HV859(2006) データシートの表示（PDF） - Supertex Inc
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HV859 Datasheet PDF : 6 Pages
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Figure 3: Typical Application Circuit for Audible Noise reduction
ON = VDD Enable
OFF = 0V
+
CDD
VDD
-
+
CIN
VIN
-
RSW
1 VDD
2 RSW-osc
VA 8
VB 7
3 REL-osc
CS 6
REL
4 GND
LX 5
HV859MG/
HV859K7
RSER
D
LX
HV859
EL Lamp
CS
Audible Noise Reduction
This section describes a method (patented) developed at Supertex
to reduce the audible noise emitted by the EL lamps used in
application sensitive to audible noise. Figure 3 shows a general
circuit schematic that uses the resistor, RSER, connected in series
with the EL lamp
How to Minimize EL Lamp Audible Noise:
The EL lamp, when lit, emits an audible noise. This is due to EL
lamp construction and it 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 C is the capacitance of the EL lamp,
and R is the external resistor, RSER, connected in series with the EL
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. 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 have another
effect on the over all performance of the Supertex EL drivers, age
compensation (patented). This addresses a very important issue,
EL lamp life that most mobile phone manufacturers are concerned
about.
As EL lamp ages, its brightness is reduced and its capacitance is
diminished. By using the RC model to reduce the audible noise
emitted by the 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 the 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 EL lamp audible noise 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.
NR040306
5
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