Compact Drivers Help LEDs Replace Halogens
May 22, 2007 2:46 PM
By DaFeng Weng, Product Definer, and Mehmet Nalbant, Director of Applications Engineering, Maxim Integrated Products, Sunnyvale, Calif.
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Fig. 1 shows a 5-W MR16 LED lamp driver circuit, composed of a rectifier bridge (formed by D1, D2, D3 and D4), two 100-µF filter capacitors (C1 and C2) and a buck converter circuit. The buck LED driver is composed of the MAX16820, buck inductor L1, power MOSFET Q1, freewheeling diode D5 and sense resistor RSENSE. The 5-W LEDs require 1 A of drive current, which the LED driver is designed to produce. The hysteretic control method is used to control the buck inductor current (which is also the LED current). The hysteretic control implemented in the MAX16820 results in a simple and robust driver with 5% LED-current accuracy.
The high-side current-sense resistor, R, sets the output current and a dedicated PWM dimming input (DIM) allows independent pulsed dimming over a wide range of light levels. The high-side current-sensing scheme and on-board current-setting circuitry minimize the number of external components, while delivering LED currents with ±5% accuracy, using a 1% sense resistor (Fig. 2).
The MAX16820 features a programmable LED current using a resistor connected between the IN and CSN pins. The following equation can be used to calculate the sense resistor value:
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where VSNSHI = 210 mV and VSNSLO = 190 mV.
The MAX16820 regulates the LED output current using an input comparator with hysteresis (Fig. 2). As the current through the inductor ramps up and the voltage across the sense resistor reaches the upper threshold, the voltage at DRV goes low, turning off the external MOSFET. The MOSFET turns on again when the inductor current ramps down through the freewheeling diode, D, until the voltage across the sense resistor equals the lower threshold.
The following equation can be used to calculate the operating frequency:
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where n = number of LEDs, VLED = forward voltage drop of one LED, and DV = (VSNSHI - VSNSLO). A convenient design tool (available at www.maxim-ic.com/MAX16819-20-Tool) can be used to calculate the inductor value.
LED current ripple is equal to the inductor current ripple (Fig. 3). In cases when a lower LED current ripple is needed, a capacitor can be placed across the LED terminals.
