LT1768CGN Linear Technology, LT1768CGN Datasheet - Page 11

• Manufacturer Part Number: LT1768CGN
• Description: IC CTRLR CCFL SGL/MULT HP 16SSOP
• Manufacturer: Linear Technology
• Type: CCFL Controllerr

Specifications of LT1768CGN

• Frequency: 300 ~ 410 kHz
• Current - Supply: 7mA
• Current - Output: 1.5A
• Voltage - Supply: 9 V ~ 24 V
• Operating Temperature: 0°C ~ 125°C
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
In linear mode (V
controlled linearly with the voltage on the PROG pin. The
equation for the VC source current in linear mode is
I
light conversion and highest efficiency, V
set to make the LT1768 normally operate in the linear
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 (V
current to five times the current out of the 1.25V refer-
enced R
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.
VC
I
I
I
I
LAMP
LAMP
V
LAMP
V
LAMP
= (V
PROG
PROG
V
V
PROG
PROG
= 9mA
= 9mA
= 1mA
= 1mA
TRACE A
TRACE B
= 1.125V
TRACE A
TRACE B
= 1.125V
PROG
= 4.5V
= 4.5V
MAX
Figure 3b. CCFL Current for Circuit in Figure 1
Figure 3a. CCFL Current for Circuit in Figure 1
RMS
RMS
RMS
RMS
pin. Setting R
– 1V)/3V (I
PWM
U
< V
RMAX
INFORMATION
PROG
U
RMAX
PROG
100 s/DIV
• 5). For the best current to
< 4V), VC source current is
1ms/DIV
to produce CCFL current
> 4V) sets the VC source
W
PWM
U
should be
mode. For example, in the circuit in Figure 1, linear mode
runs from V
equal to (3mA)(V
In PWM Mode (1V < V
is modulated between the value set by minimum current
mode and the value for I
V
its duty cycle set by the voltages on the PROG and PWM
pins and follows the equation:
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 V
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,
PWM
DC = [1 – (V
. The PWM frequency is equal to 22Hz/C
PROG
PWM
PROG
= 3V to V
PROG
– V
PROG
–1V)/1V.
PROG
= 1V to V
VC
< V
PROG
in linear mode with V
)/(V
PWM
PWM
= 4V with lamp current
), the VC source current
PROG
– 1V)] • 100%
= 3V with CCFL
LT1768
T
( F) with
11
PROG
=