RangeMAX™ LX1688
PRODUCTION DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
Copyright © 2001
Rev. 1.2, 2006-03-09
WWW.Microsemi .COM
Multiple Lamp CCFL Controller
TM
®
APPLICATION INFORMATION
APPLICATION EXAMPLE WITH LX1688
This section will highlight the features of LX1688
controller by showing a practical example. Three identical
inverter modules are connected to each other and each
module drives a single lamp. One module configured as a
master and two others configured as slaves.
A complete schematic hooked up a a master is given in
Figure 1, the schematic provides all necessary functions
such as high voltage feedback for regulation the peak lamp
voltage, short-circuit protection, open lamp sensing and
lamp current regulation needed for a typical application.
The section follows with measurement waveforms and list
of material of the actual modules. For more detail design
procedure and circuit description please refer to application
note (AN-13), which is available in Microsemi’s web site.
I
NPUT VOLTAGE
The LX1688 controller can operate at 3.3 to 5.0V
±10%, in this application all modules were driven by the
same power voltage (a constant 5.0V), which provides
VDD for controllers, and input voltage for the power
section. Notice that VDD feeds all analog signals and
VDD_P feeds only the output driver stage, these two
signals should be filtered separately (Figure 1).
S
ETTING LAMP FREQUENCY
The value of R1 determines magnitude of internal
current sources that set timing parameters. Equation (2)
gives the relationship between Lamp frequency (FLAMP)
and (RI_R), R1 in schematic. For this application we
choose R6=80.6 KΩ, which results to a lamp frequency at
62.0 KHz.
D
IMMING
The LX1688 includes highly integrated universal ‘PWM
or DC’ dim input that allows either a PWM or DC input
without requiring external conditioning.
In this application we choose Digital Dimming by
applying a PWM signal to BRITE pin.
All modules were driven by the same PWM signals, but
notice that it is possible to dim each module quite
separately.
BEPOL pin has three different modes (see table 1), in
this application it is connected to VDD which means active
high enable with active high full brightness.
The PWM signal can be varied in frequency between
48-320 HZ. No capacitor between CPWM1 and CPWM2 is
necessary.
S
ETTING MASTER/SLAVE CONFIGURATION
Simply connecting pin 14 to the ground for a master
and to the VDD for a slave will do master and slave
configuration. As shown in figure 2, module (A)
configured as master and modules (B) and (C) configured
as slaves.
S
YNCHRONIZATION OF FREQUENCY AND PHASE
To synchronize the Lamp frequency and phase of all
modules, it is required to connect the RMP_RST pin of all
the modules together and connect PHA_SYNC pin of all
the modules together.
L
AYOUT CONSIDERATION
By designing the layout in a proper way we can reduce
the overall noise and EMI for the module.
The gate drivers for MOSFETs should have an
independent loop that doesn’t interface with the more
sensitive analog control function, therefore LX1688
provides two power inputs with separate ground pins
(analog/signal), VDD feeds all analog signals and VDD_P
feeds only the output drivers, as shown in figure1 these
two pins (pin 23, 24) are separated and filtered by R14, C2
and C7. The connection of two ground pins should be at
only one point as shown in figure1.
The power traces should be short and wide as possible
and all periphery components such capacitors should be
located as closed as possible to the controller.
O
SCILLOSCOPE WAVEFORMS PICTURES
The following oscilloscope waveform pictures are
taken from the actual circuits and will show the operation
of the modules in different modes when three identical
modules are synchronized, one as a master, and two others
as slaves.
A
A
P
P
P
P
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C
A
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