Analog Devices Inc. LT3486EFE#PBF

LT3486 Dual LED Driver DC/DC Converter Product Summary

The LT3486 from Analog Devices is a highly integrated dual step-up (boost) DC/DC converter specifically designed for driving white LEDs with precise current regulation and wide dimming capabilities. Each internal converter can independently drive eight white LEDs in series, supporting up to sixteen LEDs in total from a single device. It efficiently converts input voltages ranging from 2.5 V to 24 V to the required LED forward voltage levels. Intended for applications such as notebook LCD backlighting, cell phone camera flash LEDs, automotive dashboards, and avionics displays, the LT3486 provides two fully independent 1.3A LED drivers with programmable current control and a broad PWM dimming range of up to 1000:1 without color shift. Its compact package options and robust control features make it an optimal choice for precision LED driving in space-constrained environments.

Highlighted Capabilities and Electrical Characteristics of LT3486

The LT3486 offers numerous features aimed at high efficiency and flexibility in LED driving:

Dual independent 1.3A step-up converters capable of powering up to sixteen white LEDs (eight per converter).

A wide PWM dimming range of 1000:1, ensuring fine brightness control with consistent LED color performance.

Precise ±3% LED current programming accuracy through low 200mV feedback reference voltage, minimizing resistor power dissipation.

Supports independent dimming and shutdown control on each LED driver via dedicated CTRL and PWM pins.

Fixed frequency operation programmable from 200 kHz to 2.5 MHz via an external resistor, allowing optimization between efficiency, component size, and EMI considerations.

Open LED protection with 36 V clamp voltage on outputs to prevent damage from disconnected LEDs.

Low quiescent shutdown current (<1 µA) conserves power during device disable modes.

Overtemperature protection and undervoltage lockout enhance device reliability.

Available in compact thermally enhanced 16-pin TSSOP and 16-pin DFN packages with exposed ground pads for improved thermal dissipation.

LT3486 Functional Mechanism and Control System

The LT3486 employs two identical, independent constant frequency current-mode PWM control converters sharing an internal oscillator, bandgap voltage reference, and startup bias circuits. Each converter drives a series LED string regulated to a constant current by monitoring a low voltage feedback signal set to 200 mV. This low feedback voltage reduces power loss in the current sensing resistor, thereby improving system efficiency.

At device startup, both outputs initially charge to the input voltage. Enabling the SHDN pin activates the internal bias circuits and oscillators. The CTRL pins independently control dimming and shutdown of each converter—pulling CTRL low disables the converter, while ramping CTRL from 0 to 1.8 V linearly scales LED current from zero to the programmed maximum. The PWM inputs independently modulate the converters further for extended dimming down to microampere levels by turning the converters fully on or off at programmable duty cycles and frequencies (80 Hz up to 50 kHz typical).

During each switching cycle, the power switch internal to each converter is turned on and off based on the error amplifier output which compares feedback voltage to the internal 200 mV reference, adjusting the peak inductor current accordingly. Output overvoltage protection monitors the output nodes and clamps voltage at 36 V to protect against LED open-circuit events by disabling the switcher and reducing switching frequency to minimize input current.

LT3486 Pin Layout and Package Information

The LT3486 is available in two thermally enhanced 16-pin packages—5 mm × 3 mm DFN and 4.4 mm wide TSSOP. Both designs feature an exposed thermal pad (pin 17) that must be soldered to the PCB ground plane for optimal heat dissipation. Key pins include dual switch collectors (SW1, SW2), feedback inputs (FB1, FB2), overvoltage protection pins (OVP1, OVP2), analog control inputs (CTRL1, CTRL2, PWM1, PWM2), and the shutdown input (SHDN). The device also features a reference output pin that provides a stable 1.25 V bandgap reference for precision external circuitry.

LT3486 Configuration and Dimming Techniques

LED current in each driver channel is set through external precision feedback resistors connected to FB pins. The LED current approximately equals 200 mV divided by the resistor value when CTRL voltage exceeds 1.8 V, enabling tight current accuracy and consistency. For dimming, two main approaches are available:

DC Voltage Control on CTRL Pins: A variable DC voltage from 0 V to 1.8 V linearly adjusts LED current from zero to full programmed value without engaging PWM dimming pins (which should be tied high in this mode). This method is straightforward for applications requiring smooth brightness adjustment without high-frequency switching noise.

Filtered PWM Signal Control: Applying a low-frequency PWM signal filtered by an RC network to the CTRL pins can vary the average LED current with reduced ripple effects.

Direct PWM Dimming via PWM Pins: To achieve a color-stable wide dimming range without forward current variation, a high-frequency PWM signal (from tens of Hz up to 50 kHz) can be applied to the PWM pins. This method switches the entire converter on and off rapidly, turning the LEDs fully on or off at current levels set by the CTRL pin held above 1.8 V. This approach maintains LED color fidelity by avoiding changes in forward current amplitude. The PWM pin logic low below 0.4 V disables the respective converter, while values above 0.9 V enable normal operation.

LT3486 Application Design Guidelines

Designing with LT3486 requires careful selection of external components to balance efficiency, size, and performance:

Inductors: The switching frequency selection directly impacts the inductor value and size. Typical inductors range from 4.7 µH to 22 µH, selected based on desired ripple current between 20% and 40% of peak inductor current. Inductors must have current ratings exceeding peak switch current and low DCR to minimize losses.

Ceramic Capacitors: Use X5R or X7R dielectric ceramic capacitors with sufficient voltage ratings for input and output filtering. A 4.7 µF or larger input capacitor is generally sufficient.

Schottky Diodes: Low forward voltage drop and fast recovery Schottky diodes rated 1 A or higher are preferred to reduce conduction losses and support fast switching. Low reverse leakage diodes are recommended when operating in wide PWM dimming mode to maintain output hold during OFF periods.

MOSFETs: External MOSFET choice is critical for optimal efficiency and reliability. Devices with ≥36 V drain-source voltage ratings and appropriate drain current capabilities are needed. For example, Fairchild FDN5630 is commonly used due to its 60 V rating and 1.7 A capability. For lower current applications, devices like the 2N7002 suffice.

PCB Layout: To minimize electromagnetic interference and switching noise, keep the switch node traces as short and compact as possible, separate high-current switching loops from sensitive feedback traces, and ensure the exposed pad is soldered to a solid ground for efficient heat dissipation.

LT3486 Thermal Management and Protection Mechanisms

The LT3486 is designed with robust protection and thermal management to ensure reliable operation:

Overtemperature Protection: The device includes internal thermal shutdown that disables converters if die temperature exceeds 150°C, automatically resuming operation after cooling. The maximum recommended junction temperature is 125°C.

Undervoltage Lockout: Both converters are disabled when input voltage falls below approximately 2.1 V to prevent erratic operation.

Open LED Protection: Overvoltage clamps at 36 V prevent damage from LED open-circuit conditions by disabling the affected converter and maintaining minimal switching activity.

Soft-Start: Each converter has independent internal soft-start circuitry to control inrush current during startup, ensuring smooth, controlled voltage ramp-up on LED strings.

Representative Applications Showcasing LT3486 Performance

Common LT3486 application scenarios include:

Single Li-Ion Cell Powered Backlighting: Driving sixteen white LEDs with precise current regulation and dimming from a single 3.6 V Li-ion source for notebook displays or handheld device backlights.

Camera Flash LED Driver: Providing regulated high-current drive with wide dimming capability to LED camera flashes in mobile phones.

Automotive and Avionics Displays: Reliable high current LED driving from wider input voltage rails up to 24 V with independent dimming control for instrument clusters and cockpit displays.

Performance data exhibit excellent efficiency across varying input voltages and LED currents with PWM dimming ranges up to 1000:1 without color shift. The device maintains stable operation and uniform brightness in multi-LED configurations.

Alternative or Substitute Models for LT3486

Engineers seeking alternatives or complementary devices to the LT3486 may consider other Analog Devices LED drivers or boost converters with multi-output capabilities and PWM dimming support, depending on specific application requirements such as LED current rating, input voltage range, and package size. Compatible devices may include other Linear Technology (now part of Analog Devices) LED regulators with similar dual boost converter architectures. When selecting replacements, ensure comparable features including dual independent outputs, PWM dimming capability, feedback voltage level, and package thermal performance.

Conclusion

The Analog Devices LT3486 is a sophisticated dual high-current white LED boost driver optimized for compact, efficient, and precise LED illumination applications. Its wide operating voltage range, flexible and accurate dimming controls, integrated protective features, and compact thermally enhanced packages make it ideal for demanding applications requiring uniform LED brightness and stable color performance. Thorough consideration of external component selection and PCB layout ensures optimized efficiency and reliable operation. The LT3486 continues to be a valuable component for engineers designing advanced LED lighting solutions in consumer electronics, automotive displays, and portable lighting devices.