Abstract: The AC waveform (for voltage level and frequency) across an electroluminescent (EL) lamp affects brightness, current consumption, and the color of emitted light, and adjustment of the output waveform's slew rate affects the audible noise generated by the lamp itself. Consequently, the MAX4990 circuit must be optimized according to the requirements for each application. This application note provides information regarding circuit optimization and indicates the best external components to be used with the MAX4990. IntroductionThe MAX4990 is a high-voltage DC-AC converter designed to drive electroluminescent (EL) lamps. To generate the high voltage necessary to drive an EL panel, the MAX4990 utilizes a high-frequency boost converter and a high-voltage, full-bridge output stage to generate a high-voltage AC waveform suitable for driving the EL lamp. The MAX4990's proprietary acoustic noise-reduction circuit controls the slew rate of the AC voltage driving the EL panel, thus reducing audible noise. The MAX4990 provides a DIM pin that allows the user to set the EL output voltage through a PWM or DC analog voltage, or by connecting a resistor to GND. A capacitor placed in parallel to the resistor on the DIM pin allows the user to program a slow turn-on/off time for the MAX4990. An EL lamp's brightness, current consumption, and color of emitted light are affected by the AC waveform (for voltage level and frequency) across the lamp. Adjustment of the output waveform's slew rate affects the audible noise generated by the lamp itself. Therefore, depending on requirements for each application, the MAX4990 circuit must be optimized. This application note discusses circuit optimization and selection of the external components that should be used with MAX4990. EL Lamps and Their PropertiesEL lamps are constructed of light-emitting layers of phosphor particles evenly dispersed on dielectric material. These layers are sandwiched between rear and transparent front electrodes, which are then covered by protective polymer layer. Generally, EL lamps behave as capacitors, as shown in Figure 1. Figure 1. Simplified EL lamp diagram showing its resistance and capacitance. When alternating voltage is applied to the electrodes of an EL lamp, phosphor electrons in the outermost energy level (valance band) are energized and transition to a higher energy level. However, because the higher energy level is not stable, the excited electrons return to their original energy level, thereby releasing photons. Due to even distribution of the phosphor particles, the light emitted by an EL lamp is uniform throughout the surface of the lamp. A change in frequency has an affect on the color of the emitted light, but both applied voltage and frequency affect EL lamp brightness. Raising voltage and/or frequency increases lamp brightness, but it also affects lamp life. In general, increasing lamp frequency (fEL) decreases lamp life more rapidly than increasing voltage. EL lamp life is most commonly specified in terms of time to half luminance (TTHL), or 'half-life,' which is the time it takes for the brightness of the EL lamp to decrease to half of its initial brightness at a given voltage and frequency. TTHL specifications are generally provided by the EL lamp manufacturer and specified in terms of thousands of hours. TTHL is not usually a concern for handheld products in which the lamp does not remain on for very long periods of time. In general, there are two types of EL lamps available: high-voltage EL lamps and low-voltage EL lamps. High-voltage EL lamps have a much higher threshold voltage than low-voltage EL lamps, meaning that a higher peak-to-peak voltage is required across a high-voltage EL lamp for it to start illuminating. High-voltage EL lamps are targeted to be driven with transformer-based drivers, whereas low-voltage EL lamps are meant to be driven by IC-based EL drivers. Hence, selection of the appropriate EL lamp for each application becomes the first step towards optimizing a circuit. MAX4990 Circuit OptimizationCircuit optimization consists of selecting the external components such that the required parameters for each EL panel application are met. These parameters consist of: output voltage, current draw, lamp frequency, lamp brightness requirement, lamp-generated audible noise, and output waveform shape. DC-DC ConversionHigh DC-output-voltage generation is accomplished by a boost converter. The boost converter (Figure 2) consists of: an internal DMOS switch (Q), internal switch oscillator, external inductor (LX), external fast reverse-recovery diode (D), and external high-voltage capacitor (CS). Figure 2. Boost converter for high DC-output-voltage generation. As switch Q is turned on/off, inductor LXis charged and discharged. The energy discharged through diode D is stored on capacitor CS. The switching frequency (fSW) of LXcan be set by either of the following: A combination of RSLEWresistor and CSWcapacitor values from the SLEW and SW pins (respectively) to GND, as indicated in Figure 3. Figure 3. The resistance value of the SLEW pin is used with the capacitance of the SW pin to calculate the switching frequency of LX. The LXswitching frequency can then be calculated as follows: Driving a PWM signal directly into the SW pin, as illustrated in Figure 4. Figure 4. A 90% duty cycle PWM signal is driven into the SW pin. The selection of the inductor is very important, as we need to make certain that the boost converter is capable of managing the application's required current level without saturating the inductor. Therefore, we must pay close attention to the inductor's current saturation level. The current through an inductor (ILXcan be calculated as follows: Where: VIN= Inductor supply voltage RLX= Inductor resistance LX= Inductor value RQ= Internal DMOS switch resistance fSW= Inductor switching frequency tON= (duty cycle)/fSW= Internal MOSFET switch on-time To determine whether the inductor is being saturated, the waveform on the node between LXand D (Figure 5) must be monitored if there are no current probes available. Figure 5. The red circle shows where a scope probe should be placed to detect inductor saturation. Figure 6 shows how to detect when an inductor is entering saturation. T