|LEDs have advantages and disadvantages when compared with other light sources such as incandescent or fluorescent lamps. The most significant advantages are fast turn-on, lower heat generation, lower power consumption, higher operating life, and high resistance to shock/vibration. Some of the limitations are the narrow viewing angle, near monochromatic light, limited wavelength selection, and the fact that LEDs require electronic drive circuits for operation.
LEDs need to be driven properly to ensure optimal performance and long life, and designing and implementing an effective driver is key to obtain all the benefits of LEDs. The driver's implementation must be cost effective, which is not usually achieved with discrete components but can be realized with integrated solutions.
LEDs, regardless of color, have an extremely long lifetime (often 100K hours), whenever their current and temperature limits are not exceeded. LEDs must be operated within the manufacturer's specified limits of both current and diode junction temperature in order to obtain maximum life.
Figure 1 shows a typical I-V curve for an LED. This graph in particular makes reference to the high-current LED technology recently introduced by Lumileds. The maximum forward current varies with the different type, style, and manufacturer of LEDs. Lumileds has specified the maximum forward currents at 30 mA, 75 mA, 150 mA, 350 mA, and 700 mA for differently constructed LEDs. The higher current devices have special thermally designed packages to transfer the heat to a heat sink. The same rules can apply to devices having other current ratings by simply scaling down the current and power designs.
Driving LEDs using discrete components
There are several methods to drive LEDs with discrete components. Figure 2 illustrates one of the simplest using a resistor in series with the supply voltage to limit the current. This type of methodology is simple and cheap but has several weaknesses. The most significant is that since there is not current control device, the variations in the input voltage will change the average current to the LEDs, which results in poor illumination quality and sometimes even in the degradation or total damage of the LEDs for high line-voltage conditions.
To better illustrate the problem, calculations of the current supplied to the LEDs will be made based on the circuit shown in figure 1. The normal ac line can fluctuate by 10 percent and therefore the transformer output can vary between 10.8 Vac and 13.2 Vac whenever the normal secondary voltage is 12.0 Vac. Based on this, the LED's current calculation for low, normal, and high voltage ac line is made through the following formula:
ILED = [(Vin x sq.root 2) – (3 x VLED)] / R1
Assuming that the characteristics of the LEDs are If = 350mA and Vf = 3.5V, then the resulting value for ILED for each of the voltage line conditions is as follows:
- Low line: 238 mA
- Normal line: 323 mA
- High line: 408 mA
As it can be seen, the change of the LED's current is higher than ±25% for a ±10% variation in the ac line. In the low line case, this variation causes the LEDs to dim while for the high line case it may potentially damage them due to the overheating caused by the high current. This explains why this type of drive circuit is not recommended, nor often used, because they degrade the LED's quality and life time.
Another common LED driver method using discrete components is made through a linear regulator (ON Semiconductor MC7805, MC7809 or similar), and a series medium power resistor (usually 1W or bigger). Figure 3 shows this concept. The LED's current is set by the regulated output voltage of the linear regulator and the value of the resistor R1.
Assuming that the characteristics of the LEDs are If = 350mA and Vf = 3.5V, then the resulting resistor value is as follows:
R1 = (Vout - VLEDs) / ILEDs = (9V - 7V) / 0.350A = 5.7 ohms
The power dissipation in R1 is given by:
P = I2 x R = (0.350)2 x 5.7 = 0.7 Watts
The current regulation is mostly dependent on the regulator's performance and should be expected to be good since most voltage regulators provide a good percentage of line and load regulation, usually lower than ±5%.
Although this type of concept provides good current regulation to the LEDs, it may not be optimum for applications where cost is critical, and either for those requiring enable or electronic dimming functions.
Integrated LED drivers
Integrated LED drivers provide a constant current source and are designed to replace discrete solutions for driving LEDs in ac or dc voltage applications. An integrated driver eliminates the need for individual components by combining them into a single small surface mount package (SO-8), which results in a significant reduction of both system cost and board space.
This device provides a regulated dc current to a LED array, from an ac or dc input. It can drive arrays of series or parallel-series LEDs for a wide range of applications. It has a low voltage overhead (1.4V) to facilitate its usage in low voltage applications. Its current regulating principle is made through the generation of an internal constant reference voltage (0.7V) across an external low power resistor (Rext), which sets the current independently of the input voltage supplied.
This operating principle makes it very simple to design LED circuits around the NUD4001 device. Nevertheless, there are certain design considerations such as the maximum device's power dissipation (1.13 Watts), operating ambient temperature range, device's voltage overhead and LED's array configuration that have to be taken into account before implementing this integrated driver. Figure 4 shows a typical application utilizing the NUD4001 device to drive three high-intensity white LEDs (If = 350mA and Vf = 3.5V).
The typical current regulation performance of the NUD4001 device for the circuit of figure 4 using a Rext = 2.2 ohms is shown in figure 5. At 25°C, the change of the LED's current is only 1% for an increment of 15% in the input voltage. The regulation ratings are obtained from the data shown in figure 5:
For Vin = 12 Vdc, Iout = 328 mA
For Vin = 13.8 Vdc, Iout = 331 mA
Similar regulation values are obtained at low and elevated temperatures. However, it is important to note that at low temperature, the LED's current is shifted by a factor of 5% while at elevated temperature it is lowered by a factor of 11%. These values are also obtained from the data shown in figure 5:
For Ta = 0°C and Vin = 12.5Vdc, Iout = 344 mA
For Ta = 25°C and Vin = 12.5Vdc, Iout =329 mA
For Ta = 85°C and Vin = 12.5Vdc, Iout =291 mA
This type of behavior is ideal and usually desired by LED manufacturers. This is because at high ambient temperatures the junction temperature in the LEDs increases but the reduction in current cancels this effect. At low temperatures the current may be increased by a small percentage (usually no higher than 10%) since the LED's junction temperature is colder.
PWM and enable functions
Pulse width modulation (PWM) and enable functions are very important for some LED applications, especially for those where color mixing and dimming is required. Implementing these functions into discrete LED drivers is complex and increases the system cost significantly. Most integrated LED drivers (such as the NUD4001 device) offer a PWM/enable function that is used for dimming and color mixing applications.
Figure 6 shows how to implement a PWM/enable function on the NUD4001 device. This is made by simply adding an external small signal NPN transistor connected between pin 4 and ground. The same small signal transistor can be used for an enable function for conditioning applications.
The function of Rext2 is to pull up the pin 4 of the device when the PWM signal in the base of the NPN transistor is low. The average current applied to the LED is directly dependent on the duty cycle (Iavg =Ipeak x duty cycle). And the LED's light intensity is directly dependent on the average current I(avg) applied. In the case of figure 6, the current is set to be 350 mA at 100% duty cycle and therefore, it proportionally decreases for narrower duty cycles. The PWM circuit is good for frequencies up to 10 kHz.
The same type of configuration is used for the enable function. The only difference is the way that the base of the NPN transistor is driven.
Series resistor methods: Use of these methods to drive LEDs is not recommended nor often used because they basically eliminate the valuable features of LEDs, and sometimes even cause total damage to the devices.
Linear regulators: Although the concept of linear regulators provides good current regulation for LED circuits, it may not still be optimum for applications where cost is critical, and either for those applications requiring enable or electronic dimming functions.
Integrated LED drivers: If implemented correctly, integrated drivers offer a low-cost current regulation solution for different LED circuits of ac/dc voltage. Design considerations such as the device's power dissipation, breakdown voltage and maximum current capability have to be taken into account before implementation in application circuits.
The enable and PWM features, as well as the low cost implementation, are usually determines the use of an integrated LED driver rather than a discrete solution in LED lighting applications.