White LEDs find wide use in backlighting color-LCD screens in most portable devices, such as cellular phones, PDAs, and MP3 players. Multiple LEDs often connect in series to ensure that the same current flows through every LED. To forward-bias these LEDs, a voltage of 10 to 16 V comes from an inductor-based boost regulator, such as an SP6690. However, white LEDs are behind the display, whereas boost regulators are on the main pc board, and it is important to minimize the number of interconnects. You can obtain the best results if you implement high-side and differential-current sensing. In this case, the boost regulator's output looks like a high-voltage true current source. Of course, LEDs need to connect to ground at some point, but it is unimportant where they connect. For example, the display itself can locally pick up ground. This approach allows you to effect a “single”-wire connection. The simple circuit in Figure 1 shows the implementation of the idea.
|Figure 1.||This circuit provides high-side current sensing for driving a string of white LEDs.|
R1 acts as a current-sense resistor. The diode-connected Q2 level-shifts the voltage at Node 1 and applies it to the base of Q1. These transistors come in one package and provide closely matched VBE voltage when they operate at the same current. Because the VBE values match, the emitter of Q1 is at the same voltage as Node 1. As a result, the voltage across resistor R2 matches the drop across R1 and produces Q1 emitter current that equals VR1/R2. This current flows to Q1's collector and creates a voltage drop across R3. The boost-regulator SP6690 regulates the voltage across R3 at 1.22 V, the IC's internal reference voltage. R4 provides current bias for Q2. The value of R4 allows the Q1 and Q2 collector currents to match. You calculate the value of R1 as follows:
where VOUT is the combined LED forward voltage.
The output current is
The circuit in Figure 1 sets IOUT at 20 mA, but you can adjust it by using a different R1 value. Note that you could return R4 to ground, but it instead connects to VIN. This connection removes quiescent current through the resistor and Q1/Q2 when the SP6690 is in shutdown mode.