LED driver delivers constant luminosity

The circuit in Figure 1 is similar in principle to that of a previous Design Idea (Reference 1) but offers improved, more reproducible performance. The output current is almost constant over an input-voltage range of 1.2 to 1.5V and is insensitive to variations of transistor gain. Transistors Q1 and Q2 form an astable flip-flop. R1 and C define the on-time of Q2. During that time, Q1 is off, and the voltage at the base of Q1 and the current in inductor L ramp up. When the voltage at the base of Q1 reaches approximately 0.6V, Q1 turns on, and Q2 turns off. This switching causes "flyback" action in inductor L. The voltage across the inductor reverses, and the energy stored in the inductor transfers to the LED in the form of a down-ramping pulse of current. During flyback time, voltage across the LED is approximately constant.

The voltage for yellow and white LEDs is approximately 1.9 and 3.5V, respectively. When the current through the LED falls to zero, the voltage at the collector of Q2 falls sharply, and this circuit condition triggers the next cycle. Assuming the justifiable approximation that the saturation voltage of Q2 is close to 0V and that the LEDÕs forward voltage, VD, is constant, you can easily derive the expression for the average dc current through the LED:


At first glance, IAVE depends strongly on VIN. But close examination of the logarithmic term reveals that, with a proper selection of VB, the logarithmic term can become a sharply declining function of VIN. The logarithmic term thus fully compensates for the term VIN2 in the expression. That compensation is precisely the purpose of the diode, D1, in series with the base of Q1. The circuit drives a high-brightness yellow or white LED. Table 1 shows the proper component selection for both colors. Table 1 also shows some measured results at VIN=1.35V. Because the voltage across the white LED falls from 3.9 to 3.1V during flyback, capacitor C subtracts current from the amount available to the base of Q1. This subtraction might retrigger the circuit before the current in L falls to zero. The addition of R3 and D2 solves this problem. During flyback, the current that flows through R3 compensates for the current withdrawn through C.


Reference
1. Nell, Susanne, "Voltage-to-current converter drives white LEDs," EDN, June 27, 2002, pg 84.