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SP4424NEB Datasheet(PDF) 4 Page - Sipex Corporation |
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SP4424NEB Datasheet(HTML) 4 Page - Sipex Corporation |
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4 / 12 page  SP4424DS/16 SP4424 Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 4 linear rate. As the current in the inductor builds up, the voltage across the inductor will decrease due to the resistance of the coil and the "ON" resistance of the switch: V L=VBATTERY-IRL-VSAT. Since the voltage across the inductor is decreasing, the current ramp-rate also decreases which reduces the current in the coil at the end of t ON the energy stored in the inductor per coil cycle and therefore the light output. The other important issue is that maximum current (saturation current) in the coil is set by the design and manufacturer of the coil. If the parameters of the application such as V BATTERY, L, RL or ton cause the current in the coil to increase beyond its rated I SAT, excessive heat will be generated and the power efficiency will decrease with no additional light output. The majority of the current goes through the coil and typically less than 3 mA is required for V DD of the SP4424. VDD can range from 2.2V to 5V; it is not necessary that V DD=VBATTERY. For example, an unregulated voltage source (3.3V) can be directly connected to the coil, while a regulated voltage source (2.85V) can be connected to the IC V DD pin. Coil performance is also a function of the core material and wire used -- performance variances may be noticeable from different coil suppliers. The Sipex SP4424 is tested using a 5mH/18 Ω coil from Hitachi Metals. For suggested coil sources see page 9. The f COIL signal controls a switch that connects the end of the coil at pin 3 to ground or to open circuit. The f COIL signal is a 75% duty cycle signal. During the time when the f COIL signal is high, the coil is connected from V BATTERY to ground and a magnetic field is generated in the coil. During the low part of f COIL , the ground connection is switched open, the field collapses and the voltage generated in the inductor is directed to the high voltage H-bridge switches. f COIL will send as many charge pulses as possible in 1 Lamp Cycle.{Number of Coil pulses in 1 lamp cycle =} (see figure 2 on page 6). 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 1 on page 6). The H-bridge consists of two SCR structures that act as high voltage switches. These two switches control the polarity of the lamp (capacitor) as it is charged. The SCR switches are controlled by the f LAMP signal which is the oscillator frequency divided by 2. 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 a number of voltage steps from ground to 65V (typical) on pins 4 and 5 which are 180 degrees out of phase (see figure 3 on page 6). A differential view of the outputs is shown in figure 4 on page 6. SP4424 Test Circuit HON=VDD=ON HON=0V=OFF 0.1 µF Low ESR Decoupling Capacitor CLAMP HON VDD EL2 CCOIL VSS Coil EL1 220pF SP4424 VIN=3V + – 1500pF 55nF 100 Ω Lamp Freq 1 x 2 Coil Freq. } |
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