7W LED-driver IC provides 350 mA
By Paul Rako, Technical Editor -- EDN, October 11, 2007
Targeting use in tunnel-lighting, automotive-lighting, and similar applications, the new Catalyst Semiconductor CAT4201 buck-regulator driver can deliver 7W of power to a string of white LEDs operating from a 24V rail. The device delivers 1W when driving an LED off a 6V rail and delivers as much as 350 mA to the LED. By using a buck-regulator architecture rather than linear-regulator architecture, the device achieves efficiency as high as 94%. Catalyst manufactures the device using a high-performance, high-voltage CMOS process so that it can withstand 40V transients on the input. A single pin provides dimming and control.
A designer establishes the chip’s switching frequency, which depends on the value of the inductor in the circuit rather than the resistor-to-capacitor ratio. The device allows the inductor to run out of current, so it always operates just into the discontinuous mode. The peak current is twice the average current because the current goes to exactly 0A before the part turns on again. This feature allows a resistor that sets the peak current to also establish the average current. Using low-value inductors causes the operating current to rise and fall faster than it would with high-value inductors, resulting in a faster operating frequency for a given input voltage. Using this method, you can set the switching frequency at 50 kHz to 1 MHz. In practice, an inductor value of 33 μH provides a 350-mA drive current across a wide range of input voltages. “The part detects when the inductor current goes to 0A and therefore can operate at the crossover point between continuous and discontinuous operation,” says Anthony Russell, Catalyst’s director of power-management products. “The frequency depends on the inductor value and the supply voltage.” The CAT4201 is available in a ROHS (reduction-of-hazardous-substances) TSOT-23-5 package and sells for 72 cents (10,000). A demo board is available.
