“A few years ago, manufacturers also set their forward current ratings to 20 mA for white LEDs rather than dim LEDs. Today’s white LEDs can provide higher brightness and therefore must operate at higher bias currents. Maintaining control of the LED bias point at high current operation close to the LED rating requires a new approach.
A few years ago, manufacturers also set their forward current ratings to 20 mA for white LEDs rather than dim LEDs. Today’s white LEDs can provide higher brightness and therefore must operate at higher bias currents. Maintaining control of the LED bias point at high current operation close to the LED rating requires a new approach.
A simple, common method of biasing an LED is to limit the LED’s current with a resistor in series, but this method directly affects power efficiency (defined as the ratio of LED power to total input power). For a white LED operating at 350mA, the corresponding forward voltage drop across the diode is about 3.2V. Connecting the series resistor and LED to the 5V supply works at 64% efficiency, i.e. getting 3.2V from the 5V supply. The power dissipation will generate heat, resulting in an average power dissipation of 36 mW in the series resistor and a forward current of 20 mA, which is still acceptable, but when the forward current is 350 mA, the power consumption will rise rapidly to 630 mW.
In addition, the use of series resistors causes the diode bias point and LED brightness to fluctuate with supply voltage and ambient temperature. National semiconductor‘s LM2852 is a switching buck regulator. It uses internal compensation and synchronous MOSFET switches to drive loads up to 2A. This circuit can effectively provide constant current drive for a high current LED, while at the same time Minimize the effects of supply voltage and temperature changes on LED brightness (Figure 1).
In this circuit, the LM2852 operates at about 93% efficiency, it directly controls a step-down regulator topology, keeping the current flowing through LED1 constant, which can be adjusted with potentiometer R1. The control loop of the circuit completes the current/voltage conversion, effectively regulating the output current of the circuit. In operation, the LM2852 compares its internal reference voltage with the voltage divider formed by D1, R1, and R2, and drives the control loop to maintain a constant 1.2V at its voltage detection pin. The current through the voltage divider is proportional to the current through LED1, and the current ratio tracks the circuit’s operating temperature range, since D1 and LED1 have a similar forward voltage temperature range of 2 mV/°C. Mounting D1 and LED1 on the PCB next to each other provides close enough thermal coupling for temperature compensation.
When the slider of R1 slides fully clockwise, the current through D1 is about 1 mA, and the average current through LED1 is about 500 mA. Adjusting R1 counterclockwise reduces the forward current of LED1 from 500 mA to 0A.
When scaling R1 and R2 values for different current loop gains, lowering the gain affects the conversion efficiency of the circuit, while increasing the gain makes the loop more sensitive to component tolerances. For remote brightness control, the mechanical potentiometer R1 can be replaced with a digital programming potentiometer. The specified limit continuous current is 350 mA, and the peak pulse current is 500 mA. Figure 2 shows the efficiency of the circuit as a function of input voltage. Note that the efficiency of the circuit increases as the input voltage decreases, which helps extend the operating time of battery-powered systems.
When temperature fluctuates, the rate of change of current through LED1 is less than 3% over temperature, a three-fold improvement over a series resistor current-limiting circuit (Figure 3). Although the circuit of Figure 1 is more complex than a single resistor, it also requires few components. For L1, this prototype uses Coilcraft
The selection of capacitors CIN, CSS, and COUT is outlined in the National Semiconductor LM2852 data sheet. In order to dissipate heat more efficiently, there should be a large number of copper foil pads and traces of IC1 and LED1 on the printed circuit board of the circuit. At a forward current of 350 mA, LED1 consumes 1.1W, so please refer to the manufacturer’s datasheet for their thermal design recommendations.
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