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LXE1686-0X

INVERTER EVALUATION BOARD

Microsemi Corporation 

LXE1686-0x

INVERTER EVALUATION BOARD

The LXE1686-0x inverter evaluation/design board can

be configured numerous ways for brightness control

by jumper option placement. Basic brightness control

options are as follows:

• Brightness control can be configured for linear DC

input (bypass optional U4 conditioning circuit) with the

following jumper and component changes: RJ3=N/U,

RJ4=N/U, R5=2.0K & C10=220nF

• Brightness control polarity can be reversed such that

a low level input results in maximum brightness by

placing a 10-Ohm jumper at RJ1.

• Brightness control action can be configured as a

direct function of amplitude lamp current (traditional

non-burst mode) by placing a 10-Ohm jumper at RJ2.

• Direct PWM input (wide dynamic range burst mode)

can be configured with the following jumper and

component changes: RJ3=N/U, RJ4=N/U, RJ5=10-

Ohm, RJ6=N/U, RJ7=10-Ohm, VERT SYNC

grounded, R5=2.0K, C5=10-Ohm & C7=10-Ohm

Note that configuration option component and jumper

change items can be found in the ‘Spare

Components’ section of the LXE1686-0x inverter

evaluation/design board. These components; C24

thru C27, R34 thru R35, and RJ9 thru RJ12 can all be

identified on LXE1686 schematic page.

• Direct PWM input is a highly recommended method to

achieve wide dynamic (burst mode) brightness

control, with minimum component count. The

following criteria is required for direct PWM method

brightness control:

• PWM signal logic level amplitude should be no less

than 65% of the VIN LOGIC supply voltage.

• PWM signal frequency should range from 100 to

1000Hz (lower frequency offers wider optical dynamic

dimming range).

• PWM frequency and duty should remain stable and

jitter free during inverter operation.

• PWM signal positive duty should not drop below the

minimum duty that establishes minimum desired

brightness during inverter operation.

• PWM frequency should ideally be synchronized to

application system vertical refresh rate during inverter

operation. This is particularly necessary for STN style

LCD panels, to avoid visual spook interference.

OPTIONAL OPEN LOAD TIME-OUT COMPARATOR CIRCUIT

For safety reasons, inverter design applications may

require high voltage output to be quickly disabled, in

the event of an open load condition (such as a broken

lamp). The LXE1686-0x inverter evaluation/design

board is pre-assembled with an optional comparator

circuit (U5) that provides this function. By placing a

10-Ohm jumper at RJ8, this feature will be fully

enabled for design evaluation. The preset time-out

period is approximately 2 seconds. Vary the RC time

constant values of R33 or C23 if a change in time-out

period is desired.

SHORTED OUTPUT CURRENT LIMITING CIRCUIT

For safety reasons, such as UL (or similar)

requirement, inverter design applications may require

output current to be limited to a specified value, in the

event of a shorted load condition that defeats normal

output current regulation. The LXE1686-0x inverter

evaluation/design board is pre-assembled with a

current limiting sensing circuit (Q1 & R14) that

provides this function. The preset output current

limiting value is approximately 25mA-pk (within UL

requirement). Change the value of R14 (82 Ohm)

inversely for a desired change in output current

limiting.

Channel 1 below shows P13 the top on the current

sense resistor. Channel 2 connects to P14 (VSNS),

channel 3 to P15 (VCOMP) and channel 4 to P24

(TRI_C). Since the lamp regulation circuit is defeated

the controller will proceed to the lamp strike

configuration of ramping the output frequency

following the TRI_C ramp generator.

Note that this circuit is not required for normal inverter

operation. It does however; provide protection against

inverter damage as well as user safety, in the event of

a shorted load condition that defeats normal output

current regulation.

4. OSCILLOSCOPE WAVEFORM PICTURES

The following oscilloscope waveform pictures are

provided as a reference to illustrate typical inverter

operating conditions under a typical lamp load, for test

points of visual interest. Variation in waveforms that

Copyright © 2000

Rev 1.0c, 2000-08-31


 



Page 8

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