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LXE1686-0X

INVERTER EVALUATION BOARD

Microsemi Corporation 

LXE1686-0x

INVERTER EVALUATION BOARD

combination op-amp/integrator conditioning circuit as

well.

An optional open/short lamp timeout-disable

comparator circuit is also available, and can be

implemented with one jumper placement at RJ8.

Inverter board is pre-configured to deliver from 3 to

18mArms lamp current, adjustable via the Current

ADJ on board multi-turn potentiometer (R11B).

Inverter board is pre-configured to operate from 65 to

75kHz lamp current, adjustable via the FREQ ADJ on

board multi-turn potentiometer (R10B).

Three critical inverter setup probe test points are

provided, including twenty-seven information test

points to help you design as well as understand the

LMI LX1686 inverter operating concept. Oscilloscope

pictures of typical inverter waveforms at a number of

these test points are also provided for reference.

3. BOARD SETUP PROCEDURE, CRITERIA &

OPTIONS

BASIC INPUT/OUTPUT INTERCONNECTS & MEASUREMENT

Ground panel frame to electrical system ground. If

panel is an STN style LCD, apply a blank (white)

screen video for light measurements.

Determine your method of lamp current

measurement; current probe, percentage of inverter

input power to known lamp power (~ 120%) or known

light output for a given lamp current. If using the light

output measurement, allow for CCFL temperature

stabilization (usually 20 to 40 minutes).

Setup inverter for nominal application system logic

supply voltage at ‘VIN LOGIC’ & ‘PWR GND’

terminals.

Simulate application system main power source with

a variable power supply that can be adjusted over the

full variation range of the application system at ‘VIN

POWER’ & ‘PWR GND’ terminals. Adjust ‘BRITE ADJ’

R30B full clockwise for maximum brightness

condition.

Adjust ‘FREQ ADJ’ R10B for desired lamp current

operating frequency (70kHz nominal).

Establish the lowest main input voltage (VIN POWER)

where target design lamp current, input power or light

output can be maintained. This will require an initial

high ‘Current ADJ’ setting at R11B (full clockwise) to

gain maximum inverter drive capability. Optimum

transformer turns ratio selection is achieved when

lowest main input voltage is that of the lowest

application system potential, where design target

condition are maintainable. Decrease ‘Current ADJ’

R11B until noted reference begins to decrease at

lowest input voltage condition.

Ideally, use the light output reference measurement

for final setting as follows; increase inverter input

voltage to the midrange or nominal input power

potential. Readjust ‘Current ADJ’ R11B until reference

light output level condition is re-established.

Using an O-scope probe, determine the dc potential

at P4. This DC potential is the equivalent to the

'common-mode' threshold voltage of the lamp current

regulation op-amp. Make a reference of this potential

on the scope scale gradient and place the scope

probe on TP2. Observe TP2 waveform over the entire

main input supply range. The end goal will be to

achieve an AC waveform peak on P2 that doesn't

exceed the DC reference potential, but ideally rides

just below it. In the scope shot below the bar indicates

the level at P4 (VDD) channel 1 is TP2 (ISNS) and

channel 4 show the output current low return side.

Note that the peak value of TP2 is below the P4 level.

In this scope shot the main input supply was at the

nominal input voltage, not the extremes.

RECOMMENDED LAMP CURRENT OPERATING FREQUENCY

Recommended operating frequencies for the LMI

magnetic types: 70 to 100kHz for 2.4 and 4.0Watt, 65

to 75kHz for 7.0Watt. C6 may be reduced in value

(120pF provided as spare component C26) to obtain

a higher operating frequency range.

Lamp current regulation

To reduce TP2 peak amplitude, increase C13 (2.2nF)

value or vise-versa (this will change lamp current

setting, requiring readjustment of ‘Current ADJ’

R11B). Observe light output over the entire inverter

input supply range. Light output variations over low to

high supply condition may be balanced out by varying

the ratios of R11A + R11B to R18. Often R18 is not

needed to accomplish balanced light output, but when

Copyright © 2000

Rev 1.0c, 2000-08-31


 



Page 6

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