Chapter 4: Linear Power Supplies
Linear power supplies for driving LEDs are preferred for a number of reasons. The complete absence of any EMI radiation is one important technical reason. Lowest cost is an important commercial reason. However, they also have disadvantages: in some applications they have low efficiency and hence the introduction of thermal problems; in other applications, such as when powered from the AC mains supply, they have the disadvantage of large size.
4.1.1 Voltage Regulators
Many voltage regulators are based on the LM317 originally from National Semiconductor, but which is now made by a number of manufacturers. Inside the LM317 are: (1) a power switch, which is an NPN transistor; (2) a voltage reference set to produce 1.25V and (3) an operational amplifier (op-amp) to control the power switch, as shown in Figure 4.1. The op-amp tries to keep the voltage at the output equal to the voltage at the adjust (ADJ) pin minus the reference voltage.
Figure 4.1: LM317 Regulator
To produce a certain output voltage, a feedback resistor is connected from the output (OUT) to the ADJ pin and a sink resistor is connected from the ADJ pin to ground, thus creating a potential divider. Usually the feedback resistor is set to 240 ohms, in order to draw a minimum of 5mA from the regulator and help to maintain stability.
A capacitor on the output terminal also helps with stability. The output voltage is given by the equation:
Note, I ADJ = 100 ?A, worst case.