A dc-ac inverter circuit generates the high-voltage ac signal required to drive an electroluminescent (EL) panel. An EL panel is a strip of plastic that’s coated with a phosphorous material. When a high-voltage ac signal, which is at least 40 V or greater, is applied across this panel, it emits light. The brightness of this light depends on the amplitude and frequency of the voltage waveform applied across the panel. As the voltage or frequency of the driving signal increases, the brightness of the lamp increases.
EL panels can be used to backlight LCD displays, keypads, or other types of user interfaces. EL lamps typically consume less power than LEDs, which suits them for backlighting battery-powered products, such as pagers, calculators, cellular phones, and so on.
The circuit shown uses the SP4425 IC along with a few other components to convert a 1.5-V battery supply to a high-voltage square-wave output (see the figure). This squarewave output is applied across the EL panel, causing it to illuminate. The SP4425 IC contains an internal oscillator that drives an internal high-power bipolar junction transistor switch. The oscillator frequency is set by C2, which is 180 pF, and is approximately 22 kHz. The switch is connected between one end of the 470-µH inductor and ground. The other end of the inductor is connected to the battery voltage.
When the switch is turned on, a low impedance path is provided between the inductor and ground. This causes the current flowing through the inductor to increase. As the current through the inductor increases, energy is stored in the inductor in the form of magnetic flux. When the switch is turned off, this stored energy is transferred through a diode (D1) to a 0.1-µF capacitor. This process is repeated continually, causing the voltage across the capacitor to increase with each cycle. For a one square-inch-size lamp, the voltage across C1 will increase to a dc level in the range of 50 to 70 V. This voltage is then fed back into the IC at pin 4.
Within the IC is an internal H-bridge circuit and a frequency divider. The frequency divider divides the oscillator frequency down by a factor of 64. Therefore, the frequency divider’s output is approximately 343 Hz. The H-bridge along with the frequency divider convert the dc voltage at pin 4 into a 343-Hz square wave. The voltage amplitude of this square wave is 50 to 70 V, and is approximately equal to the voltage across C1.
Two complementary square-wave outputs are provided by the IC at pin 5 and pin 6. Both outputs are equal in amplitude, but are 180° out of phase with each other. The EL lamp is connected between these two complementary outputs, resulting in a differential voltage across the lamp that’s twice the amplitude of a single square-wave output. The brightness of the lamp typically can range from 3 to 5 foot-lamberts, depending on the dc resistance of the inductor.
This dc-ac inverter circuit is intended for battery-powered applications, in which the display is only required to be illuminated for short periods of time. Therefore, S1 is connected to pin 8 and is used to enable or disable the circuit. When S1 is closed and pin 8 is pulled up to the supply voltage, the IC is enabled. When S1 is opened, an internal pulldown resistor in the IC causes the voltage at pin 8 to be pulled to ground. This disables the IC and puts the circuit in a low-current standby mode. C3 is used as a decoupling capacitor for the IC supply pin. The supply pin must be sufficiently bypassed because large current transients are drawn from the battery when the circuit is functioning.