Application Note 36 Micrel Application Note 36 MIC4826/7 Electroluminescent Display Drivers by William Mai and Andrew Cowell This application note covers the MIC4826/7 Electroluminescent (EL) lamp drivers and designing with EL lamps. With most phosphors, the spectrum of emitted light will tend to shift towards blue with an increase in excitation frequency. Color can be controlled by selecting the phosphor type, by adding fluorescent dyes in the phosphor layer, by using a color filter over the lamp, or a combination of these processes. EL lamp brightness increases approximately with the square of applied voltage. Increasing frequency, in addition to affecting hue, will also increase EL lamp brightness, but with a more linear relationship. Many EL lamp manufacturers provide performance characteristics informing designers on the relationships of frequency, voltage, and EL lamp brightness for their EL lamps. Increased voltage and/or frequency, however, adversely affect lamp life. Higher frequencies generally decrease lamp life moreso than increased voltages. EL lamps, unlike other types of light sources, do not abruptly fail. Instead, their brightness gradually decreases through use. Due to the nature of the devices that EL lamps are used in, this is normally not a concern. The MIC4826 and MIC4827 allow the user to select the EL frequency and voltage driving the lamp to give the user maximum flexibility during the design process. Electroluminescent Displays - The Basics The design of an EL lamp circuit begins with the selection of a lamp. A typical lamp will exhibit a capacitance on the order of 2nF to 3.5nF per square inch. When a high voltage AC signal is applied across the electrodes of an EL lamp, an electric field is generated across the plates of the lamp. This electric field excites the phosphor atoms to a higher energy state. When the electric field is removed, the atoms fall back to a lower energy state, emitting photons as visible light. The wavelength of the emitted light is determined by the type of phosphor used and the frequency of the excitation voltage. Figure 1 shows a typical bridge configuration that is applied to the EL electrodes to generate the AC signal. Typical AC voltages applied to the EL lamp are 50V to 250 VPK-PK, with a frequency of 50Hz to 1KHz. L1 220mH VIN 1 CIN D1 VDD 5 RSW COUT SW 2 RSW Switch Oscillator 6 CS Q1 8 REL VA Q2 V EL Oscillator VREF Transparent Front Protective Cover Transparent Front Electrode Phosphor Dielectric Rear Electrode Rear Protective Cover EL LAMP Q3 7 Figure 3. Typical EL Lamp Construction VB 3 Q4 REL 4 How the MIC4826/7 Drives the EL Display To generate the high voltages needed for driving EL lamps, MICREL drivers employ switch-mode converters using a boost converter to generate the high voltages needed. Following the boost converter is an H-bridge driver, this applies the peak-to-peak voltage across the EL lamp at a user selectable frequency. The MIC4826 provides 160 VPP while the MIC4827 provides 180VPP for bigger EL lamps. Figure 1 shows the internal block diagram of the MIC4826 and MIC4827. The CS pin is the high voltage output of the boost converter, which is half the peak-to-peak voltage across the EL lamp. The second stage is the H-bridge circuit that switches the boost voltage across the EL lamp. Both the switching frequency of the boost converter and the switching frequency of the EL lamp can be adjusted independently. GND Figure 1. MIC4826/7 Block Diagram VA (50V/div) The basic AC signal applied to the EL lamp across VA and VB, the two electrode pins, can be seen in Figure 2. VA – VB (50V/div) VB (50V/div) VIN = 3.0V L = 220µH COUT = 0.01µF Lamp = 2in2 RSW = 332k REL = 3.32M TIME (2ms/div) Figure 2. Typical AC Signal Applied to EL lamp Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com September 2001 1 MIC4826/4827 Application Note 36 Micrel EL Frequency The EL lamp frequency is controlled via an external resistor connected between REL pin and VDD pin of the device. As the the resistor value decreases the lamp frequency increases. The EL frequency range is 60Hz to 1000Hz, with an accuracy of ±20%. By using the below equation and a known value resistor, the EL frequency can be determined. MIC4826/7 Basic Operation The MIC4826 is a high voltage EL driver with an AC output voltage of 160V peak-to-peak. The MIC4827 is a higher voltage EL driver with an AC output voltage of 180V peak-topeak. Both parts are capable of driving EL lamps up to 6 in2 (typically). Input supply current for the MIC4826/7 are typically 21µA. The MIC4826 and MIC4827 have a shutdown current of 100nA. Both high voltage EL drivers have two internal oscillators to control the switching MOSFET and the H-bridge driver. The internal oscillators’ frequency can be individually programmed through the external resistors to maximize the efficiency and the brightness of the lamps. Referring to Figure 1, initially power is applied to VDD. The internal feedback voltage is less than the reference voltage causing the internal comparator to go low which enables the switching MOSFET’s oscillator. When the switching MOSFET turns on, current flows through the inductor and into the switch. The switching MOSFET will typically turn on for 90% of the switching frequency. During the on time, energy is stored in the inductor. When the switching MOSFET turns off, current flowing into the inductor forces the voltage across the inductor to reverse polarity. The voltage across the inductor rises until the external diode conducts and clamps the voltage at VOUT+VD1. The energy in the inductor is then discharged into the COUT capacitor. The internal comparator continues to turn the switching MOSFET on and off until the internal feedback voltage is above the reference voltage. Once the internal feedback voltage is above the reference voltage, the internal comparator turns off the switching MOSFET’s oscillator. When the EL oscillator is enabled, VA and VB switch in opposite states to achieve a 160V peak-to-peak AC output signal for the MIC4826 and 180V peak-to-peak for the MIC4827. The external resistor that connects to the REL pin determines the EL frequency. Switching Frequency The switching frequency of the converter is controlled via an external resistor between RSW pin and VDD pin of the device. The switching frequency increases as the resistor value decreases. The switching frequency range is 8kHz to 200kHz, with an accuracy of ±20%. By using the below equation and a known value resistor, the switching frequency can be determined. fSW (kHz) = fEL (Hz) = A typical EL frequency for a portable device is 100 to 400Hz, depending on display size and type. Inductor Selection In general, smaller value inductors, which can handle more current, are more suitable to drive larger size lamps. As the inductor value decreases, the switching frequency (controlled by RSW) should be increased to avoid inductor saturation, or the input voltage should be increased. Typically, inductor values ranging from 220µH to 560µH can be used. Murata offers the LQH3C series up to 560µH and LQH4C series up to 470µH, with low DC resistance. A 220µH Murata (LQH4C221K04) inductor is recommended for driving a lamp size of 3 square inches. It has a maximum DC resistance of 4.0Ω. Diode The application circuits specify the 1N4148 or equivalent. It has a forward current of 100mA and a typical forward voltage of 930mV. For applications that are not cost driven, a fast switching diode with lower forward voltage and higher reverse voltage can be used to enhance the efficiency. Output Capacitor Low ESR capacitors should be used at the regulated boost output (CS pin) of the MIC4826/7 to minimize the switching output ripple voltage. Selection of the capacitor value will depend upon the peak inductor current, inductor size, and the load. MuRata offers the GRM40 series with up to 0.015.µF at 100V, with a X7R temperature coefficient in 0805 surfacemount package. Typically, values ranging from 0.01µF to 0.1µF at 100V can be used for the regulated boost output capacitor. Remote Enable Remote enable is implemented by connecting RSW and REL to a signal that swings between ground and VDD. When the remote enable is at ground, the power conversion and lamp drive oscillators are halted and the driver becomes disabled. When the remote enable signal is at VDD, the oscillators function normally and the driver is enabled. Since RSW and REL are typically high resistances, loading of the remote enable signal is minimal. However, to avoid interactions between the power conversion and lamp drive oscillators, the remote enable signal should be from a CMOS output of less than 20KΩ. 36 RSW (MΩ) There is a trade off in inductor size versus system efficiency. Normally EL displays are in portable equipment and size is of the utmost importance. A typical switching frequency recommended is 108kHz, giving a recommended typical inductor value of 220µH. See the “Pre-designed Circuits” section for complete information. MIC4826/4827 360 REL (MΩ) 2 September 2001 Application Note 36 Micrel Split Supplies Some applications require a high lamp drive capability but operate from a 1.5V source. The MIC4826/7 family provides high lamp drive, but does not operate directly from a 1.5V source. A technique using split supplies overcomes this challenge. Many applications that operate from a 1.5V supply employ a voltage booster to provide a nominal 3V. Although this 3V, low current supply usually cannot deliver enough power to drive an EL lamp, a split-supply driver circuit circumvents this obstacle. See Circuit 3 in the “Pre-designed Circuits” section for full information Pre-designed Circuits L1 D1 220µH Vishay Telefunken Murata MCL4148 LQH4C221K04 Li-Ion Battery VIN 3.0V to 4.2V C2 10µF/6.3V Murata GRM42-6X5R106K6.3 COUT 0.01µF/100V GRM40X7R103K MIC4826 C1 0.22µF/10V Murata GRM39X7R224K10 R2 3.32M R1 322k 1 VDD SW 5 2 RSW CS 6 3 REL VB 7 4 GND VA 8 3in2 LAMP IIN VA–VB FEL Lamp Size 3.3V 20mA 160VPP 100Hz 3in2 VA — VB (50V/div) VB (50V/div) VA (50V/div) VIN TIME (2ms/div) Circuit 1. EL Driver for PDA Application (3in2 Lamp) September 2001 3 MIC4826/4827 Application Note 36 Micrel L1 220mH Murata LQH4C221K04 VIN 2.5V to 5.5V C2 10mF/6.3V Murata GRM42-6X5R106K6.3 D1 Diodes BAV20WS COUT 0.1mF/100V GRM42-2X7R104K100 MIC4826 R2 3.32M R1 332k 1 VDD SW 5 2 RSW CS 6 3 REL VB 7 4 GND VA 8 EL LAMP LSI X533-13 IIN VA–VB FEL Lamp Size 3.3V 14mA 160VPP 100Hz 2in2 VA — VB (50V/div) VB (50V/div) VA (50V/div) VIN TIME (2ms/div) Circuit 2. EL Driver for 2in2 Lamp Using 1 Cell Li-Ion Battery or 5V Fixed Input Voltage MIC4826/4827 4 September 2001 Application Note 36 Micrel L1 220µH Murata LQH4C221K04 VIN 1.5V C2 10µF/6.3V Murata GRM42-6X5R106K6.3 VDD 1.8V to 5.5V R1 C1 0.01µF/50V Murata GRM40-X7R103K50 442k R2 3.32M D1 Diodes BAV20WS COUT 0.01µF/100V GRM40X7R103K100 MIC4826 1 VDD SW 5 2 RSW CS 6 3 REL VB 7 4 GND VA 8 EL LAMP IIN VDD IDD VA–VB FEL Lamp Size 1.5V 22mA 3.0V 36µA 160VPP 100Hz 1.6in2 VA — VB (50V/div) VB (50V/div) VA (50V/div) VIN TIME (2ms/div) Circuit 3. Split Supply Applications September 2001 5 MIC4826/4827 Application Note 36 Micrel L1 220µH Murata LQH4C221K04 VIN 1.8V to 3.3V C2 10µF/6.3V Murata GRM42-6X5R106K6.3 D1 Diodes BAV20WS COUT 0.1µF/100V GRM42-2X7R104K100 MIC4827 R2 3.32M R1 1M 1 VDD SW 5 2 RSW CS 6 3 REL VB 7 4 GND VA 8 EL LAMP METROMARK 12607-N IIN VA–VB FEL Lamp Size 3.0V 31mA 180VPP 104Hz 5.3in2 VA – VB (50V/div) VB (50V/div) VA (50V/div) VIN TIME (2ms/div) Circuit 4. EL Driver for Remote Control Lamp Using 2 Cell Alkaline Batteries MIC4826/4827 6 September 2001 Application Note 36 Micrel L1 220µH Murata LQH4C221K04 VIN 2.4V to 5.5V C2 10µF/6.3V Murata GRM42-6X5R106K6.3 D1 Diodes BAV20WS COUT 0.033µF/100V GRM42-6X7R333K100 MIC4827 R2 3.32M R1 332k 1 VDD SW 5 2 RSW CS 6 3 REL VB 7 4 GND VA 8 EL LAMP LSI X533-13 IIN VA–VB FEL Lamp Size 3.3V 18mA 180VPP 104Hz 2in2 VA – VB (50V/div) VB (50V/div) VA (50V/div) VIN TIME (2ms/div) Circuit 5. EL Driver for 2in2 Lamp with 180VPP Voltage September 2001 7 MIC4826/4827 Application Note 36 Micrel MICREL INC. TEL 1849 FORTUNE DRIVE SAN JOSE, CA 95131 + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 2001 Micrel Incorporated MIC4826/4827 8 September 2001