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SP4480EY Datasheet(PDF) 6 Page - Sipex Corporation |
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SP4480EY Datasheet(HTML) 6 Page - Sipex Corporation |
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6 / 13 page  SP4480DS/06 SP4480 Dual Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 6 Increasing Light Output EL lamp light output can be improved by con- necting a fast recovery diode from the COIL pin to the CAP pin. The internal diode is bypassed resulting in an increase in light output at the EL lamp. We suggest a fast recovery diode such as the industry standard 1N4148. The optimal value of C INT will vary depending on the lamp parameters and coil value. Lower C INT values can decrease average supply current but higher C INT values can increase lamp bright- ness. This is best determined by experimenta- tion. A rule of thumb is larger coils (1mH) are paired with a smaller C INT (680pF) and smaller coils (470 µH) are paired with a larger C INT (1800pF). Changing the EL lamp Output Voltage Waveform Designers can alter the sawtooth output voltage waveform to the EL lamp. Increasing the ca- pacitance of the integration capacitor, C INT, will integrate the sawtooth waveform making it ap- pear more like a square wave. driver over the operating parameters. Dual H-Bridge The H-Bridge consists of two SCR structures and two NPN transistors that control how the lamp is charged. Setting ELEN1 to HIGH activates the EL1 and EL2 outputs. Setting ELEN2 to HIGH activates EL3 and EL4 out- puts. The EL driver illuminates the lamp by applying the high voltage supply of the boost converter to the lamp terminals through the H- Bridge and then switching the terminals polarity between the high voltage supply and ground at a constant frequency. This applies an AC volt- age to the lamp that is twice the peak output voltage of the boost driver. An AC voltage greater than the 40V across the terminals of the lamp is typically necessary to adequately illu- minate the EL lamp. Both EL drivers may be operated simultaneously but with decreased light output from the EL panels. DESIGN CONSIDERATIONS Inductor Selection If limiting peak current draw from the power supply is important, small coil values (<1mH) may need a higher oscillator frequency. Induc- tor current ramps faster in a lower inductance coil than a higher inductance coil for a given coil switch on time period, resulting in higher peak coil currents. It is important to observe the saturation current rating of a coil. When this current is exceeded, the coil is incapable of storing any more energy and then ceases to act as an inductor. Instead, the coil behaves according to its series DC resistance. Since small coils (<1mH) have in- herently low series DC resistance, the current can peak dramatically through a small coil dur- ing saturation. This situation results in wasted energy not stored in the magnetics of the coil but expressed as heating which could lead to failure of the coil. Generally, selecting a coil with lower series DC resistance will result in a system with higher efficiency and lamp brightness. Lamp Effects EL lamp parameters vary between manufactur- ers. Series DC resistance, lighting efficiency and lamp capacitance per area differ the most overall. Larger lamps require more energy to illuminate. Lowering the oscillator frequency allows more energy to be stored in the coil during each coil switch cycle and increases lamp brightness. The oscillator frequency can be lowered to a point where the lamp brightness then begins to drop because the lamp frequency must be above a critical frequency (approx. 100Hz) to light. Lamp color is affected by the switching frequency of the EL driver. Green EL lamps will emit a more blue light as EL lamp frequency increases. Noise Decoupling on Logic Inputs If ELEN1 or ELEN2 are connected to traces susceptible to noise, it may be necessary to connect bypass capacitors of approximately 10nF between ELEN1 and VSS and ELEN2 and VSS. If these inputs are driven by a microprocessor which provides a low impedance HIGH and LOW signal, then noise bypassing may not be necessary. |
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