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SP4491UEB Datasheet(PDF) 7 Page - Sipex Corporation |
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SP4491UEB Datasheet(HTML) 7 Page - Sipex Corporation |
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7 / 13 page  Rev. 12/19/00 SP4491 Two Panel Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 7 Since small coils (<1mH) have inherently low series DC resistance, the current can peak dra- matically through a small coil during 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 bright- ness. 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 fre- quency of the EL driver. Green EL lamps will emit a more blue light as EL lamp frequency increases. Noise Decoupling on Logic Inputs If EL1/EL2 or ON/OFF are connected to traces susceptible to noise, it may be necessary to connect bypass capacitor of approximately 10nF between EL1/EL2 and VSS, and ON/OFF and VSS. If these inputs are driven by a micropro- cessor which provides a low impedance HIGH and LOW signal, then noise bypassing may not ber be necessary. 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 brightness. This is best determined by experimentation. 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 capaci- tance of the integration capacitor, C INT, will inte- grate the sawtooth waveform making it appear more like a square wave. Printed Circuit Board Layout Sugges- tions The EL driver's high-frequency operation makes PCB layout important for minimizing electrical noise. Keep the IC's GND pin and the ground leads of C1 and C INT less than 0.2in (5mm) apart. Also keep the connections to the COIL pin as short as possible. To maximize output power and efficiency and minimize output ripple volt- age, use a ground plane and solder the IC's VSS pin directly to the ground plane. EL Lamp Driver Design Challenges There are many variables which can be opti- mized for specific applications. The amount of light emitted is a function of the voltage applied to the lamp, the frequency at which is applied, the lamp material, the lamp size, and the inductor used. Sipex supplies characterization charts to aid the designer in selecting the optimum circuit configuration. Sipex will perform customer application evalu- ations, using the customer's actual EL lamp to determine the optimum operating conditions for specific applications. For customers consider- ing an EL backlighting solution for the first time, Sipex is able to provide retrofits to non-backlit products for a thorough electrical and cosmetic evaluation. Please contact your local Sipex sales Representative or the Sipex factory directly to initiate this valuable service. |
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