Circut Ideas: LED driver lights three LED strings and provides constant voltage supply

Dec. 13, 2007
LED applications are booming.

Typical uses today include backlighting for LCD TVs, projectors, computer displays, camera flashes, and vehicle brake lights. All need constant-current LED drivers that are economical and offer high dimming ratio, high efficiency, and precise adjustable current. To save space, some LED drivers are designed to light multiple strings of LEDs.

For instance, Linear Technology’s LT3476 can drive four LED strings, totaling 100 W. The chip is basically a constant-frequency, currentmode regulator. In applications where there are leftover driver channels, the extra channels can be applied to the task of producing regulated electrical power. This eliminates the need for extra circuitry otherwise used with a separate power supply.

In the accompanying diagram, an LT3476 drives three constant-current RGB LED strings. Its fourth channel serves as a constant-voltage source. A typical application might be in an LCD projector which needs three color LED strings for red, blue, and green. There are six 1-A LEDs in each string, and each string is driven in a buck-mode topology. The LCD panel also needs a bias voltage source, and the fourth channel can provide this bias.

To use one of the LT3476 channels as a voltage source, the CAP and the LED pin of the channel must be tied to the top voltagesense resistor R1. Channel four is a boost circuit that generates a 15.5-V output at 700 mA from a 9-V supply with 90% efficiency.

A point to note is that the CAP and LED pins are the inputs to a voltage-feedback-error amplifier in the chip. This amplifier regulates the current through the sense resistor RSNS and hence through the LED string. In a classical voltage-regulator design, it is safe to assume the voltagefeedback- error amplifier draws no current (10-nA range) from the output. This assumption is not valid in the circuit shown here. The CAP pin and the LED pin bias current of the LT3476 are on the order of 70 μA. This current level is necessary for the high-side current sensing to properly drive the LEDs. As a result, the value of the sense resistor R1 must be low enough to limit the error caused by the bias current. The recommendation is to set R1 at less than 15 Ω. Given VOUT and R2, R1 can be calculated using the equation:

R1 = R2/(VOUT/0.105 V–1)

In this equation, 0.105 V is the regulated sense voltage between CAP and LED when the VADJ pin is tied to the Ref pin directly.

— Hua (Walker) Bai, Applications Engineer, Linear Technology Corp.

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