Optimization of the MAX4990 High-Voltage DC-AC Converter for EL Lamps

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.

Introduction

The 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 Properties

EL 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 (f_EL) 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 Optimization

Circuit 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 Conversion

High 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 (L_X), external fast reverse-recovery diode (D), and external high-voltage capacitor (C_S).


Figure 2. Boost converter for high DC-output-voltage generation.

As switch Q is turned on/off, inductor L_X is charged and discharged. The energy discharged through diode D is stored on capacitor C_S.

The switching frequency (f_SW) of L_X can be set by either of the following:

1. A combination of R_SLEW resistor and C_SW capacitor 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 L_X.
   
   The L_X switching frequency can then be calculated as follows:
   
   
   
   
2. 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 (I_LX can be calculated as follows:



Where:
V_IN = Inductor supply voltage
R_LX = Inductor resistance
L_X = Inductor value
R_Q = Internal DMOS switch resistance
f_SW = Inductor switching frequency
t_ON = (duty cycle)/f_SW = Internal MOSFET switch on-time

To determine whether the inductor is being saturated, the waveform on the node between L_X and 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. The voltage at the bottom of the waveform is not rising linearly and is bent upward, indicating that the inductor is starting to saturate.


Figure 6. The inductor is starting to saturate when the waveform is bent upward instead of rising linearly.

The waveform for an inductor that is in the normal range of operation appears similar to that in Figure 7. The voltage at the bottom of the waveform is rising linearly, thus showing that the inductor is not being saturated.


Figure 7. No inductor saturation is occurring, as evidenced by the bottom of the waveform rising linearly.

To prevent a given inductor from saturating, a shorter on-time (faster f_SW) must be used. This can easily be done by using a smaller C_SW from the SW pin to GND, or by using a higher frequency PWM signal.

We also need to make sure that L_X has enough time to discharge its stored energy to C_S through the external diode so as to prevent inductor saturation. This is done by selecting the appropriate f_SW for L_X.

The diode's reverse-recovery time becomes important, as the diode must contain all the energy transferred from L_X in C_S. Therefore, a diode with a fast reverse-recovery time, such as the BAS21 or BAS20, is recommended for use with the MAX4990.

The C_S value needs to be selected based on the lamp size and the amount of ripple that can be tolerated on the output waveform. Typically, a C_S value of about 1/10 of the EL lamp capacitance is recommended. For any given f_SW, larger C_S values reduce the ripple on the output waveform. In general, higher inductor switching frequencies are recommended when smaller C_S values are selected.

Output Waveform Across the EL Lamp

The MAX4990 uses a high-voltage, full-bridge output stage to convert the high voltage generated by the boost converter to an AC waveform suitable for driving the EL panel. Output voltage, the output waveform's slew rate, slow turn-on and turn-off, and the output waveform's frequency are adjustable to provide the highest degree of freedom when optimizing the circuit so that the best results can be obtained for each application.

Optimization of the EL driver circuit is dependent upon many different variables. The best way to optimize a circuit is to prioritize the variables for each application. For example, because EL lamps are used for backlighting purposes, lamp brightness can often be a fixed value, thereby reducing the set of variables and simplifying the optimization process.

An EL lamp's brightness and color of emitted light are dependent upon the output voltage frequency. Depending on the lamp color, different frequency ranges must be used.

The first step toward optimizing the output waveform is to select the appropriate f_EL based on the desired lamp color.

Lamp frequency is adjusted by either:

1. A combination of the R_SLEW and C_EL capacitor values from the SLEW and EL pins (respectively) to GND. The f_EL can be calculated as follows:
   
   
   
   
2. Applying a clock signal to the EL pin. In this case, the f_EL is ¼th of the clock frequency.

Lamp brightness intensifies as frequency is increased, as illustrated in Figure 8. Keep in mind that, as the f_EL is increased, a higher amount of current from a given input voltage is required to drive a given lamp size.


Figure 8. An EL lamp's brightness and frequency increase concurrently.

Once the f_EL is selected, the peak-to-peak voltage (V_P-P) across the lamp must be selected to achieve the desired brightness. The AC V_P-P is controlled by:

1. A combination of R_DIM and R_SLEW, which can be calculated as follows:
   
   
   
   
2. A PWM signal applied to the DIM pin. The PWM signal's duty cycle is internally translated from 0 to 1.22V.
3. A DC voltage applied to the DIM pin. Increasing the DC voltage from 0.35V to 1.3V increases the V_P-P across the lamp from 70V to 250V.

EL lamp brightness increases as the output voltage across the lamp is increased (see Figure 9). Applying a higher voltage across the lamp requires that a higher current is drawn from the inductor's input voltage source.


Figure 9. EL lamp brightness also increases as V_P-P increases.

For applications that are sensitive to audible noise, the output waveform's slew rate is very important, as it enables the user to shape the output waveform to reduce the audible noise generated by the EL lamp.

The output waveform's slew rate is controlled through R_SLEW, which is connected from the SLEW pin to GND. This slew rate can be calculated as follows:



An ideal output slew rate that minimizes audible noise generated by the lamp is dependent upon the lamp size and mechanical enclosure. Therefore, resonant frequency of the enclosure must be taken into account when selecting f_EL for a given lamp size.

A sinusoidal waveform is believed to be optimal for providing the least amount of audible noise generated by the lamp. However, a sinusoidal waveform also generates less brightness than a square waveform at a given voltage level and frequency. Hence, each circuit must be optimized according to the requirements for its individual application.

As the slew rate of the output waveform is decreased, audible noise generated by the EL lamp is decreased and a lower amount of current is drawn. However, a slower slew rate does decrease lamp brightness, as seen in Figure 10.


Figure 10. Lamp brightness is decreased as the slew rate slows.

To recover the brightness lost due to decreasing the slew rate, V_P-P across the lamp needs to be increased. This can easily be done by increasing the desired output voltage and using a slower f_SW for a given input voltage. A slower f_SW increases the inductor on-time, which allows the storage of more energy in the inductor and the transfer of a higher amount of energy per cycle to C_S.

For applications sensitive to audible noise, it is not recommended to allow the output waveform to enter regulation (i.e., the output waveform should not have a plateau at the top of it). After selecting the slew rate for the application, the output voltage must be adjusted to get the best waveform shape across the lamp.

When using MAX4990, the optimum waveform for applications sensitive to audible noise is thought to be the waveform shape shown in Figure 11.


Figure 11. This waveform is optimal for applications sensitive to audible noise.

The MAX4990 has yet another feature that makes it even more desirable for many applications—its slow turn-on/turn-off time provides a gradual EL lamp turn-on (fade in) and turn-off (fade out).

This slow turn-on/turn-off feature is controlled by connecting a capacitor, C_DIM, in parallel with R_DIM to GND (Figure 12).


Figure 12. C_DIM is connected in parallel with R_DIM to GND.

The slow turn-on/turn-off time is a result of slow voltage decay across R_DIM and C_DIM, and it can be approximated as follows:





Figure 13 shows the affect of using C_DIM in parallel with R_DIM for slow turn-on/turn-off of the EL lamp.


Figure 13. Slow turn-on and turn-off are shown for the MAX4990 when C_DIM is connected in parallel with R_DIM to GND.

Optimizing the MAX4990 Circuit

To optimize an EL lamp circuit when using the MAX4990, the following steps must be taken:

1. Use R_SLEW = 100kΩ. R_SLEW may be changed later to optimize/minimize the audible noise generated by the lamp.
2. Select an inductor value based on available input voltage and lamp size. In general, when using the MAX4990 and the lamp sizes it is capable of driving, inductor values of 100µH to 330µH range are recommended. Select a specific inductor from your preferred manufacturer for your application (please refer to page 11 of the MAX4990 data sheet for some recommended inductors).
3. Set the initial f_SW by selecting an appropriate C_SW value that prevents inductor saturation and provides the required output voltage across the lamp. It is recommended to start with a faster f_SW by selecting a smaller C_SW value and then increasing this value to make sure that the inductor is not entering saturation.
4. Select an initial C_EL value that would provide the preferred f_EL for the minimum R_SLEW value.
5. To obtain the desired brightness, set the output voltage by selecting an appropriate R_DIM value.
6. Select a C_S value from 1nF to 10nF such that the ripple at the top of the output waveform is minimal. In general, larger C_S values must be used for larger lamp sizes. If a specific C_S value is to be used, readjust f_SW to minimize the ripple at the top of the output waveform. In most cases, a C_S value that is 1/10th of the value of the EL lamp capacitance is sufficient.
7. Select the appropriate output slew rate (dV/dt) based on your application's audible noise requirement by using the appropriate R_SLEW value. Audible noise decreases as the waveform's slew rate across the lamp is reduced.
8. Adjust the C_SW and C_EL values to get the appropriate lamp and inductor switching frequencies based on the selected R_SLEW value. In most cases, only an adjustment of C_EL is needed to get the desired f_EL.
9. Set the output voltage again by adjusting the R_DIM value to achieve the desired brightness.
10. Repeat steps 2 through 8 until you satisfy the current-draw requirement of the specific input voltage for your application.
11. Select C_DIM based on the set R_DIM value to achieve the desired slow turn-on/turn-off time.

Using MAX4990 Evaluation Kit for Circuit Optimization

The MAX4990 evaluation kit (EV kit) provides the first order of approximation for optimizing an actual application circuit. This EV kit enables the user to set the f_SW, f_EL, output voltage, and output waveform's slew rate through a separate on-board variable capacitor and resistors.

The on-board 555 timer enables the user to choose different inductor switching frequencies, as well as f_EL, without changing the C_SW and C_EL values and without the R_SLEW value having any affect on these frequencies.

It is recommended that the following equipment is used for optimizing the MAX4990 circuit:

1. Power supply
2. Multimeter in series with the power supply to measure the current draw
3. 2-channel oscilloscope to monitor the waveform and its shape across the lamp (connect one channel to V_A and another to V_B; if desired, view the differential voltage across the lamp by adding V_A to the inverted signal from V_B)
4. Brightness measurement meter or specifications from the lamp manufacturer regarding the voltage and frequency required to obtain certain brightness

The following procedure details how to use the MAX4990 EV kit to optimize a circuit when driving a 20nF green lamp to provide 37cd/m² with f_EL = 400Hz when V_DD = V_BATT = 3.6V. Note that the inductor used on the EV kit is a 220µH inductor from TOKO (D312C series, part number 1001BS-221M), and its C_S value is 3.3nF (150V).

To optimize the circuit using the 555 timer on the MAX4990 EV kit, the following steps must be taken when placing the jumper on JU4 and JU5:

1. Set VR1 to 0Ω with JU1 in positions 2 and 3. Put the JU4 and JU5 jumpers in place.
2. Using a 3.6V supply, power the EV kit with an EL lamp connected to it and JU8 in place.
3. Use VR3 to set the f_EL to 400Hz.
4. Use VC1 to set the current draw to its minimum.
5. Use VR2, with JU3 in positions 1 and 2, to set the output voltage at a level that indicates the circuit is not in regulation. This means that the output waveform does not have a plateau at the top of it, as seen in Figure 14.
   
   
   Figure 14. The MAX4990 EV kit is not in regulation.
   
   The following was measured for V_DD = 3.6V:
   I_DD = 26mA, brightness = 24.6cd/m2, R_SLEW = 100kΩ (VR1 + R1 on the EV kit for R1 = 100kΩ), R_DIM = 73.8kΩ
   
   
6. Measure f_SW. This can be done by expanding the timescale of either V_A or V_B to the point that you can see the small steps on the output waveform, as in Figure 15.
   
   
   Figure 15. The step time can be measured to determine f_SW.
   
   Measure the width of a step (t_STEP) by looking at top 1/3 of either V_A or V_B to calculate:
   
   
   
   
7. Change f_SW through VC1 to increase the output voltage to a level that approximately provides the desired brightness (Figure 16).
   
   
   Figure 16. A sufficient brightness level has been reached.
   
   The following was measured for V_DD = 3.6V: I_DD = 46mA, brightness = 35.2cd/m², R_SLEW = 100kΩ, R_DIM = 73.8kΩ
   
   
8. Check the waveform to make sure that the inductor is not saturated, as shown in Figure 17.
   
   
   Figure 17. The waveform should show no signs of inductor saturation.
   
   
9. Calculate f_SW as in equation 8 from step 6 (Figure 18).
   
   
   Figure 18. Measure step time to determine f_SW.
   
   
10. For applications sensitive to audible noise, continue by repeating step 5 (Figure 19).
    
    
    Figure 19. The waveform has no plateau, showing that the device is not in regulation.
    
    The following was measured for V_DD = 3.6V:
    
    I_DD = 53mA, brightness = 38.5cd/m², R_SLEW = 100kΩ, R_DIM = 87kΩ
    
    
11. With JU1 in positions 2 and 3, use VR1 to change the slew rate of output waveform's rising and falling edges. Note that the faster slew rate (dV/dt) provides higher brightness, but it also causes the EL lamp to generate a higher audible noise (Figure 20).
    
    
    Figure 20. Slew rate adjustment.
    
    The following was measured for V_DD = 3.6V:
    I_DD = 45mA, brightness = 33.5cd/m², R_SLEW = 120.9kΩ, R_DIM = 87kΩ
    
    
12. Repeat step 5 to make certain the device is not in regulation (Figure 21).
    
    
    Figure 21. Again, the waveform is checked to make certain the device is not in regulation (no waveform plateau).
    
    The following was measured for V_DD = 3.6V: I_DD = 52mA, brightness = 38cd/m2, R_SLEW = 120.9kΩ, R_DIM = 114.7kΩ
    
    
13. Repeat step 8 to check for inductor saturation (Figure 22).
    
    
    Figure 22. As can be seen from the waveform, the inductor is not saturated.
    
    
14. Repeat step 11 to change the slew rate (Figure 23).
    
    
    Figure 23. Slew rate adjustment is required to eliminate waveform plateau.
    
    The following was measured for V_DD = 3.6V:
    I_DD = 36mA, brightness = 27.7cd/m², R_SLEW = 178kΩ, R_DIM = 87kΩ
    
    
15. Repeat step 5 to make certain the device is not in regulation (Figure 24)
    
    .
    Figure 24. No plateaus at the top of the waveform indicates that the device is not in regulation.
    
    The following was measured for V_DD = 3.6V:
    I_DD = 49mA, brightness = 36.9cd/m², R_SLEW = 178kΩ, R_DIM = 181.8kΩ
    
    
16. Repeat steps 7, 8, 9, 11 and 5 (in this order) until you achieve the desired brightness with the appropriate waveform shape for your application (Figures 25 and 26).
    
    
    Figure 25. Step 8 is repeated to check for inductor saturation.
    
    
    Figure 26. Repeating step 5 shows that the device is not in regulation.
    
    The following was measured for V_DD = 3.6V:
    I_DD = 56mA, brightness = 37.5cd/m², R_SLEW = 260.5kΩ, R_DIM = 239.4kΩ
    
    
17. Calculate f_SW using equation 8 from step 6 (Figure 27).
    
    
    Figure 27. As in step 6, step time can be measured to determine f_SW.
    
    Using equation 8:
    
    
    
    
18. Measure the R_DIM and R_SLEW values.
    
    R_DIM = 239.4kΩ
    R_SLEW = 260.5kΩ
    
    
19. Note that the R_SLEW value would be the value on VR1 + 100kΩ, as there is a 100kΩ resistor used on the EV kit in series with VR1. Make sure that power is disconnected from the EV kit when measuring the VR1 and VR2 values.
    
    
20. Having measured f_SW, f_EL (400Hz), and both the R_SLEW and R_DIM values, calculate the C_SW and C_EL values to be used in your circuit using equations 1 and 3:
    
    
    
    
    
    
21. Select the closest available capacitor values to the values calculated for your circuit. Some minor adjustments to R_SLEW and R_DIM may be required based on the selected C_SW and C_EL values.
    
    
22. Select C_DIM based on the set R_DIM value to achieve the desired slow turn-on/turn-off time.
    
    If the user replaces the inductor on the EV kit with an off-the-shelf inductor, the circuit will need to be re-optimized based around the new inductor. Additionally, changes in lamp size and available input voltage will require circuit optimization as well.
    
    When using the 555 timer, if f_SW needs to be altered in order to optimize the circuit, the user must make sure that f_SW has a duty cycle (DC) range of 88% to 90%. The DC range is set based on the following equation: