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SP4438UEB Datasheet(PDF) 7 Page - Sipex Corporation |
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SP4438UEB Datasheet(HTML) 7 Page - Sipex Corporation |
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7 / 19 page  7 Rev.11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation DESCRIPTION The SP4438 Electroluminescent Lamp Driver is a low-cost low voltage device ideal for the replacement of LED backlighting designs in cell phones, PDAs and other portable designs desiring low acoustic noise. The SP4438 contains a DC-AC inverter that can produce an AC output of 145V P-P (typical) from a +2.7V to +3.3V input voltage. An internal block diagram of the SP4438 can be found in Figure 2. The SP4438 is built on Sipex's dielectrically isolated BiCMOS process that provides the isolation required to separate the high voltage AC signal used to drive the EL lamp from the low voltage logic and signal processing circuitry. This ensures latch-up free operation in the interface between the low voltage CMOS circuitry and the high voltage bipolar circuitry. The SP4438 is ideal for applications driving EL lamps to backlight LCD displays and keypads, used in cellular radios. A total of only six external components are required for the standard operation of the SP4438: an inductor, a fast recovery diode, three capacitors and a resistor. A diagram of the SP4438 in a typical application can be found in Figure 1. Electroluminescent Technology An EL lamp is a strip of plastic that is coated with a phosphorous material which emits light (fluoresces) when a (>40V) AC signal is applied across it. Long periods of DC voltages applied to the lamp tends 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 offers low power single chip driver circuits specifically designed to drive small to medium sized electroluminescent panels. Market Applications Electroluminescent backlighting is ideal when used with LCD displays, keypads, or other backlit readouts. Its main use is to illuminate displays in dim to dark conditions for momentary periods of time. EL lamps consume less power than LEDs or incandescent bulbs making them ideal for battery powered products. Also, EL lamps are able to evenly light an area without creating any undesirable "hot spots" in the display. THEORY OF OPERATION The SP4438 is a DC-AC inverter made up of: 1. The Oscillator/Frequency Divider, 2. The Coil/Boost Converter, 3. The Switched H- bridge Network, and 4. The Precision Bridge Control Logic. Further details of each element follow. The Oscillator/Frequency Divider The oscillator provides the SP4438 with an on- chip clock used to control the coil switch (f COIL) and the H-bridge network (f LAMP ). Although the oscillator frequency can be varied to optimize the lamp output, the ratio of f COIL/fLAMP will always equal 128. Figure 2 shows the oscillator output driving the coil and through 7 flip flops, driving the lamp. The suggested oscillator frequency is 32kHz for f COIL. The oscillator output is internally divided down by 7 flip flops to create a second internal control signal at 250Hz for f LAMP. The Coil/Boost Converter The supply V COIL can range from +2.7V to +9V. See figure 4 on page 6. V COILshould be chosen such that I COIL does not exceed the maximum coil current specification. The majority of the current goes through the coil and is typically much greater than I DD. The inductor is an external component connected from V COIL to the COIL pin of the SP4438. Energy is stored in the coil according to the equation E L = 1/2 x L x IP 2 where I P, to the first approximation, is the product I P = (tON) x ((VBATT - VCE)/L) where t ON is the time it takes for the coil to reach its peak current, V CE is the voltage drop across the internal NPN transistor and L is the |
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