HV857MG Supertex, HV857MG Datasheet - Page 7

• Manufacturer Part Number: HV857MG
• Description: Display Drivers HVCMOS
• Manufacturer: Supertex

Specifications of HV857MG

• Driver Type: EL Lamp Drivers
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: MSOP-8
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Minimization of 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 wave-
form 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 capaci-
tance of the EL lamp, and R is the external resistor, R
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 per-
formance 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.
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1235 Bordeaux Drive, Sunnyvale, CA 94089
SER
,
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Effect of Series Resistor on EL Lamp
Audible Noise and Brightness
As EL lamp ages, its brightness is reduced and its capaci-
tance 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).
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.
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Tel: [phone]
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www.supertex.com
HV857