Microsemi

Integrated Products, Power Management

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

Page 10

Copyright

 2001

Rev. 1.1a, 2003-03-21

RangeMAX™

LX1688

I N T E GRA T ED

PR ODUC T S

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.

Input 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).

Setting lamp frequency

The value of R1 determines magnitude of internal

current sources that set timing parameters. Equation (2)

choose R6=80.6 Kohm, which results to a lamp

frequency at 62.0 KHz.

Dimming

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.

Setting 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.

Synchronization 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.

Layout 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.

Oscilloscope 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.