LED driver solutions for portable applications

IntroductionLED

driving has rapidly become an increasingly important application area for power conversion technology. In addition to the requirements for higher efficiency and lower quiescent current, LED driving has many more sophisticated requirements, such as LED matching, dimming, white light balance, etc. There are also some basic architectural issues, such as whether the LEDs are connected in series or parallel, and whether high-side or low-side shutdown is performed. Certain specific LED implementations can achieve different degrees of success, such as the well-known inductive boost solutions and charge pump multipliers. Fractional charge pumps, 4-switch buck-boost solutions, and multi-inductor solutions have been ignored. The latter is also known as "SIMO", which means single inductor multiple output, and this technology is expected to play an increasingly important role in the future as white light LED backlighting is replaced by more complex RGB products.

Introduction to LEDs and How They Work

The trend in portable products is gradually moving towards more multimedia applications. This trend requires the use of higher resolution displays that can support millions of colors. The traditional method of lighting displays has been vacuum fluorescent tubes, but LEDs have recently been widely adopted. LEDs are much smaller in size, which is very beneficial for portable products, and LEDs also consume less power and are far more reliable than vacuum fluorescent lamps. However, maintaining constant light intensity and color is the biggest challenge facing this lighting technology. Understanding how white light LEDs work helps understand how to ensure consistent intensity and color.

Because LEDs are semiconductor devices, they have unique characteristics compared to other light sources, the most notable of which is the nonlinear relationship between current and light intensity. Figure 1 shows this relationship for some typical LEDs.

Figure 1 Nonlinear relationship between current and light intensity for some typical LEDs

The second significant characteristic concerns the forward voltage drop of the LED. Unlike incandescent bulbs, LEDs are not purely resistive loads. The forward voltage drop varies with the color of the LED. Generally speaking, red LEDs have a forward voltage of 2.2V and green LEDs have a forward voltage of 3.1V. White and blue LEDs have the same forward voltage, typically 3.3V.

Providing a constant voltage and current to these LEDs in portable devices is a challenge. The power supply must be able to self-regulate to accommodate the decreasing battery voltage, otherwise the light intensity will vary with the battery voltage. Therefore, these devices require very specific power supplies.

Driver Selection

There are three common architectures for keeping the LED current and voltage constant. The first is an inductive boost regulator for a series LED configuration. The second is still the same inductive boost regulator, but for a parallel LED configuration. The last is a capacitive charge pump. Each of these architectures has its advantages, but only one will provide the greatest benefit for a given application.

Inductive Boost Regulator

The basic operating principle of the inductive boost architecture (such as Fairchild Semiconductor's FAN5608) is to use the current storage capability of the inductor. The inductor can resist the change of current, both positive and negative. The effect of this resistance on the voltage drop across the device can be expressed as follows:

This simple formula shows the working principle of the boost converter. The transistor turns on and current begins to flow through the inductor, then the transistor turns off. Since the current cannot drop to zero instantly, it continues to flow through the diode. The current gradually decreases and the di/dt becomes negative, resulting in a negative voltage across the inductor.

Using Kirchhoff's voltage law, the output voltage can be calculated.

Vin·ton+(Vin-Vout)·toff=0The

above equation can be rearranged as

Here D represents the ON duty cycle. Since D ranges from 0 to 1, the output voltage is always higher than the input voltage. The output voltage is proportional to the duty cycle, so in order to produce a higher voltage, the duty cycle must be increased. The FAN5608 uses this method to achieve a maximum output of up to 18V. This allows driving up to 4 or 5 series LEDs. For parallel configurations, the FAN5608 can generate up to 40mA.

Capacitor Charge Pumps

Charge pumps use capacitors to store energy and can boost input voltages by 1, 1.5, or 2 times. Through a switch array and a clock, capacitors can be alternately charged in parallel and discharged in series to increase the output voltage. Figure 2 explains this principle well.

Figure 2

The maximum output voltage of the regulator depends on the number of capacitors and the time allotted for charging and discharging. Fairchild Semiconductor's FAN5607 uses two capacitors and has three modes: 1×, 1.5×, and 2×. The device can provide up to 30mA of current to each of four white LEDs over an input voltage range of 2.4V to 5.5V.

LED Topology

Using an inductive boost converter, the LEDs can be driven in series or in parallel. A series array ensures that the current through all LEDs is the same, thus ensuring the same light intensity. The disadvantage of this approach is that the output voltage of the driver must equal or exceed the sum of the forward voltages of all LEDs. In some applications, this can be as high as 24V, requiring a silicon process with a breakdown voltage exceeding 24V, which generally increases the cost of the device. Second, the efficiency of the boost converter is also affected as the output voltage increases. Table 1 shows a comparison of the power required to produce the same amount of light from four white LEDs for three different topologies. If efficiency is a high priority, the series topology is not a good choice.

Although the converter does not need to boost the voltage too high (e.g., 3.3V) to drive the parallel array, the parallel topology requires current regulation for each LED. Since the light intensity of the LED varies with the current, the current in all LEDs needs to be matched to keep the light intensity of each LED stable. This adds complexity and cost to the system. The advantage of the parallel topology is high efficiency. From the data in Table 1, it can be seen that the efficiency of the FAN5608 in parallel mode is slightly higher than that in series mode.

Charge pumps are mainly used to drive parallel arrays because the output voltage is related to the number of charged capacitors. Charge pumps have some advantages. They generally require less board space because the capacitors can be as small as 0402 packages. This is a significant advantage, especially when the end product is a portable device. For portable radio products, another benefit is less EMI generation. Even with the use of shielded inductors, the EMI noise generated by inductive boost regulators exceeds that of ordinary charge pumps. This is an important consideration for portable receivers such as mobile phones. The FAN5607 generates very little EMI noise, which makes it very suitable for driving white light LEDs in mobile phone displays. However, if board space and EMI requirements are not too stringent, a charge pump may not be the right solution. This is because, for this solution, efficiency is sacrificed to reduce size. Charge pumps are not the most efficient step-up regulators, so this effect must be taken into account when calculating battery power consumption. Dimming Methods

Dimming

is useful for varying the intensity of lighting to achieve power consumption goals or aesthetic values. There are two common methods for dimming LEDs. The first is to simply adjust the current. Small changes in current cause small changes in LED intensity, which is a very easy to control process. The second method uses a pulse width modulated clock to change the LED's ON duty cycle. The average current through the LED decreases as the duty cycle decreases. The main consideration for this method is the clock frequency, which must be high enough to not be noticeable flicker. It is generally necessary to be 1kHz or higher. Both linear regulation and pulse width modulation affect the color of white LEDs, but in opposite ways.

Most white LEDs are just blue LEDs with a phosphor coating. The electrons in the phosphor are excited by short wavelength light, emitting white light. The color or chromaticity of a white LED will change as the light amplitude, peak wavelength, or spectral shape changes. These factors will change as the junction temperature changes. Dimming with linear current regulation will make white LEDs more yellow because the phosphor is more effective when the current is reduced. Dimming with pulse width modulation will make the LED more blue because the phosphor is less effective. This effect is due to the peak wavelength shifting to shorter wavelengths.

LEDs are an efficient way to illuminate displays in portable devices. Because they use semiconductor technology, they require unique regulation methods. Charge pumps and inductive boost regulators provide the best power solutions, but each has its own advantages and should be considered for specific applications. The importance of efficiency, minimal EMI emissions, and smaller size all dictate the need for the right driver. Another important factor is the dimming method. A

combination

of pulse width modulation and linear regulation provides a stable dimming method while minimizing color variation. Ensuring that LEDs provide constant light is not a challenge, but the solution should be tailored to the application in question to maximize its benefits.