Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 9

Copyright

Rev. 1.0a, 2006-11-13

RangeMAX

™

LXMG1621-xx

TM

®

WIDER DIMMING APPLICATION

The following application defines techniques capable of

delivering dimming ranges in the 250:1 range. As is widely

understood, these techniques will provide general capabilities

and actual system performance will vary with panel design,

CCFTs, ambient temperature and a number of other variables

outside the control of the inverter. These methods can be used

in conjunction with other techniques such as lamp heating and

matching.

Wide ratio (250:1) dimming can be accomplished using the

Microsemi inverter in two ways:

1. By varying the input voltage on the brightness pin as

indicated in Figure 9.

Caution must be exercised when

applying negative voltage to the brightness control input.

Applying more then –300mV to any inverter input will cause

inverter malfunction (see Absolute Maximum Ratings).

-100mV

0V

2.5V

.070

.14

0.7

7

V

Figure 9 – Average Lamp Current vs. V

U9

T1

R48

R62

R63

R60

R31

R57

R54

R53

R51

Q11

R89

C37

R61

Q10

Backside of Module

U7

Figure 10 – Locating The Resistor on the LXMG1621-xx

2.

By making a resistor value change on the module.

Remove R61 for maximum dimming range or increase R61

value to desired minimum dim range setting (see figure 10).

Care should be exercised since at a low enough dim setting the

inverter will be unable to detect that the lamp has started and

will initiate lamp strike (kickoff) voltage. This will result in

lamp flicker. For repeatable low light levels the BRITE input

DC supply must be a very clean stable voltage source, at low

dim inputs.

If you plan to completely remove R61 it is recommended that

you ensure that a minimum voltage above zero remain on the

BRITE input to prevent the above flicker problem.

This

minimum voltage may need to be adjusted for each individual

inverter module. If you are using a pot to control the BRITE

input then a separate trim pot on the low side would accomplish

the same goal.

Both methods discussed will provide a lower duty cycle

operation than is necessary in a 100:1 dimming application.

Careful consideration should be made with regard to display

quality at these dimming levels. At very low brightness levels,

even very small amounts of noise on the VBRITE line can cause

flicker on the display, so special care must be given to

grounding, filtering, and shielding the inverter from the digital

logic and clock signals.