feature

Patented sound noise reduction

Patented Lamp Aging Compensation

190 VPP output voltage for higher brightness

Patented High Efficiency Output Timing

Single Cell Li-Ion Compatible

150nA Shutdown Current

Wide input voltage range 1.8 to 5.0V

Individually adjustable lamps and converters

frequency

Output voltage regulation

Distribution capacity

Available in 8-lead MSOP and DFN packages

application

LCD backlight, mobile phone, PDA, handheld wireless communication products, global positioning system (GPS)

General Instructions

The Supertex HV857 is a high voltage driver electroluminescent (EL) light up to 5.0 square inches designed for driving. This input supply voltage range is 1.8 to 5.0V. The device uses an inductor and a minimum number of passive components. The nominal regulated output voltage indicator applied to the EL is ±95V. By connecting the RSW Osc to the VDD/GND resistor. The HV857 has two internal oscillators, a switching MOSFET, and a high voltage EL lamp driver. The switching frequency MOSFET is connected between the RSW Osc pin and the power supply pin VDD. The EL lamp driver frequency is set by an external resistor connected between the RELOsc pin and the VDD pin. External sensors are connected to the LX and VDD pins for distribution applications or VIN. AU$0.003 - 0.1µF capacitor connected between CS and ground. El lamp is connected between VA and VB. The switching MOSFET charges the external inductor and puts it into the capacitor. The voltage at CS will initiate an increase. Once the voltage at CS reaches the nominal 95V, the switching MOSFET turns off to save power. This outputs VA and VB are configured as an H-bridge and are switching in opposite states to ±95V through the EL lamp.

DC Characteristics (continued)

The inductor used is a 220µH Murata inductor with a maximum DC resistance of 8.4Ω, part number LQH32CN221K21.

Typical performance curve of Figure 1 (EL lamp=3.0in2, VDD=3.0V)

The EL lamp frequency is controlled by an external REL resistor connected between REL Osc and VDD. Lamp frequency increases as REL decreases. As the frequency of the EL lamp increases, the current drawn from the battery will increase and the output voltage VCS will decrease. The color of this EL lamp depends on its frequency. A 2 megohm resistor will provide a lamp frequency of 205 to 275Hz. Reducing the REL resistance by a factor will increase the lamp frequency by a factor of 2.

The switching frequency of the converter is controlled by an external resistor RSW between RSW Osc and VDD of the device. The switching frequency increases as RSW decreases. With a given inductor, as the switching frequency increases, the current drawn from the battery will decrease and the output voltage VCS will decrease

Inductor LX boosts low input voltage by inductive flyback. When the internal switch is turned on, the sensor is charging. When the internal switch is closed, the charge stored in the inductor will be transferred to the high voltage capacitor CS. The energy stored in the capacitor is connected to the internal H-bridge and thus to the EL lamp. Generally speaking, the smaller the inductance value, the larger the current it can handle, and the more suitable it is for driving larger size lamps. When the inductance value decreases, the switching frequency of the inductor (controlled by RSW) should be increased to avoid saturation. A 220µH Murata (LQH32CN221) inductor is generally recommended with a series DC resistance of 8.4Ω. In order to have the same inductance value but lower series DC resistance, an inductor with a lower RSW resistance value is required to prevent high current consumption and inductor saturation. As the size of the EL lamp increases, more current will be drawn from the battery to keep the high voltage across the EL lamp. Input power (VIN x IIN) also increases. If the input power is greater than the package power dissipation, it is recommended to use an external resistor in series with the lamp side to help reduce package power dissipation.

Distribution configuration

The HV857 can also be used for handheld device operation from an adjustable voltage battery. This is shown in Figure 2. The regulated voltage can be used to run the internal logic of the HV857. The current amount is 150µA maximum required to run the internal logic with VDD of 3.0V. Therefore, the regulated voltage can easily provide current without being loaded. The HV857 can easily enable and disable the control signals on the RSW and REL resistors with logic, as shown in Figure 2 below. Control signals can come from a microprocessor. The control signal must track the VDD supply. RSW and REL are usually very high values. Therefore, there are only 10 when the logic signal is in the logic high (enable) state. When the microprocessor signal is high, the device is enabled, and when the signal is low, it is disabled.

Figure 2: Distribution and enable/disable configuration

muffler

This section describes sound-sensitive lamps in applications where Supertex reduces audible noise from ELs. Figure 3 shows a general circuit schematic using a resistor, RSER, in series with an EL lamp.

Figure 3: Typical Muffler Application Circuit

Minimization of audible noise for EL lamps

When the EL light is on, it makes an audible noise. This is due to the construction of EL lamps and it creates a major problem for applications where EL lamps can be placed close to the ear like a cell phone. The noisiest waveform is a square wave Let's say the quietest waveform is a sine wave. After extensive research, Supertex has developed a waveform that is quieter than a sine wave. The waveform takes the shape of a rising time constant of about 2RC and a falling time constant of 2r C, where C is the capacitance of the EL lamp and R is the external resistor, RSER, in series with the EL lamp. This waveform has been shown to produce less noise than a sine wave. The sound produced by the EL lamp can be set to the desired level based on the value of the series resistor used with the lamp. It is important to note that using this resistor will reduce the voltage across the lamp. Reducing the overvoltage of this light will also have another effect on the overall performance of the Supertex EL driver, namely age compensation (patented). This solves a very important EL lamp life that most cell phone manufacturers care about. Effects of Series Resistance on EL Lamps Audible Noise and Brightness As EL lamps age, their brightness decreases and capacitance decreases. By using the RC model to reduce the audible noise emitted by the EL lamp, the passing lamp increases as the capacitance decreases. So the increase in voltage will compensate for the brightness. Therefore, it will extend the half-life (half of the original brightness) of the EL lamp. Increasing the series resistance value with the lamp will reduce the audible noise and brightness of the EL lamp. This is because the output voltage across the lamp will decrease and the output waveform will have more rounded edges.