Analog Devices Inc. LT3999EDD#PBF

Overview

LT3999EDD#PBF from Analog Devices (formerly Linear Technology) is a monolithic, high-voltage push-pull DC/DC transformer driver IC designed for isolated power generation in compact, low-noise applications. It integrates dual 1A current-limited switches operating out of phase, supports programmable switching frequency up to 1MHz, features duty cycle control for line regulation, and delivers stable isolated output in a 3mm × 3mm DFN package with exposed GND pad.

For engineers reviewing the LT3999EDD#PBF datasheet, LT3999EDD#PBF pinout, LT3999EDD#PBF application, or LT3999EDD#PBF equivalent, key selection criteria include input voltage range (2.7V–36V), programmable current limit (0.4A–1.7A), soft-start capability via ILIM/SS pin, cross-conduction prevention, and thermal shutdown protection - all critical for medical, industrial, and data acquisition isolated supplies.

Technical Context

The LT3999EDD#PBF implements a fixed-frequency, two-phase push-pull topology where SWA and SWB drive opposite ends of a center-tapped transformer primary. Its oscillator operates at twice the effective switch frequency (e.g., RT = 49.9kΩ sets f_SW = 300kHz per switch, f_OSC = 600kHz), enabling precise harmonic placement when synchronized to an external clock up to 1MHz.

Duty cycle is controlled via OVLO/DC and RDC pins, allowing VIN-dependent adjustment to stabilize VOUT across wide input ranges - a feature used with post-regulation LDOs to minimize power loss. Protection includes precision UVLO (1.25V threshold, 125mV hysteresis), OVLO (1.25V threshold), and non-overlap timing (70ns) to prevent shoot-through.

Key Specifications

Pinout & Package

The LT3999EDD#PBF uses a 10-lead (3mm × 3mm) plastic DFN package with exposed GND pad (Pin 11), requiring soldering to PCB ground for thermal and electrical integrity. Pin pitch is 0.5mm; exposed pad dimensions are 1.65mm × 2.15mm.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar push-pull transformer driver applications.

Compared with LT3439EMS#PBF and LT1533CS#PBF, the LT3999EDD#PBF provides broader input range (2.7V–36V), active duty cycle compensation for line regulation, and a space-saving 3mm × 3mm DFN package - making it optimal for next-generation compact, wide-input isolated supplies where programmability and thermal efficiency are critical.

Availability

LT3999EDD#PBF is available at Aetrix Electronics and suitable for medical instrument isolation, industrial distributed power, and data acquisition systems requiring stable component supply with full industrial temperature support (–40°C to +125°C).

Supply support for LT3999EDD#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, Inc. (acquired Linear Technology in 2017) is a global leader in high-performance analog, mixed-signal, and power management semiconductors, serving precision, industrial, automotive, and communications markets.

The LT3999 product line was designed specifically for compact, low-noise isolated DC/DC conversion using push-pull topology - targeting applications demanding galvanic isolation, wide input range, and programmable protection without external controllers.

FAQ

What is the maximum switching frequency supported by the LT3999EDD#PBF?

The LT3999EDD#PBF supports a programmable switching frequency up to 1MHz per power switch (i.e., 2MHz oscillator frequency). This is set by the RT resistor: for example, RT = 12.1kΩ yields 1MHz switch frequency. The SYNC pin also accepts external clocks up to 1MHz for precise harmonic alignment.

Does the LT3999EDD#PBF require external components for basic operation?

Yes - the LT3999EDD#PBF requires external components including a center-tapped transformer, output rectifier diodes, input/output capacitors, and resistors for RT (frequency), RBIAS (bias), and optionally RDC (duty cycle) and ILIM/SS (current limit/soft-start). A minimum 4.7µF low-ESR ceramic capacitor on VIN is mandatory.

How does duty cycle control work on the LT3999EDD#PBF?

Duty cycle control on the LT3999EDD#PBF uses the OVLO/DC and RDC pins: OVLO/DC is biased from VIN via a resistor divider, and RDC connects to ground. The resulting duty cycle (20%–48%) adjusts dynamically with VIN to stabilize output voltage - enabling pseudo-regulation before post-LDO stages and reducing LDO power dissipation.

What is the purpose of the exposed pad on the LT3999EDD#PBF DFN package?

Pin 11 of the LT3999EDD#PBF is an exposed thermal pad that must be soldered to the PCB ground plane. It serves dual roles: lowering thermal resistance (θ_JA = 43°C/W, θ_JC = 5.5°C/W) for reliable 125°C operation and providing low-inductance grounding to minimize switching noise coupling into sensitive analog circuits.

Can the LT3999EDD#PBF be synchronized to an external clock, and what are the requirements?

Yes - the LT3999EDD#PBF SYNC pin accepts a 2V or greater square wave up to 1MHz. To avoid metastability, the internal oscillator frequency (set by RT) must be 10%–50% lower than the external sync frequency. The rising edge of the sync signal initiates clock discharge, enabling deterministic harmonic placement in EMI-sensitive applications.