HV859 Ver la hoja de datos (PDF) - Supertex Inc
Número de pieza
componentes Descripción
Fabricante
HV859 Datasheet PDF : 8 Pages
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Fig. 3: Typical Application Circuit for Audible Noise Reduction
ON = VDD
OFF = 0V
Enable
+
VDD
-
CDD
+
CIN
VIN
-
RSW
1 VDD HV859 VA 8
2 RSW-Osc
VB 7
3 REL-Osc
REL
4 GND
CS 6
LX 5
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. Fig.
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.
Doc.# DSFP-HV859
C073013
Supertex inc.
5
www.supertex.com
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