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Basic knowledge and drivers of standard and white LEDs

For many years, light-emitting diodes (LEDs) have been widely used in status displays and dot-matrix display panels. Now, not only can you choose between blue and white light products that have just recently been developed (commonly used in portable devices), but you can also choose among the already existing green, red and yellow light products. For example, white LEDs are considered to be the ideal backlight source for color displays. However, attention must be paid to the inherent characteristics of these new LED products and the need to design appropriate power supplies for them. This article describes the characteristics of new and old types of LEDs, as well as the performance requirements for drive power supplies.

Standard Red, Green and Yellow LEDs
The simplest way to make an LED work is to use a voltage source connected to the LED by connecting a resistor in series. As long as the operating voltage (VB) is kept constant, the LED will emit a constant intensity of light (although the light intensity decreases as the ambient temperature rises). The light intensity can be adjusted to the desired intensity by changing the resistance of the series resistor.

For 5mm diameter standard LED, Figure 1 gives its forward conduction voltage (VF) and forward current (IF) as a function of the curve Note that the LED's forward voltage drop increases with the increase in forward current. Assuming that a green LED operating at a forward current of 10mA should have a constant operating voltage of 5V, the series resistance RV is equal to (5V-VF,10mA)/10mA = 300Ω. The forward turn-on voltage is 2V, as shown in the graph (Fig. 2) under typical operating conditions given in the data sheet.

Figure 1. Standard red, green and yellow LEDs have a forward conduction voltage range of 1.4V to 2.6V. When the forward current is below 10mA, the forward conduction voltage changes by only a few hundred millivolts.

Figure 2. Series resistors and voltage regulators provide a simple way to drive LEDs.

These commercially available diodes are made of GaAsP (Gallium Arsenide Phosphide). Easy to control and well known to the vast majority of engineers, they offer the following advantages:

The resulting color (emission wavelength) in the forward current, operating voltage, as well as changes in ambient temperature to maintain considerable stability. A standard green LED emits a wavelength of approximately 565 nm with a tolerance of only 25 nm. Since the color difference is very small, no problems will occur when driving several such LEDs in parallel at the same time (as shown in Figure 3). Normal variations in the positive conduction voltage will produce a slight difference in light intensity, but this is of secondary importance. Differences between LEDs from the same manufacturer and batch can usually be ignored.
Forward current as high as about 10mA, the forward voltage varies very little. The change is about 200mV for red LEDs and about 400mV for other colors (as shown in Figure 1).
In contrast, for forward currents below 10mA, blue and white LEDs have even less forward voltage variation. They can be driven directly from inexpensive lithium batteries or three NiMH batteries.

Fig. 3. The figure gives the structure of driving several red, yellow or green LEDs in parallel at the same time with very small color differences or brightness differences.

As a result, the current consumption for driving standard LEDs is very low. If the LEDs are driven at a voltage higher than their maximum forward voltage, boost converters or complex and expensive current sources are not required.

LEDs can even be driven directly from lithium batteries or 3 NiMH batteries, as long as the diminished brightness due to battery discharge is sufficient for the application.

Blue LEDs
LEDs emitting blue light have not been available for a long time, and design engineers have only been able to use the existing colors: red, green and yellow. Early "blue" devices were not true blue LEDs, but incandescent lamps surrounded by blue scattering material.

A few years ago, the first "true blue" LEDs were developed using pure silicon carbide (SiC) material, but their luminous efficacy was very low. The next generation of devices using gallium nitride substrate, its luminous efficiency can reach several times the initial product. The current crystal epitaxial material used to manufacture blue LEDs is indium gallium nitride (InGaN). Emission wavelength range of 450nm to 470nm, indium gallium nitride LED can produce five times the light intensity of gallium nitride LED.

White LEDs
LEDs that actually emit white light do not exist. Such a device is very difficult to manufacture because LEDs are characterized as emitting only one wavelength. White does not appear on the spectrum of colors; an alternative is to synthesize white light using different wavelengths.

A trick is used in white LED design. The InGaN substrate, which emits blue light, is covered with a conversion material that emits yellow light when excited by blue light. Thus a mixture of blue and yellow light is obtained, which appears white to the naked eye (as shown in Figure 4).

Figure 4. The emission wavelength (solid line) of a white LED includes peaks in the blue and yellow light regions, but appears white to the naked eye. The relative light sensitivity of the naked eye (dashed line) is shown.

The color of a white LED is defined by the color coordinates; the values of the X and Y coordinates are calculated according to the requirements of the International Commission on Illumination (CIE) Specification 15.2. The datasheet for white LEDs usually details the change in color coordinates caused by an increase in forward current (as shown in Figure 5).

Figure 5. Changes in forward current change the color coordinates of a white LED (OSRAM Opto Semiconductors' LE Q983) and, consequently, the quality of the white light.

Unfortunately, LEDs with InGaN technology are not as easy to control as standard green, red, and yellow light.The display wavelength (color) of InGaN LEDs changes with forward current (as shown in Figure 6). For example, the color variation presented by white LEDs arises from the different concentrations of the conversion material and the wavelength variation of the blue light emitting InGaN material with the forward voltage. The change in color can be seen in Figure 5, where a shift in the X and Y coordinates implies a change in color (as mentioned earlier, white LEDs do not have a well-defined wavelength.)

Figure 6. Increased forward current changes the color of a blue LED by changing its emission wavelength.

When the forward current is as high as 10mA, the change in forward voltage is large. The amount of variation ranges from about 800mV (some diode models vary a bit more). The change in operating voltage due to battery discharge will therefore change the color because the change in operating voltage changes the forward current. At a forward current of 10mA, the forward voltage is approximately 3.4V (this value will vary from supplier to supplier, ranging from 3.1V to 4.0V). Similarly, the current-voltage characteristics vary considerably from one LED to another. Driving LEDs directly from batteries is difficult because most batteries will discharge to a voltage below the minimum forward conduction voltage required by the LED.

Driving White LEDs in Parallel
Many portable or battery-powered devices use white LEDs for backlighting. In particular, PDA color displays require white background light to restore the desired color, which should be very close to the original. Future 3G cell phones support picture and video data, which also require white backlighting. Digital cameras, MP3 players and other video and audio devices also include displays that require white backlighting.

In the vast majority of applications, a single white LED is not enough and several LEDs need to be driven simultaneously. specific operations must be employed to ensure that their intensity and color are consistent, even when batteries are discharged or other conditions vary.

Figure 7 gives the current-voltage curves for a set of randomly selected white LEDs. Loading a 3.3V voltage (upper dashed line) across these LEDs produces forward currents in the range of 2mA to 5mA, resulting in white light of varying brightness. The Y-coordinate variation in this region (shown in Figure 5) is dramatic and can lead to unrealistic display colors. Similarly, the LEDs have different light intensities, which can produce uneven brightness. Another issue is the minimum supply voltage required. The LEDs require a voltage higher than 3V to drive them, below which several LEDs may be completely dimmed.

Figure 7. curve shows the considerable difference between the current-voltage characteristics of different white LEDs, even randomly selected LEDs from the same product lot. driving several such LEDs in parallel with a constant 3.3 V will therefore result in white light of varying brightness (upper dashed line).

Li-ion batteries provide an output voltage of 4.2V when fully charged, which drops to a nominal 3.5V for a short period of operation. as the battery is discharged, its output voltage drops further to 3.0V. if the white LEDs were driven directly from the battery, as shown in Figure 3, the following problems would arise:

First, when the battery is fully charged, all the diodes are lit, but will have different light intensities and colors. As the battery voltage drops to its nominal voltage, the light intensity diminishes and the differences between the white lights become greater. Therefore, the designer must consider the values of the battery voltage and the diode forward voltage, and instead needs to calculate the resistance of the series resistor. (Some of the LEDs will go out completely as the battery is completely discharged.)

Charge pumps with current control
The goal of an LED power supply is to provide an output voltage that is high enough and loads the same current on LEDs connected in parallel. Note (as shown in Figure 5) that if all the LEDs in a parallel configuration have the same current, then all the LEDs will have the same color coordinates.Maxim offers a charge pump with current control to achieve this goal (MAX1912).

Figure 8 shows three LEDs in parallel, the charge pump has a large range, you can increase the input voltage to 1.5 times. Earlier charge pumps could simply multiply the input voltage, while the newer technology offers better efficiency. Increase the input voltage to just the right level to drive the LED operation. A network of resistors connected to SET (pin 10) ensures consistent current to all LEDs. The internal circuitry keeps the SET level at 200mV so that the current flowing through each LED can be calculated ILED = 200mV/10Ω = 20mA. If certain diodes require lower currents, more than 3 LEDs can be driven in parallel at the same time, and the output current of the MAX1912 can be up to 60mA. Further applications and diagrams can be found in the MAX1912 datasheet.

Figure 8. The IC internals include a charge pump and current control. The charge pump provides sufficient drive voltage for the white LEDs, while the current control ensures uniform white light by loading each LED with the same current.

Simple Current Control
White LEDs can be easily driven if the system provides a level above the diode's forward conduction voltage. For example, digital cameras usually include a +5V supply. In that case, there is no need for a boost function because the supply voltage is sufficient to drive the LEDs. for the circuit shown in Figure 8, a matched current source should be selected. For example, the MAX1916 can drive three LEDs in parallel at the same time (as shown in Figure 9).

Figure 9. A single external resistor (RSET) sets the value of the current flowing through each LED. Simple brightness control (dimming function) can be realized by loading a pulse width modulation signal on the enable pin (EN) of the IC.

Simple operation: Resistor RSET sets the current loaded to the connected LEDs. This method takes up very little PCB space. In addition to the IC (small 6-pin SOT23 package) and a couple of bypass capacitors, only one external resistor is required. the IC has excellent current matching, with a 0.3% difference between different LEDs. This structure provides the same color area, so each LED has a consistent white light brightness.

Dimming Changes Light Intensity
Some portable devices adjust the brightness of their light output according to ambient light conditions, while others reduce their light intensity through software after a short idle time. This requires that the LEDs have dimmable light intensity, and such adjustments should affect each forward current in the same way to avoid possible color coordinate shifts. Uniform brightness can be obtained by controlling the current flowing through the RSET resistor using a small digital-to-analog converter.

A 6-bit resolution converter, such as the MAX5362 with an I²C interface or the MAX5365 with an SPI™ interface, is capable of providing 32 levels of brightness adjustment (as shown in Figure 10). Since the forward current affects the color coordinates, the LED white light will change with the light intensity. But this is not a problem because the same forward current will cause every diode in the group to emit the same light.

Figure 10. The digital-to-analog converter controls the dimming of the LEDs by consistently changing the forward current of the LEDs.

The dimming scheme that keeps the color coordinates from shifting is called pulse width modulation. It can be implemented by most power supply devices that can provide either enable or disable control. For example, by pulling down the EN level to disable the device, the MAX1916 can limit the leakage current through the LED to 1µA, resulting in zero emitted light. Pulling the EN level high manages the controllable LED forward current. If a pulse-width modulation signal is applied to the EN pin, the brightness is proportional to the duty cycle of that signal.

Since the forward current flowing through each LED is constantly consistent, the color coordinates do not shift. However, the naked eye will feel the change in light intensity brought about by the change in duty cycle. The human eye cannot distinguish frequencies above 25Hz, so switching frequencies of 200Hz to 300Hz are a good choice for PWM dimming. Higher frequencies can cause problems, as the color coordinates change during the short time interval used to switch the LEDs on and off.The PWM signal can be provided by the microprocessor's I/O pins or by its peripherals. The level of two degrees that can be provided depends on the byte length of the counting register used.

Switch-mode boost converter with current control
In addition to the previously mentioned charge pump (MAX1912), boost converters with current control can also be realized. For example, the MAX1848, a switch-mode voltage converter, can produce an output voltage of up to 13V, enough to drive three LEDs in series (as shown in Figure 11). This approach is perhaps the cleanest because all series-connected LEDs have exactly the same current. the LED current is determined by RSENSE in conjunction with the voltage loaded on the CTRL input.

Figure 11. A switch-mode boost converter can drive several series-connected LEDs. these LEDs all have the same forward current, which (for example) is controlled by the digital-to-analog converter via the CTRL input.

The MAX1848 can be used to implement the dimming function according to either of the methods described earlier. The forward current through the LED is proportional to the voltage loaded on the CTRL pin. Since the MAX1848 enters shutdown mode when the voltage loaded on CTRL falls below 100mV, this also enables the PWM dimming function.

OVERVIEW
White LEDs can be driven in parallel if the uniformity of the white light emission can be ensured by equalizing the LED forward currents.To drive the LEDs, a controlled current source or a step converter with current control should be selected. Such a combination with several standard products can be realized using charge pumps or switching boost converters.