LT1768IGN Linear Technology, LT1768IGN Datasheet - Page 12

• Manufacturer Part Number: LT1768IGN
• Description: IC CTRLR CCFL SGL/MULT HP 16SSOP
• Manufacturer: Linear Technology
• Type: CCFL Controllerr

Specifications of LT1768IGN

• Frequency: 300 ~ 410 kHz
• Current - Supply: 7mA
• Current - Output: 1.5A
• Voltage - Supply: 9 V ~ 24 V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
LT1768
Choosing R
The value for R
V
maximum allowable current specified by the lamp manu-
facturer.
The voltage for the PWM pin should then be set so that the
LT1768 normally operates in linear mode. A typical value
for V
region to 50% of the V
The value for R
the minimum manufacturer specified lamp current or
enable a wide dimming range. If a minimum specified
current is desired, the V
12
and is dependant on the operating mode. For dual lamp
displays, the transfer function for minimum current mode
(I
mode (I
The transfer functions discussed above are between R
and R
current. Due to the differences between the average and
RMS functions, the actual overall transfer function be-
tween actual lamp current and R
empirically determined, and is dependant on the particular
lamp/display housing combination used. For example, in
the circuit of Figure 1 setting R
16.8 , sets the minimum and maximum RMS lamp
currents for the example display to 1mA and 9mA per lamp
respectively. Figure 4 shows the lamp current vs program-
ming voltage for the circuit in Figure 1.
PROG
DIO
I
CCFL
/I
PWM
MIN
RMIN
(mA)
to 4.5V then adjusting R
DIO
Figure 4. Lamp Current vs PROG Voltage for
the Circuit in Figure 1
current and average lamp current not RMS lamp
9mA
6mA
0mA
is approximately 2.5V, which limits the PWM
) is equal to 10A/A, and for maximum current
/I
RMAX
RMAX
OFF
CURRENT
RMIN
0.5
RMAX
MIN
and R
) is equal to 100A/A.
1.0
U
0%
(FREQ = 220Hz)
should be chosen to either produce
should be determined by setting
PROG
PWM
RMIN
PROG
INFORMATION
V
U
PROG
100%
3V
input voltage range.
(V
LINEAR
(V)
and V
should be set to 0.75V and
RMIN
PWM)
MIN
RMAX
4.0
/R
PWM
W
to 10k and R
CURRENT
MAX
MAX
5.0
1768 F04
to produce the
current must be
U
RMAX
MAX
to
R
dimming range is desired, V
and R
ratio. Care must be taken when adjusting R
duce extreme dimming ratios. The minimum lamp current
set by R
thermometering (uneven illumination) will occur. If the
desired dimming ratio can’t be achieved by adjusting
R
connecting the R
current is set to less than the open lamp threshold current
(approximately 125 A), the FAULT pin will be activated for
PROG voltages between 0.5V and 1V.
The values chosen for R
critical in determining the lifetime of the display. It is
imperative that proper measurement techniques, such as
those cited in the references, be used when determining
R
Lamp Fault Modes and Single Lamp Operation
The DIO pin diodes that conduct on the positive cycle are
used to detect open lamp fault conditions. If the current
in either of the DIO pins on the positive half cycle is less
than 125 A due to either an open lamp or lamp lowside
short to ground, for a minimum of 1 PWM cycle, then the
FAULT pin will be activated and the lamp programming
current into the VC pin in high level PWM mode, linear
mode, and maximum current mode, will be reduced by
approximately 50%. Halving the VC source current will cut
the total lamp current to approximately one half of its
programmed value. This function insures that the maxi-
mum lamp current level set by R
even under fault conditions. If the current in both of the
DIO pins on the positive cycle is less than 125 A, and the
VC pin hits its clamp value (indicating an open lamp or
lamp lowside short to ground fault condition) for a mini-
mum of 1 PWM cycle, the gate drive will be latched off. The
latch can be cleared by setting the PROG voltage to zero or
placing the LT1768 in shutdown mode.
Since open lamp fault conditions produce high voltage AC
waveforms, it is imperative that proper layout spacings
between the high voltage and DIO lines be observed.
Coupling capacitance as low as 0.5pF between the high
RMIN
RMIN
RMAX
RMIN
, the minimum lamp current can be set to zero by
adjusted to produce the specified current. If a wide
and R
RMIN
adjusted to produce the required dimming
RMIN
must be able to fully illuminate the lamp or
MIN
values.
pin to the V
RMAX
PROG
and R
RMAX
REF
should be set to 0.75V
pin. If the minimum
will not be exceeded
RMIN
are extremely
RMIN
to pro-