST#PBF is available at Aetrix Electronics and suitable for battery backup systems, sealed lead-acid charger interfaces, and industrial 5V motor control rails requiring stable component supply with guaranteed long-term availability and traceable sourcing.

Supply support for LT1123CST#PBF includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Analog Devices acquired Linear Technology in 2017 and maintains full product support, datasheets, and application engineering for legacy Linear parts including the LT1123 series.

The LT1123 product line was designed specifically to enable low-cost, high-current, ultralow-dropout linear regulators using discrete PNP transistors-targeting industrial, automotive, and telecom power systems where efficiency, thermal manageability, and reliability are critical.

FAQ

What is the maximum output current achievable with LT1123CST#PBF?

The LT1123CST#PBF itself does not deliver output current-it drives an external PNP transistor. When paired with the MJE1123 and properly heatsunk, the LT1123CST#PBF-based regulator guarantees 4A output current with dropout voltage ≤0.75V. Output current is ultimately limited by the PNP's SOA, heatsinking, and RD resistor selection-not the LT1123CST#PBF's internal rating.

Can LT1123CST#PBF be used with PNP transistors other than MJE1123?

Yes, LT1123CST#PBF can interface with other PNP transistors, but performance guarantees (e.g., 4A/0.75V dropout) apply only with the MJE1123. For alternate PNPs, users must characterize beta vs. I_C, V_CE(sat) vs. I_B, and thermal behavior-and recalculate RD, RB, and heatsink requirements accordingly. Linear Technology provides design formulas in the LT1123CST#PBF datasheet for this purpose.

What is the role of the RD resistor in LT1123CST#PBF circuits?

The RD resistor in LT1123CST#PBF circuits limits base drive current to the external PNP, directly setting both output current limit and dropout voltage. It also reduces power dissipation in the LT1123CST#PBF by dropping voltage between DRIVE pin and PNP base. Values range from 0Ω (direct drive) to 270Ω depending on input voltage and desired I_OUT; selection charts are provided in the LT1123CST#PBF datasheet.

Does LT1123CST#PBF require an input bypass capacitor?

Yes, LT1123CST#PBF requires a minimum 10µF aluminum electrolytic capacitor at the input (VIN), especially if located >6" from the main filter capacitor. This stabilizes the DRIVE pin supply and prevents oscillation during transient load steps. Larger values (e.g., 100µF) improve stability margin in high-noise environments or long-trace layouts.

How does thermal limiting work in LT1123CST#PBF?

LT1123CST#PBF includes an internal thermal shutdown circuit that disables the DRIVE pin when junction temperature reaches 125°C. To protect both the LT1123CST#PBF and the external PNP, the device should be mounted thermally adjacent to the PNP (e.g., same heatsink). This ensures coordinated thermal limiting-preventing destructive overheating of either component during overload or poor ventilation.