Analog Devices Inc. LT3991IMSE-5#TRPBF

Overview

LT3991IMSE-5#TRPBF from Analog Devices (formerly Linear Technology) is a 55V, 1.2A monolithic step-down DC/DC regulator with fixed 5V output, 2.8µA quiescent current in regulation, and internal compensation. It uses current-mode control with a 0.44Ω NPN switch, supports 200kHz–2MHz switching frequency via RT pin, and delivers power for automotive battery regulation and industrial supplies requiring ultralow standby consumption.

For engineers reviewing the LT3991IMSE-5#TRPBF datasheet, LT3991IMSE-5#TRPBF pinout, LT3991IMSE-5#TRPBF application, or LT3991IMSE-5#TRPBF equivalent, key selection criteria include its 5V fixed output accuracy (±1.4% over temperature), 700nA shutdown current, Power Good flag with 9% threshold hysteresis, and compatibility with 3mm × 3mm DFN or 10-pin MSOP packages - critical for space-constrained, high-reliability embedded systems.

Technical Context

The LT3991IMSE-5#TRPBF implements constant-frequency current-mode PWM control with internal slope compensation and an active VC clamp for peak-current limiting. Its oscillator is programmable via RT-to-GND resistor (200kHz–2MHz) and synchronizable externally (250kHz–2MHz), while Burst Mode® operation reduces no-load input current to 2.8µA with output ripple <15mV_P-P.

It integrates a bipolar NPN main switch driven by BOOST voltage, a dedicated BD pin for external Schottky diode anode connection, and a 1V-enable threshold with 30mV hysteresis. The PG open-drain flag asserts when VOUT reaches 91% of regulation, validated only when VIN > 4.3V and EN is high.

Key Specifications

Pinout & Package

LT3991IMSE-5#TRPBF is housed in a thermally enhanced 10-lead plastic MSOP package with exposed GND pad (Pin 11) requiring PCB soldering for thermal performance (θ_JA = 45°C/W, θ_JC = 10°C/W).

Key Features

Applications

Equivalent & Alternatives

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

Compared with LTC3630EMSE-5#PBF and TPS54302DDAR, LT3991IMSE-5#TRPBF uniquely combines 55V input tolerance, 5V fixed output, ultralow 2.8µA quiescent current, and integrated PG flag - making it optimal for automotive and industrial systems demanding robustness, simplicity, and zero-maintenance standby operation.

Availability

LT3991IMSE-5#TRPBF is available at Aetrix Electronics and suitable for automotive battery regulation, industrial sensor power, and portable medical instrumentation requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LT3991IMSE-5#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 and maintains full product support, qualification, and manufacturing continuity for all LT® parts.

The LT3991 family was designed specifically for high-input-voltage, ultralow-quiescent-current DC/DC conversion in automotive, industrial, and battery-powered systems where reliability under extreme voltage transients is non-negotiable.

FAQ

What is the guaranteed output voltage accuracy of the LT3991IMSE-5#TRPBF over temperature?

The LT3991IMSE-5#TRPBF delivers a fixed 5.0V output with ±1.4% accuracy over its full –40°C to 125°C junction temperature range, as specified in the Electrical Characteristics table. This accuracy is achieved using an internal 10MΩ feedback divider and trimmed bandgap reference, eliminating external resistor tolerance errors. The LT3991IMSE-5#TRPBF does not require external feedback components, simplifying design and improving long-term stability.

Does the LT3991IMSE-5#TRPBF require an external catch diode?

Yes, the LT3991IMSE-5#TRPBF requires an external Schottky catch diode connected between SW and VOUT (anode to SW, cathode to VOUT). The BD pin is tied to the diode anode, and the BOOST pin drives the internal NPN switch. Unlike synchronous buck regulators, LT3991IMSE-5#TRPBF uses an external diode for freewheeling - enabling higher voltage operation and simplified thermal management. Diode reverse leakage must be <1µA to preserve ultralow quiescent current.

How does the Power Good (PG) flag behave during startup and input undervoltage?

The PG flag on the LT3991IMSE-5#TRPBF is an open-drain output that remains low until VOUT reaches 91% of 5.0V (i.e., 4.55V) and stays asserted only when VIN > 4.3V and EN is high. During cold-crank or brown-out events where VIN drops below 4.3V, PG is invalidated regardless of VOUT level - preventing false "power OK" signals. This behavior is hardwired and requires no external configuration. The LT3991IMSE-5#TRPBF PG function is thus intrinsically safe for automotive and industrial sequencing.

Can the LT3991IMSE-5#TRPBF operate without the SS (soft-start) capacitor?

Yes, the LT3991IMSE-5#TRPBF can operate with SS pin floating, which disables soft-start and results in immediate full-current startup. However, this may cause inrush current exceeding 1.2A and induce voltage overshoot or system reset in sensitive downstream circuitry. For reliable operation with capacitive loads or low-impedance sources, a 10nF–100nF capacitor from SS to GND is recommended. The LT3991IMSE-5#TRPBF SS pin is only actively discharged when EN is low - ensuring controlled restart after shutdown.

What is the minimum input voltage required for the LT3991IMSE-5#TRPBF to regulate 5V output?

The LT3991IMSE-5#TRPBF requires a minimum input voltage of 4.3V to initiate regulation, as defined by its UVLO threshold. Below this, the IC remains disabled even if EN is high. Once enabled, sustained regulation depends on duty cycle limits: for example, at 2MHz switching frequency, minimum VIN rises to ~6.2V due to t_ON(MIN) constraints. The LT3991IMSE-5#TRPBF datasheet specifies 4.3V as the absolute minimum start-up voltage, verified across temperature and process corners.