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HV857

High Voltage EL Lamp Driver for Low Noise Applications

Supertex Inc 

HV857 Application Note

Mobile Phone Circuit for Audible Noise Reduction:1

The following table provides EL lamp audible noise and brightness for circuits which were designed based on typical EL lamp sizes for

Mobile phone applications. See Figure 1, Table 3.

Table 1

Circuit

Lamp Size +

Series R

Audible

Noise

2.6in2 + 0K

2.6in2 + 25K

1

2.6in2 + 50K

2.6in2 + 75K

2.6in2 + 100K

1.7in2 + 0K

1.7n2 + 25K

1.7in2 + 50K

2

1.7in2 + 75K

1.7in2 + 95K

1.7in2 + 120K

35.1dBA

32.0dBA

29.2dBA

26.7dBA

23.3dBA

32.0dBA

28.3dBA

26.0dBA

24.4dBA

22.9dBA

21.0dBA

Note: 1. All values are nominal.

Lamp Brightness

ft-lm

8.09

6.93

5.00

3.83

2.80

6.90

6.35

5.72

4.85

4.20

3.42

Cd/m2

27.7

23.7

17.1

13.1

9.56

23.59

21.73

19.55

16.60

14.35

11.69

Supply Voltage

VDD

VIN

3.0V 3.0V

3.0V 3.0V

Lx Supply Lamp

Current Frequency

20.6mA

23.3mA

23.5mA

22.6mA

21.3mA

13.4mA

15.5mA

16.6mA

16.9mA

16.5mA

15.6mA

250Hz

250Hz

How to Minimize EL Lamp

Audible Noise:

The EL lamp, when lit, generates an audible noise. This is due

to EL lamp construction which creates a major problem for

applications where the EL lamp can be close to the ear such as

cellular phones. The noisiest waveform is a square wave and the

quietest waveform has been assumed to be a sine wave.

After extensive research, Supertex has developed a waveform

that is quieter than a sine wave. The waveform takes the shape

of approximately 2RC time constants for rising and 2RC time

constants for falling, where the C is the capacitance of the lamp

and R is the external resistor used in series with one side of the

lamp. This waveform has been proven to generate less noise

than a sine wave.

The audible noise from the EL lamp can be set at a desired level

based on the series resistor value used with the lamp. We have

chosen two commonly used lamp sizes for the mobile phones to

demonstrate the effect of series resistor on the audible noise

generated by the EL lamp. It is important to note that use of this

resistor will reduce the voltage across the lamp. Reduction of

voltage across the lamp will also has another effect on the overall

performance of the Supertex EL drivers, age compensation

(patent pending). This addresses a very important issue. EL

lamp life is an important design concern to mobile phone

manufacturers.

As an EL lamp ages, its brightness is reduced and its capacitance

is diminished. By using the RC model to reduce the audible noise

generated by an EL lamp, the voltage across the lamp will

increase as its capacitance diminishes. Hence the increase in

voltage will compensate for the reduction of the brightness. As a

result, it will extend an EL lamp’s half-life (half the original

brightness).

Effect of Series Resistor on EL

Lamp Audible Noise and

Brightness:

Increasing the value of the series resistor with the lamp will

reduce the audible noise of an EL lamp as well as its brightness.

This is due to the fact that the output voltage across the lamp will

be reduced and the output waveform will have rounder edges.

8

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