New solutions for powering LEDs in portable applications

The trend in portable products is bent toward increasingly more multi-media applications. This trend has mandated the use of higher resolution displays with millions of colors. The traditional methods used to light the display was the vacuum fluorescent bulb, but recently the most widely adopted method of lighting these displays has been the use of LEDs. LEDs are much smaller in size which is beneficial for portable products, and they consume less power. They are also far more reliable compared to vacuum fluorescent. Maintaining a consistent light intensity and color presents the biggest challenge associated with this lighting method. Understanding how white LEDs function will provide clues as to how to assure the intensity and color are consistent.

LED Operation:

Because an LED is a semiconductor it has unique characteristics when compared to other lighting sources. The most notable characteristic is the non-linear relationship between current and light intensity. Figure 1 illustrates this relationship for some typical LEDs.

The second most notable characteristic is the forward voltage drop associated with an LED. Unlike an incandescent bulb, an LED is not a purely resistive load. The magnitude of the forward voltage varies with the color of the LED. A typical red LED has a forward voltage of 2.2V, and a typical green LED has a forward voltage of 3.1V. The white LEDs are the blue LEDs have the same forward voltage which typically is 3.3V. Providing a consistent voltage and current to these LEDs in a portable device presents a challenge. The power supply needs to adapt to the decreasing battery voltage, otherwise the light intensity will vary with the battery voltage. These devices require a very specific power supply.

Driver Alternatives:

There are three common architectures that are used to maintain both a constant current and a constant voltage for LEDs. The first of these is an inductive boost regulator with the LEDs in a serial configuration. The second architecture is the same inductive boost with the LEDs in a parallel configuration. The last architecture is a capacitive charge pump. There are advantages to each of these architectures, but only one of them will provide the most benefit to a given application.


Inductive Boost Regulator:

The basic operation of the inductive boost, such as the FAN5608 from Fairchild, utilizes the current storage capability of an inductor. An inductor resists changes in current flow, both negatively and positively. That resistance affects the voltage drop across the device. This is expressed as the following ratio:

The simple schematic provides an illustration of how the boost converter functions. The transistor turns on to start the current flowing in the inductor. Then the transistor is turned off. Since the current cannot instantaneously decrease to zero, it continues to flow through the diode. The current gradually decreases, so di/dt becomes negative resulting in a negative voltage across the inductor.

Using Kirchokff’s Voltage Law, the output voltage can be calculated.

V in.ton + (Vin - Vout).toff = 0

This can be rearranged as

where D is the on duty cycle. Since D can range from 0 to 1, the output voltage will always be higher than the input voltage. The output voltage is directly proportional to the duty cycle, so in order to produce a higher voltage the duty cycle needs to increase.

The FAN5608 uses this method to increase the input voltage to as high as 18V. That allows for up to four to five LEDs in series. The FAN5608 can also produce up to 40mA for a parallel configuration.

Capacitive Charge Pump:

A charge pump uses capacitors to store energy and boost the input voltage by a factor of 1, 1.5, or 2. Using an array of switches and a clock the capacitors are alternatively charged in parallel and discharged serially to produce a boost in the output voltage. This is better explained by the following illustration.

The maximum output voltage of this regulator is dependent upon the number of capacitors and the time allotted for charging and discharging. The Fairchild FAN5607 utilizes two capacitors and has three modes, 1X, 1.5X, and 2X. This part can provide up to 30mA through each of four white LEDs over a input voltage range of 2.4V to 5.5V.

LED Topology:

With an inductive boost converter the LEDs can be either serially driven or parallel driven. The serial array assures that the current through all LEDs is identical which assures the same intensity. The unfortunate necessity associated with a serial array is the output voltage of the driver must equal or exceed the summation of the forward voltages of all the LEDs. In some applications that can be as much as 24V. That higher voltage requires the use of a silicon process that has a breakdown voltage in excess of 24V which typically impacts the cost of the part. Secondly, the efficiency of a boost converter suffers as the output voltage increases. Figure 6 demonstrates the variation in power required by three different topologies to produce the same amount of light from four white LEDs. If efficiency is a primary concern, the serial topology is not the appropriate choice.

Although the converter doesn’t need to boost the voltage very high (i.e. 3.3V) to drive a parallel array, a parallel topology requires current regulation for each LED. Since the intensity of an LED varies with current, the current in all of the LEDs needs to be matched in order to have consistent intensity from each LED. This adds complexity and cost to the system. The advantage of the parallel topology is the efficiency. The data shown in figure 6 on the FAN5608 in the parallel mode and serial mode indicates the efficiency is slightly better in the parallel mode.

Charge pumps are primarily restricted to driving a parallel array because the output voltage is dependent on the number of charge capacitors used. There are some benefits associated with charge pumps. Typically they require less board space because the capacitors can be a small as an 0402 package size. This can be a compelling feature especially when the end product is a portable. The added benefit for portable radios is the lower EMI generated. Even with a shielded inductor, the inductive boost regulators generate more EMI noise than the typical charge pump. This is an important consideration in portable receivers such as a cellular phone. The FAN5607 generates very little EMI noise which makes it ideal for driving the white LEDs in a cell phone display. If board space and EMI are not a concern, a charge pump may not be the appropriate solution. The tradeoff for smaller size is efficiency. The charge pump is not the most efficient boost regulator, so it is important to consider this impact when calculating the power draw on the battery.

Dimming Methods:

It can be beneficial to vary the intensity of the lighting for either power consumption or aesthetic value. There are two common methods for dimming LEDs. The first method is a simple regulation of the current. Small changes in current create small changes in the intensity of an LED. That makes this a very easy process to control. The second method is to use a pulse width modulated clock to vary the ON duty cycle of the LEDs. The average current through the LEDs is lowered as the duty cycle is decreased. The key consideration with this method is the frequency of the clock. The frequency needs to be high enough that no flickering is perceived. Generally 1kHz or higher is sufficient. Both linear regulation and pulse width modulation have an affect on the color of a white LED, but they have an opposite affect.

The vast majority of white LEDs are simply a blue LED that has been coated with phosphor. The electrons in the phosphor are excited by the short wavelength light, and they radiate a white light. The color, or chromaticity, of a white LED will change as a result of a change in light amplitude, peak wavelength, or the shape of the spectrum. These will change as the junction temperature changes. Employing a linear current regulation dimming method will cause the white LED to become more yellow because the phosphor becomes more efficient as the current decreases. Employing a pulse width modulation dimming method will cause the LED to become more blue because the phosphor becomes less efficient. This effect is due to a shift in the peak wavelength to a shorter wavelength.

Both the FAN5607 and the FAN5608 allow for the implementation of either dimming method. Both devices allow for a varying analog input that will linearly regulate the current. Both parts have an enable that can be pulsed to turn the output on and off. The ideal dimming method would employ a combination of both methods to minimize the color shift.

Conclusion:

The use of LEDs is an efficient method to light the display in portable devices. Since they are a semiconductor, they require a unique means of regulation. Charge pumps and inductive boost regulators provide the best source of power but there are advantages to each that should be considered for a specific application. The importance for efficiency, minimal EMI radiation, and smaller size will indicate the appropriate driver to use. Another important factor is the dimming method. The combination of pulse width modulation and linear regulation provides a consistent dimming method while minimizing the shift in color. Providing consistent light from LEDs is not a challenge, but the solution should be tailored to the application so the maximum benefit can be realized.

Click here for the illustrations:

Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6