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SP4405UEB Datasheet(PDF) 6 Page - Sipex Corporation |
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SP4405UEB Datasheet(HTML) 6 Page - Sipex Corporation |
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6 / 20 page  SP4405DS/18 SP4405 Low Voltage Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 6 35kHz. During the time when the f COIL signal is HIGH, the coil is connected from V BATTERY to ground and a charged magnetic field is created in the coil. When the f COIL signal is LOW, the ground connection is switched open, the field collapses, and the energy in the inductor is forced to flow toward the high voltage H-bridge switches. f COIL will send an array of charge pulses (see Figure 4) to the lamp. Each pulse increases the voltage drop across the lamp in discrete steps. As the voltage potential approaches its maximum, the steps become smaller (see Figure 3). The Switched H-Bridge Network The H-bridge consists of two SCR structures that act as high voltage switches. These two switches control the polarity of how the lamp is charged. The SCR switches are controlled by the f LAMP signal which is the oscillator frequency divided by 128. For a 48.6kHz oscillator, the 7 flip flops will drive f LAMP= 380Hz. When the energy from the coil is released, a high voltage spike is created triggering the SCR switches. The direction of current flow is determined by which SCR is enabled. One full cycle of the H-bridge will create 64 voltage steps from ground to 80V (typical) on EL1 and EL2 (pins 4 and 5, respectively) which are 180 degrees out of phase (see Figure 5). A differential representation of the output is shown in Figure 6. The feedback loop connects the H-bridge network to the error comparator through an internal resistor network. When the error comparator senses a lamp voltage below regulation (< 80V P typical), the error comparator enables the oscillator to perform step-up conversions to the lamp output. When the error comparator senses lamp voltages above regulation (> 80V P typical), it disables the oscillator minimizing power consumption until the lamp voltage drops below regulation. With a varying supply voltage, the EL lamp brightness will appear constant as a direct result of this regulated output voltage. Fine Tuning Performance Circuit performance of the SP4405 can be improved with some of the following suggestions: Increase EL Lamp Light Output: By connecting a fast recovery diode from COIL (pin 3) to C INT (pin 7), the internal diode of the switched H-bridge network 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, be used for D1. This circuit connection can be found in Figure 2. Changing the EL Lamp Output Voltage Waveform: Designers can alter the sawtooth output voltage waveform to the EL Lamp. Increasing the capacitance of the integration capacitor C INT will integrate the sawtooth waveform making it appear more like a square wave. Printed Circuit Board Layout Suggestions: The SP4405's high-frequency operation makes PC layout important for minimizing ground bounce and noise. Keep the IC's GND pin and the ground leads of C1 and C INT in Figure 2 less than 0.2in (5mm) apart. Also keep the connections to COIL (pin 3) as short as possible. To maximize output power and efficiency and minimize output ripple voltage, use a ground plane and solder the IC's V SS (pin 2) directly to the ground plane. EL Lamp Driver Design Challenges There are many variables which can be optimized for specific applications. The amount of light emitted is a function of the voltage applied to the lamp by the inductor, the frequency at which it is applied, the lamp material and the lamp size. Sipex supplies characterization charts to aid the designer in selecting the optimum circuit configuration (see Figures 7 to 25). Sipex will perform customer application evaluations, using the customer's actual EL lamp to determine the optimum operating conditions for specific applications. For customers considering an EL backlighting solution for the first time, Sipex is able to offer retrofitted solutions to the customer's existing LED or non-backlit product for a thorough electrical and cosmetic evaluation. Please contact your local Sales Representative for Sipex or the Sipex factory directly to initiate this valued service. |
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