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SP4423CU Datasheet(PDF) 4 Page - Sipex Corporation |
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SP4423CU Datasheet(HTML) 4 Page - Sipex Corporation |
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4 / 13 page  SP4423DS/14 SP4423 Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 4 The on-chip oscillator of the SP4423 can be overdriven with an external clock source by removing the C OSC capacitor and connecting a clock source to pin 8 (Cap 2). The clock should have a 50% duty cycle and range fromV DD-1V to ground. An external clock signal may be desirable in order to synchronize any parasitic switching noise with the system clock. The maximum external clock frequencies that can be supplied is 400kHz. The coil is an external component connected from V BATTERY to pin 3 of the SP4423. Energy is stored in the coil according to the equation E L=1/2LI 2, where I is the peak current flowing in the inductor. The current in the inductor is time dependent and is set by the "ON" time of the coil switch: I=(V L/L)tON, where VL is the voltage across the inductor. At the moment the switch closes, the current in the inductor is zero and the entire supply voltage (minus the V SAT of the switch) is across the inductor. The current in the inductor will then ramp up at a 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- IR L-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 Sipex SP4423 is final tested using a 20mH/70 Ω coil from CTC. For suggested coil sources see page 10. The supply V DD can range from 2.2 to 6.0V. It is not necessary that Vdd = V BATTERY. VBATTERY should not exceed max coil current specification. The majority of the current goes through the coil and is typically much greater than I DD. 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, switching at 1/8 the oscillator frequency. For a 64kHz oscillator f COIL is 8kHz. 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. During the low part of f COIL, 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 16 of these charge pulses 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 shorter (see Figure 1 on page 9). 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 256. For a 64kHz oscillator, f LAMP=250Hz. 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 16 voltage steps from ground to 80V (typical) on pins 4 and 5 which are 180 degrees out of phase with each other (see Figure 3 on page 9). A differential representation of the outputs is shown in Figure 4 on page 9. To minimize AC interference it is advisable to use a decoupling filter capacitor between V DD and ground. Electroluminescent Technology What is electroluminescence? An EL lamp is basically a strip of plastic that is coated with a phosphorous material which emits light (fluoresces) when a high voltage (>40V) which was first applied across it, is removed or reversed. Long periods of DC voltages applied to the material tend to breakdown the material and reduce its lifetime. With these considerations in mind, the ideal signal to drive an EL lamp is a high voltage sine wave. Traditional approaches to achieving this type of waveform included discrete circuits incorporating a transformer, transistors, and several resistors and capacitors. This approach is large and bulky, and cannot be implemented in most hand held equipment. Sipex now offers low power single chip driver circuits specifically designed to drive small to medium sized electroluminescent panels. All that is required is one external inductor and capacitor. |
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