Analog Devices Inc. LT3645IUD#TRPBF

Overview

LT3645IUD#TRPBF from Analog Devices (formerly Linear Technology) is a dual-output DC/DC regulator integrating a 500mA synchronous step-down buck converter and a 200mA low-dropout linear regulator (LDO) in a single 16-lead 3mm × 3mm QFN package. It operates from 3.6V to 36V input, delivers 3.3V/5V buck outputs and 0.8V–8V LDO outputs, and features 750kHz switching frequency, <2μA shutdown current, and integrated overvoltage lockout up to 40V - enabling robust power management for automotive image sensor and microcontroller supply rails.

For engineers reviewing the LT3645IUD#TRPBF datasheet, LT3645IUD#TRPBF pinout, LT3645IUD#TRPBF application, or LT3645IUD#TRPBF equivalent, key selection criteria include its dual-regulator architecture with independent enable controls (EN/UVLO and EN2), 310mV LDO dropout at 200mA, PGOOD-indicated regulation status via NPG pin, and thermal performance enabled by exposed GND pad soldering - all critical for industrial and automotive power sequencing designs.

Technical Context

The LT3645IUD#TRPBF implements a constant-frequency current-mode buck regulator with internal bipolar NPN switch and BOOST-driven gate drive, paired with a separate bipolar NPN-based LDO powered from VCC2. Its 750kHz oscillator supports frequency foldback during soft-start and overload, while cycle-by-cycle current limiting and DA-pin diode-current sensing prevent runaway under short-circuit conditions.

Regulation relies on two independent error amplifiers: one referencing 0.8V at FB for the buck output, and another referencing 0.797V at FB2 for the LDO output. Both feature ramped reference soft-start (900μs for buck, 600μs for LDO), and the NPG open-collector output asserts low only when both FB ≥ 720mV and FB2 ≥ 720mV - confirming simultaneous regulation of both outputs.

Key Specifications

Pinout & Package

The LT3645IUD#TRPBF is housed in a thermally enhanced 16-lead (3mm × 3mm) plastic QFN package with exposed GND pad (Pin 17), requiring PCB soldering for thermal and electrical integrity. θ_JA = 58.7°C/W, θ_JC = 7.1°C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual-output regulator applications.

Compared with LT3641IUD#TRPBF and TPS63020DSJR, the LT3645IUD#TRPBF uniquely combines high-voltage buck regulation with a precision LDO in one package - eliminating discrete post-regulators and simplifying layout for noise-sensitive dual-rail systems, though it lacks buck-boost capability and operates only above 3.6V input.

Availability

LT3645IUD#TRPBF is available at Aetrix Electronics and suitable for automotive camera modules, industrial PLCs, and embedded vision systems requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LT3645IUD#TRPBF 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) is a global leader in high-performance analog, mixed-signal, and power management semiconductors, serving aerospace, automotive, industrial, and communications markets.

The LT3645IUD#TRPBF belongs to Linear's high-voltage monolithic regulator family, designed specifically for robust, compact power solutions in automotive and industrial environments where wide input range, integrated protection, and dual-rail flexibility are essential.

FAQ

What is the operating temperature range for the LT3645IUD#TRPBF?

The LT3645IUD#TRPBF is specified for –40°C to +125°C junction temperature operation and is guaranteed across this full range. Its thermal design - including exposed GND pad soldering and θ_JA = 58.7°C/W - enables reliable performance in under-hood automotive and industrial control cabinet environments without derating up to 125°C ambient when properly heatsinked.

How does the NPG pin indicate regulation status on the LT3645IUD#TRPBF?

The NPG pin on the LT3645IUD#TRPBF is an open-collector output that pulls low only when both the buck output (FB ≥ 720mV) and LDO output (FB2 ≥ 720mV) are within 90% of their target regulation voltages. It remains high-impedance otherwise, allowing wired-OR connection to system reset controllers - a key feature for safe power sequencing in the LT3645IUD#TRPBF.

Can the LT3645IUD#TRPBF's LDO operate independently of the buck regulator?

Yes - the LT3645IUD#TRPBF's LDO can operate independently: it requires EN/UVLO > 1.23V *and* EN2 > 1.3V to activate. If EN/UVLO is high but EN2 is pulled low (<0.5V), the LDO remains disabled while the buck continues functioning. This enables selective power gating of analog/peripheral rails without affecting main logic supply - a defined behavior of the LT3645IUD#TRPBF.

What is the minimum input voltage required for the LT3645IUD#TRPBF to start up reliably?

The LT3645IUD#TRPBF requires a minimum input voltage of ~3.4V to exit undervoltage lockout, but actual startup depends on output load and configuration. For a 5V buck output, typical minimum startup VIN is ~5.5V at light load and ~6.8V at 500mA; for 3.3V output, it is ~4.4V and ~5.2V respectively - due to BOOST capacitor charging constraints and duty-cycle limits, as documented in the LT3645IUD#TRPBF datasheet Figure 4a/4b.

Does the LT3645IUD#TRPBF require an external Schottky diode for the BOOST circuit?

The LT3645IUD#TRPBF includes an internal Schottky diode between VCC2 and BOOST, so no external diode is needed if VCC2 ≥ 2.5V. However, if VCC2 is supplied below 2.5V (e.g., 1.8V or 2.2V), an external Schottky (e.g., BAS70) must connect VCC2 (anode) to BOOST (cathode) to ensure adequate BOOST voltage - a requirement explicitly defined in the LT3645IUD#TRPBF application notes.