Linear Technology’s LT®3466 is a dual step-up converter capable of driving up to 20 white LEDs with a programmable constant current.The device is also capable of driving asymmetric LED strings with independent dimming and shutdown control for multipanel LCD backlight applications.
The LT3466 IC uses a constant-current, step-up architecture to directly regulate LED current, providing consistent light intensity and color across all LEDs regardless of variations in their forward voltage drop. Important features including soft-start,open LED protection, adjustable switching frequency and DC dimming control are integrated into the part, making the LT3466 LED driver an ideal solution for space-constrained portable devices such as cellular phones, PDAs and digital cameras.
Circuit described here is capable of driving two sets of ten,in-series LEDs with 12mA of constant current per converter.IC LT3466 is a dual channel white LED driver capable of driving up to 20 white LEDs from a single cell Li-Ion (3-5 VDC) input. The device features 44V internal power switches,internal Schottky diodes, adjustable switching frequency, DC dimming control, automatic soft-start, open LED protection and optimized internal compensation.As shown,the circuit works from single Li-Ion (3V) battery or 5V wall adapter.Inrush current during start-up is well controlled due to the internal soft-start circuitry of LT3466.The circuit is also protected with an internal 39V output clamp when the LEDs are disconnected or fail open.Efficiency is above 80% over a wide LED current range of 5mA to 12mA, reaching 83% at 12mA.In this ciruit,the operating frequency was set to 400kHz using a 147k resistor at RT terminal.
LT3466 features single pin shutdown and dimming control for each converter.The LED current in the two drivers can be set independently by modulating the CTRL1 and CTRL2 pins.There are two different types of control methods - DC voltage dimming and filtered PWM signal dimming.Note that,the LED currents are proportional to the voltages at the CTRL1 and CTRL2 pins.DC voltage dimming is achieved by reducing the voltage on the CTRL pin.As the CTRL voltage decreases, the feedback voltage across the LED current programming resistor decreases from 200mV to 0V,thus reducing the LED current from ILED to 0A.PWM dimming works similar to DC voltage dimming except that the DC voltage input to the CTRL pins comes from an RC-filtered PWM signal.The corner frequency of RC components should be much lower than the frequency of the PWM signal for proper filtering.
Courtesy: Linear Technology® http://www.linear.com
