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LX1695

Switched Royer CCFL Inverter Monitor IC

Microsemi Corporation 

LX1695

TM

®

Switched Royer CCFL Inverter Monitor IC

PRODUCTION DATA SHEET

THEORY OF OPERATION

DIGITAL DIMMING PWM

A DC voltage to PWM converter provides an accurate

digital dimming brightness control by varying Royer on

time from 100% to as low as 3.125%. Minimum duty cycle

is implemented by causing the Royer output signal to be

high any time the DAC clock count is less than 8. Since the

DAC is dual slope, this insures duty will be at least 16 out

of the full 512 counts per cycle (See figure 3).

The PWM includes an on chip oscillator that provides

dimming burst rates between 30 and 600Hz. The oscillator

frequency is trimmed to ±3% accuracy (+20 to +65°C) to

prevent unwanted display artifacts that can be caused by the

lamp dimming frequency beating with the displays video.

Burst frequency can be controlled in two ways: An

external resistor from DIM_FREQ to ground sets the

frequency of the on chip oscillator. The formula for

calculating a given ROYER output frequency based on the

DIM_FREQ resistor to ground is as follows:

(1 ROYER Output Frequency -184.32E-6)

R DIM_FREQ =

151.23E-9

As an example, if a ROYER output frequency of 120Hz is

desired then:

(1 120 - 184.32E-6)

R DIM_FREQ =

151.23E-9

= 53885

The closed nominal 1% resistor value would be 53.6K,

nominally yielding a just slightly higher than 120Hz output.

Dimming frequency becomes the oscillator frequency

divided by 512 or a logic level pulse supplied through a 10K

resistor to the DIM_FREQ pin overrides the internal timing

circuits causing the dimming frequency to be input

frequency divided by 512. The 10K external series resistor

limits current into the ESD structure at the DIM_FREQ pin.

The duty cycle at the Royer output is directly and linearly

proportional to the DC level of signal BRITE_IN. Two

(2.0) volts corresponds to 100% duty and zero volts

corresponds to minimum duty. Minimum duty is internally

limited to 3.125% even if BRITE_IN is zero volts

High input impedance (>10 MΩ) at the BRITE_IN pin

makes it easy to set up minimum and maximum duty cycle

outputs using only a few external resistors. The input pin is

also directly compatible with Microsemi’s LX1970 and

1971 ambient light sensors that provide automatic

brightness control.

START-UP FAULT DELAY

Open lamp detection is disabled for a programmable

period after power turn-on, giving the Royer oscillators

sufficient time to ignite all lamps. An external resistor and

capacitor at pin FLT_DLY controls this time. The capacitor

begins to charge at power on, and its exponential voltage

rise is compared to a 2.1 volt reference to signal the end of

fault delay interval. This condition is latched and then the

external capacitor is discharged by an on chip NMOS

transistor. Discharge time is about 10% of charge time, and

the capacitor value can be up to 10uF. The resistor will

typically be less than 1 megohm.

OPEN LAMP DETECTION

The open lamp detection circuit can sense if any lamp in

the entire array is not conducting and shut the system off to

prevent possible catastrophic system failure. Lamp current

sensing is gated only during Royer on time and is delayed

from its leading edge so that reliable detection is provided,

even while dimming with very low duty cycles. Delay is 48

counts of the DAC clock beginning when Royer goes high.

This gives the actual Royer oscillators time to come up to

full power before testing for a broken lamp. At low

dimming levels when the output duty cycle less than 12.5%

open lamp sensing is internally disabled. This corresponds

to about 0.25V on the BRITE_IN pin.

An external R/C time out at pin FLT_DLY programs a

delay after power-on to mask fault detection while the lamps

are igniting. Typically this time-out is in the one to two

second range, but can be as long as 5 seconds. Maximum

recommended value of the resistor is 1 megohm to prevent

error due to leakage current on the PCB, and low leakage

ceramic capacitors are recommended.

Typically this time-out is in the one to two second range,

but can be as long as 5 seconds.

Copyright © 2004

Rev. 1.0a, 2006-01-06

Microsemi

Integrated Products Division

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

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