60V output buck, boost, or buck-boost LED controller
By Paul Rako, Technical Editor -- EDN, June 10, 2008
Linear Technology Corp’s new LT3755 LED controller has a 4.5 to 40V input-voltage range. It drives external FETs to illuminate as many as 14 1A white LEDs from a nominal 12V input, delivering in excess of 50W. The part uses a high-side-current sense, enabling designers to use it in boost, buck, buck-boost, SEPIC (single-ended-primary-inductance-converter), and flyback topologies. The LT3755 can deliver efficiencies of more than 94% in boost mode. A frequency-adjustment pin permits the user to program the frequency between 100kHz and 1MHz. The device also features programmable undervoltage lockout with hysteresis. Shutdown current is less than 1 μA.
The LT3755 uses True Color PWM dimming, which delivers constant LED color with dimming ranges as great as 3000-to-1. Alternatively, you can use the control pin for a 10-to-1 analog dimming range. A fixed-frequency, current-mode architecture provides stable operation over a range of supply and output voltages. A ground-referenced voltage feedback pin serves as the input for several LED-protection features, making it possible for the converter to operate as a constant-voltage source.
The LT3755 has an open-status pin, and the LT3755-1 replaces the status pin with a frequency-synchronization pin. Both parts come in 16-pin, 3×3-mm QFN packages and sell for $2.75 (1000). The parts are also available in industrial-temperature-rated versions.
