POWER AND LINEAR IC SOLUTIONS

Power Management IC Answers Automotive LED Challenges

The move towards the use of LEDs for automotive headlamps has created a need for compact, efficient driver circuits. Linear Technology’s LT3755 is designed to satisfy this.

The design parameters for headlamp drivers are exacting. They must be capable of operating from the automotive power bus and be both cost- and space-effective. They are key to delivering long LED operating life, by ensuring that the LED current and temperature limits are not exceeded.

Most headlamp applications require approximately 50W of LED current. The LT3755 can boost the automotive bus voltage (nominal 12V) to as high as 60V to drive up to 14 1A LEDs connected in series. The device can attain an efficiency as high as 93%, eliminating the need for heat sinking the power components, and enabling a very compact footprint.



Although the LT3755 is most commonly used in boost mode, its high-side current-sense design enables it to also be configured in buck mode, buck-boost mode or flyback topology, depending on the application’s specific requirements.

The LT3755 drives a low-side external N-channel MOSFET from an internally regulated 7V supply. The fixed frequency, current-mode architecture provides stable and precise operation over a wide range of supply and output voltages.

The device offers a constant-current source, which is imperative for LED driver ICs to enable constant LED brightness even when the input voltage varies. The LT3755’s maximum input voltage of 40V enables it to regulate LED current and voltage when the primary bus is subjected to the 40V transients commonly seen in a load-dump condition.

A ground referenced voltage-feedback pin serves as the input for protection features such as open LED protection and allows the converter to be configured as a constant voltage source. Operating frequency is pin-selectable from 100kHz to 1MHz, allowing the user to optimise efficiency, performance and overall solution size. External synchronisation is available as an option.



The LT3755’s True Color™ feature allows PWM dimming ratios as high as 3000:1 with no change in colour of the emitted light. This enables brightness to be constantly adjusted for a wide variety of ambient conditions. Because Linear Technology’s high-current LED drivers are current-mode regulators, they do not directly modulate the duty cycle of the power switch. Instead, the feedback loop controls the peak current in the switch during each cycle. This improves loop dynamics and provides inherent cycle-by-cycle current limiting.

Features

• Integrated LED controller
• 3000:1 true colour PWM dimming
• Wide input-voltage range: 4.5V to 40V
• Output voltage up to 60V
• Constant-current and constant-voltage regulation
• 100mV high-side current sense
• Multiple modes of operation
  • Boost; buck; buck-boost
  • Flyback topology
  • Single Ended Primary Inductor Converter (SEPIC)
• Adjustable frequency: 100kHz to 1MHz
  • Optional external synchronisation
• Programmable undervoltage lockout with hysteresis
• Less than 1µA shutdown current
• Thermally enhanced 16-Lead QFN (3mm × 3mm) package

Applications

• Headlamps
• Direction indicators
• General-purpose high-power LED control
• Industrial applications

For further information, including a copy of the LT3755 Datasheet, please click here