White LED Driver Minimizes Space, Maximizes Efficiency and Flexibility

Introduction

White LEDs are the LCD backlight of choice for portable equipment due to their small profile, exceptional ruggedness and high luminosity. Li-Ion batteries, also rugged and small, are the popular power choice for the same portable equipment. The white LEDs have a nominal forward voltage between 3V and 4V and a Li-Ion battery has a nominal voltage of 3.6V, thus requiring circuitry to step-up the Li-Ion battery’s voltage. The problem is, stepping up tends to reduce efficiency, which in turn reduces battery life. The LTC3205 solves this problem by powering up in direct-connect mode rather than step up mode. Once powered up, it uses a dropout detector for each white LED to determine if any are running out of drive. As the first white LED just begins to lose current, the chip automatically engages a very powerful 2: 3 mode step-up charge-pump. Including 9 precision LED current sources in all, the LTC3205 has enough LED pins and sufficient strength to power a 4-LED main display, a 2-LED sub display and a 3-LED RED, GREEN and BLUE “happy-light”.

Figure 1 shows the block diagram of the LTC3205. The chip is controlled by a simple 3-wire serial interface. The 16-bit register provides two control bits for the main display and two control bits for the sub display, giving exponentially-spaced brightness control for each. The remaining 12 bits are divided among the RED, GREEN and BLUE outputs giving 16 shades for each LED and a total of 4096 total colors for the happy light. The RGB LEDs are pulse-width modulated for brightness control while the white LED currents are linearly controlled. A separate logic reference pin (DVCC) allows logic levels from a microcontroller to be supplied at virtually any voltage above or below the battery voltage.

Figure 1. Block diagram of the LTC3205.

When powered from a Li-Ion battery, the power management section of the LTC3205 connects the LEDs (via the CPO pin) to the battery. Its 2:3 charge-pump only soft-starts when a main or sub display LED has insufficient drive or when the RGB current sources are used. The fractional-ratio charge-pump ensures that the efficiency is high even when the device is in charge-pump mode. The patented constant frequency architecture keeps input noise to a minimum by regulating current on both charge-pump phases.

Two precision servo amplifiers provide inputs to set the reference currents for the LEDs. The main/sub displays and the RGB display are controlled independently for maximum flexibility.

Flexibility Limited only by Imagination

Because there are so many white LED applications, it’s important to arrange each circuit for optimum performance. Given that the LTC3205 has four individual settings for its main display, four individual settings for its sub display and 16 individual settings for each of its color LED pins, the applications it can serve are virtually unlimited.

The LTC3205’s primary application provides regulated currents to a 3- or 4-LED main display, a 1- or 2-LED sub display and 4096 colors to an RGB display. Nevertheless, it is possible to arrange the LTC3205 to provide power to an 8-LED keyboard display as shown in Figure 2. Alternatively, it can be used to power a 4-LED camera light as shown in Figure 3.

Figure 2. 8-LED keyboard display.

Figure 3. 4-LED camera light.

Brightness control can be achieved in a number of ways for each of the displays. The RGB display has 16 built in settings per LED. No external hardware or signals are required. The main and sub displays have four settings each, but these can be easily multiplied by adding a digital control signal to switch a reference resistor in or out as shown in Figure 4. With techniques like this, the number of brightness settings can be increased rapidly to 7 or 13 for the main and sub displays. Brightness can also be controlled by analog means as shown in Figure 5.

Figure 4. Alternative digital brightness control.

Figure 5. Alternative analog brightness control.

Efficiency

Because the LTC3205 only enables its charge-pump as needed, it spends the majority of each battery cycle in direct-connect mode. Since the LED voltages are so close to the battery voltage, true efficiency (PLED/PBATTERY) is maximized, as is battery life. Figure 6 shows an example of achievable efficiency as a function of battery voltage for a 4-LED application running at 15mA per LED.

Figure 6. 4-LED main panel efficiency vs battery voltage.

To achieve this high level of efficiency, the LED current sources are designed to deliver accurate current with as little as 120mV of compliance voltage. Furthermore, the 0.8Ω pass switch in direct-connect mode drops only 48mV with 60mA of display power. Both of these features are required to keep the LTC3205 in direct-connect mode as long as possible.

Conclusion

The LTC3205, designed specifically for portable backlighting applications, provides all of the necessary current regulation, power circuitry and control logic to deliver efficient and accurate power to a large number of LEDs in a portable product. To further reduce board level complexity, it uses only four 0603 sized ceramic capacitors keeping the total solution height under 1mm. A straightforward serial interface reduces the number of wires needed to control all of the LEDs. Given its feature set, the LTC3205 packs an amazing amount of backlighting horsepower, flexibility and performance into a very small 4mm × 4mm footprint.

作者

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Steve Martin