Electronic Design

Transformerless EL display driver uses lower-voltage 555 IC

The 555 timer has been manufactured in greater volumes than any other linear IC. Regarded by many as one of the most successful devices of all time, the 555 timer is perhaps only equaled in popularity by the 741 op amp. The success of the part can be attributed to its flexibility, performance, and ability to satisfy the timing requirements of a wide range number of applications. Design demands have changed, though, and the performance of the standard 555 is rapidly becoming inadequate.

The original bipolar parts are used in many applications employing 5-V supplies or higher; while today’s requirements often use 3.3- and 3.0-V supplies or lower. The move to low-power circuits and the increased use of battery power also mean that the original 555 quiescent current rating of 5 to 10 mA now proves to be a limiting factor. By producing the 555 in CMOS, manufacturers have already managed to reduce power consumption and operating voltage, with parts developed requiring 250-µA quiescent current and operating at 2 V.

Zetex’s ZSCT1555 uses bipolar technology capable of operation from a single-cell battery. The new part can operate down to 0.9 V. Assuming a 5-V supply, a typical CMOS solution draws 170 µA, while the new timer consumes only 140 µA; at 1.5 V, this becomes 75 µA. Shown is the ZSCT1555 quiescent current versus supply voltage (Fig. 1).

Electroluminescent (EL) panels are becoming increasingly popular for backlighting portables due to the advantages they offer over CCFL, LED, and incandescent displays. EL panels are more robust, feature a longer life expectancy (approximately 10,000 hours), have uniform light dispersion, and consume less power. They’re usually employed for very low light level viewing, suitable for any equipment featuring an LCD display, including mobile phones, pagers, meters, portable PCs, and automotive instrumentation.

Traditional EL driver circuits feature a flyback transformer design to generate the high ac voltage required to energize the panel. This approach is expensive when the cost of the transformer is considered along with the larger pc-board area it requires. The circuit shown in Figure 2 eliminates the need for a transformer, takes advantage of the ZSCT1555’s low power and the efficiencies of the discrete switching transistors.

The design uses two combined switching circuits. The first generates a high voltage, approximately 200 V using a boost converter. The voltage is chosen according to EL panel size and desired brightness by varying the frequency; the EL panel presents a capacitive load. The second circuit converts the high voltage to an 800-Hz ac signal to drive the EL panel.

The two ZSCT1555 timers are utilized as clocks for the switching transistors. High efficiency is ensured by the high-speed switching characteristics of the bipolar transistors. Advanced transistor design gives the lowest saturation voltage, minimizing power dissipation.

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