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Auto-biasing white LED drivers reduce overall power By Manfred Plankensteiner Engineering Manager Energy Management and Audio Product Group Dialog Semiconductor E-mail: manfred.plankensteiner @diasemi.com The growth in use of mobile phone handsets with color displays continues to place heavy demands on batter y power. Since cellphones and other portable devices, such as communicators and PDAs with wireless capability, are constantly used for multiple functions, there is significant demand on the backlighting requirements of the LCD module, which needs to be illuminated all the time. While there are many manufacturers of standalone white LED drivers, one technique for currentcontrol boost converters automatically optimizes current efficiency, regardless of the number of LEDs it supports. All color displays need some form of backlight, which is usually enabled by white LED drivers. To ensure uniform brightness over the complete display, the equipment normally uses multiple LEDs. In most applications four LEDs are used, either in parallel or in series. If connected in parallel, each LED can be controlled and adjusted separately. The disadvantage of this approach is that there are more connections to the LCD module and a tighter tolerance requirement with respect to the currents in each LED. On the other hand, LEDs connected in serial will guarantee that each sees exactly the same current. Another advantage of the serial approach is that it decreases system cost, since only two connections to the LCD module are needed. However, the drawback is that it is not possible to independently control the LEDs. With parallel LEDs, there is a chance that a mismatch in currents will occur, depending on the matching of the resistors and for ward voltage of the LED. With serial LEDs, the currents are identical, producing more uniform backlighting. Only the forward voltage variation has to be taken into account, which will influence the overall brightness but is not seen as a local effect. In Li-ion battery systems, white LEDs always need a boost converter. Parallel LEDs require a minimum of 5V. However, by increasing the boost to a much higher voltage system, the cost goes down because fewer contacts are needed to connect the LCD module to the LED driver. Lowering complexity, cost An important consideration is how to optimize power consumption. Where standalone LED drivers are used, this has implications for both the cost and overall power consumption of the entire system. Integrating several functions with similar requirements onto the same piece of silicon produces several design and system efficiencies. There is an argument for integrating the white LED driver within the power management system itself, because of the similarities of the functional blocks and their demands. For example, both functions require similar voltage levels and therefore can be implemented using a similar process. Both features also require direct connection to the battery, which calls for a 5V-compatible process. The output voltage of a white LED driver can go up to 20V, a voltage needed for charger inputs in power-management systems. Integrating the white LED driver within the power-management system can further reduce system costs. Driver similarities Several functional blocks used in a typical white LED driver are identical to those that powermanagement circuits require. For example, both need a reference voltage, current biasing, an oscillator and digital control. A current-controlled boost does not sense the output voltage for regulation; it senses the voltage generated by a current flowing through a sense resistor. This sense voltage is compared with an internal refer- with 40mA output current. In making these calculations, a typical efficiency of 90 percent for the boost converter is taken into account. For a voltage-controlled boost, the output voltage must be high enough to support LEDs with maximum forward voltage. This means that for four LEDs, the output voltage must be above 18.5 V to guarantee that the LEDs will be driven correctly in all operating conditions. However, it is unlikely each LED will demonstrate its worstcase forward voltage. Typically, for four LEDs, only 14.4V would be required. Therefore 4.1V x 20mA = 0.08W is dissipated in the current sink— power that is not converted into emitted light. ence voltage and the converter automatically adjusts the boost output voltage until the reference voltage and the sense voltage are equal. By using a small reference voltage (100 mV), the dissipated power in the sense resistor is negligible. With this approach, the boost converter creates a voltage just high enough to ensure that the current is driven through the LEDs, independent of the number and forward voltage of the diodes. Therefore, all transferred energy from the boost is dissipated mainly in the LEDs—plus a small amount needed in the sense resistor. An additional advantage of the current-controlled approach is the boost converter’s efficiency. Higher output volt- Voltagecontrolled boost Currentcontrolled boost Typical efficiency overall (%) 64, 80 Vforwardmax of LED 4, 5 Vforwardtyp of LED 3, 6 Number of LEDs 1 Vboost N 5 4, 5 3, 6 2 9, 5 90 68, 21 4, 5 3, 6 3 14 90 69, 43 4, 5 3, 6 4 18, 5 90 70, 05 4, 5 3, 6 1 3, 7 90 87, 57 4, 5 3, 6 2 7, 3 90 88, 77 4, 5 3, 6 3 10, 9 90 89, 17 4, 5 3, 6 4 14, 5 90 89, 38 Efficiency boost (%) 90 Source: Dialog Semiconductor The efficiency for a white LED driver is higher using a current-controller boost converter than a voltage-controlled converter. ages normally result in lower efficiency. However, in this case, as the converter only boosts to the voltage needed, the efficiency of the boost is optimized to the actual operating mode. In most applications, the brightness of the backlight is controlled via pulse-width modulation, which can also be used for dimming. A serial interface is needed to program the start value, end value and dim time. If a fixed-voltage boost converter is compared with a current-controlled boost converter, it is possible to highlight the differences in efficiency. In this case, the consumed power out of the battery to the dissipated power in the LEDs has to be compared. Figure shows a comparison of the overall efficiency of the current-controlled and voltagecontrolled white LED driver In addition, a voltage-controlled boost converter needs to be programmed to the number of LEDs used. This can be done with a serial interface or dedicated devices for the different numbers of LEDs needed. The current-controlled boost converter eliminates the need for output-voltage programming as the circuit automatically boosts the required voltage to ensure LED current f low. Hence, this approach consumes less power than a standard circuit high-voltage step-up converter with fixed-output voltage, which needs to boost to the worst-case voltage. It is only in the worst case—when the LEDs have maximum forward voltage—that the current-controlled boost has to create the high voltage. In all other operating modes, however, it will always adjust to the actual needs.