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LXE1686-0X

INVERTER EVALUATION BOARD

Microsemi Corporation 

LXE1686-0x

INVERTER EVALUATION BOARD

2. BEFORE GETTING STARTED

LCD PANEL CHARACTERISTICS

The LCD panel manufacturers specification for your

application should define CCFL (CCFT) lamp load

parameters critical for proper inverter design. These

parameters are usually defined as follows for

minimum, typical and maximum ratings:

• Lamp current (mA rms)

• Lamp (run) voltage (V rms)

• Lamp power (Watt rms)

• Lamp current frequency (kHz)

• Lamp strike voltage (cold) at rated temperature (°C)

• Lamp connector pin-out orientation (hot and cold

CCFL lead assignment)

• Lamp lifetime anticipated if lamp current is main-

tained within a specified range (Hours)

• Center screen or five point average light output at a

reference lamp current (cd/m² or Nit)

WHAT YOU SHOULD KNOW ABOUT LCD PANEL MANU-

FACTURES SPECIFICATIONS:

Several of the CCFL characteristics are often those

specified by the CCFL manufacturer based on their

burn-in screening process. Lamp (run) voltage

parameters may be padded for manufacturing

process tolerance and may take into account ‘near

end of life’ deviation values. Lamp strike voltage

values may represent a bare lamp condition without

benefit of the final LCD panel assembly. Lamp run

voltage ratings may be up to 20% higher and strike

voltage ratings may be up to 100% higher than the

actual value for a CCFL mounted within an LCD

assembly.

Many panel manufactures specify minimum lamp

current and maximum lamp frequency ratings based

on limitations of analog dimming methods. This is

specified mainly to guarantee acceptable light output

uniformity. Linfinity’s digital wide range dimming

technique eliminates low light level uniformity issues

based on minimum lamp current or maximum

operating frequency. Additionally, Linfinity’s lamp life

tests show no significant lamp life degradation due to

wide range digital dimming.

Lamp wiring polarity has a significant effect on

inverter performance and should be carefully

observed. The ‘hot’ lead, or shortest lead in the LCD

panel assembly, is usually the color pink or red. The

‘cold’ lead, or longest lead in the LCD panel

assembly, is usually the color white. The ‘cold’ lead,

or return lead, is also the lead that lamp current

measurement is taken from in the LCD panel

specification.

An important note (one that is often overlooked during

development phase) is that the LCD panel metal

framework around the CCFL must be grounded to

insure proper inverter operation as well as preventing

electrical shock and reducing EMI emission.

Light transmission through the LCD is typically

specified for a white spectrum emission perpendicular

to the panel’s center, at a specified lamp current. This

also includes the presence of video (blank page) at a

specified contrast bias setting (if applicable). Larger

panel’s, greater than 7” diagonal, may specify light

transmission as the average of four corner screen

locations and the center. The presence of video and

contrast bias is critical for proper light measurement

using STN type LCD. However, light transmission

through a TFT type LCD may only be affected as little

10% with or without video. Touch screens will reduce

light emission as a whole and should be taken into

consideration as to how this affects the LCD panel

specification.

STN type LCD typically uses a split screen vertical

frame rate that often requires vertical synchronization

with the LINFINITY inverter’s burst mode dimming, so

as to avoid visual ‘spook’ disturbance. LINFINITY

wide range digital inverters provide the ‘Vsync’ feature

for this purpose. Often, TFT type LCD can be

operated with LINFINITY inverter burst mode

technique without need of vertical synchronization. In

either case, experimentation is recommended to

determine the need or the proper vertical

synchronization rate to eliminate undesirable visual

‘spook’ disturbances.

APPLICATION SYSTEM POWER

Your application system power may be derived from a

battery or typical AC to DC power source. In either

case, fixed regulated supplies are likely in use for

logic devices. The LINFINITY inverters low power

logic can also share from one of these supplies, if it is

fixed between 3.3V to 5.5V. Ideally, a 4.5V to 5.5V

supply should be considered, as it will allow for a

greater selection of lower cost power FET devices

required to power the CCFL output transformer. This

voltage range also allows for better dynamic range in

the inverter lamp current regulation scheme.

The main inverter power can be drawn directly from

the battery or main power source. However, it is not

Copyright © 2000

Rev 1.0c, 2000-08-31


 



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