LT1768I View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1768I

High Power CCFL Controller for Wide Dimming Range and Maximum Lamp Lifetime

Linear Technology 

LT1768

APPLICATIONS INFORMATION

per lamp to 1mA for all PROG voltages but limits the

dimming ratio to 6:1.

Trace B in Figures 3a and 3b shows Figure 1’s CCFL

current waveform operating at 1mA in PWM mode.

Maximum current mode (VPROG > 4V) sets the VC source

current to five times the current out of the 1.25V refer-

enced RMAX pin. Setting RRMAX to produce CCFL current

equal to the manufacturer’s maximum rating in this mode

insures no degradation in the specified lamp lifetime. For

example, setting R4 in the circuit in Figure 1 to 16.2k sets

the maximum CCFL current to 9mA (9mA is only a typical

maximum lamp current used for illustration, consult lamp

specifications for the actual value). Trace A in Figure 3a

and 3b shows Figure 1’s CCFL current waveform operating

at 9mA in maximum current mode.

TRACE A

VPROG = 4.5V

ILAMP = 9mARMS

TRACE B

VPROG = 1.125V

ILAMP = 1mARMS

1ms/DIV

Figure 3a. CCFL Current for Circuit in Figure 1

TRACE A

VPROG = 4.5V

ILAMP = 9mARMS

TRACE B

VPROG = 1.125V

ILAMP = 1mARMS

100µs/DIV

Figure 3b. CCFL Current for Circuit in Figure 1

In linear mode (VPWM < VPROG < 4V), VC source current is

controlled linearly with the voltage on the PROG pin. The

equation for the VC source current in linear mode is

IVC = (VPROG – 1V)/3V (IRMAX • 5). For the best current to

light conversion and highest efficiency, VPWM should be

set to make the LT1768 normally operate in the linear

mode. For example, in the circuit in Figure 1, linear mode

runs from VPROG = 3V to VPROG = 4V with lamp current

equal to (3mA)(VPROG–1V)/1V.

In PWM Mode (1V < VPROG < VPWM), the VC source current

is modulated between the value set by minimum current

mode and the value for IVC in linear mode with VPROG =

VPWM. The PWM frequency is equal to 22Hz/CT(µF) with

its duty cycle set by the voltages on the PROG and PWM

pins and follows the equation:

DC = [1 – (VPWM – VPROG)/(VPWM – 1V)] • 100%

The LT1768’s PWM mode enables wide dimming ratios

while reducing the high crest factor found in PWM only

dimming solutions. In the example of Figure 1, PWM

mode runs from VPROG = 1V to VPROG = 3V with CCFL

current modulated between 0mA and 6mA. The PWM

modulation frequency is set to 220Hz by capacitor C3.

When combined, these five modes of operation allow

creation of a DC controlled CCFL current profile that can be

tailored to each particular display. With linear mode CCFL

current control over the most widely used current range,

and PWM mode at the low end, the LT1768 enables wide

dimming ratios while maximizing CCFL lifetimes.

Lamp Feedback Current

In a typical application, the DIO1/2 pins are connected to

the low voltage side of the lamps. Each DIO pin is the

common connection between the cathode and anode of

two internal diodes (see Block Diagram). The remaining

terminals of the diodes are connected to PGND. Bidirec-

tional lamp current flows into the DIO1/2 pins and their

diodes conduct alternately on the half cycles. The diode

that conducts on the negative cycle has a percentage of its

current diverted into the VC pin. This current nulls against

the VC source current specified by the Multimode Dim-

ming section. A single capacitor on the VC pin provides

both stable loop compensation and an averaging function

to the halfwave-rectified lamp current. Therefore, current

into the VC pin from the lamp current programming

section relates to average lamp current.

The overall gain from the resistor current to average lamp

current is equal to the gain from the Multimode Dimming

block divided by the gain from the DIO pin to the VC pin,

11

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