LT1786FCS#TR Linear Technology, LT1786FCS#TR Datasheet - Page 15

• Manufacturer Part Number: LT1786FCS#TR
• Description: IC REG SW CCFL SMBUS PROG 16SOIC
• Manufacturer: Linear Technology

Specifications of LT1786FCS#TR

• Applications: Converter, CCFL
• Voltage - Input: 4.5 ~ 30 V
• Number Of Outputs: 1
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -

APPLICATIONS
Simplified Lamp Current Programming
A programming block in the LT1786F controls lamp
current, permitting either grounded lamp or floating lamp
configurations. Grounded configurations control lamp
current by directly controlling one-half of actual lamp
current and converting it to a feedback signal to close a
control loop. Floating configurations control lamp current
by directly controlling the Royer’s primary-side converter
current and generating a feedback signal to close a control
loop.
Previous backlighting solutions have used a traditional
error amplifier in the control loop to regulate lamp current.
This approach converted an RMS current into a DC voltage
for the input of the error amplifier. This approach used
several time constants in order to provide stable loop
frequency compensation. This compensation scheme
meant that the loop had to be fairly slow and that output
overshoot with start-up or overload conditions had to be
carefully evaluated in terms of transformer stress and
breakdown voltage requirements.
The LT1786F eliminates the error amplifier concept
entirely and replaces it with a lamp current programming
block. This block provides an easy-to-use interface to
program lamp current. The programmer circuit also
reduces the number of time constants in the control loop
by combining the error signal conversion scheme and
frequency compensation into a single capacitor. The con-
trol loop thus exhibits the response of a single pole
system, allows for faster loop transient response and
virtually eliminates overshoot under start-up or overload
conditions.
Lamp current is programmed at the input of the program-
mer block, the I
regulator and accepts a DC input current signal of 0 A to
100 A from the DAC. This input signal is converted to a
0 A to 500 A source current at the CCFL V
programmer circuit is simply a current-to-current con-
verter with a gain of five.
The I
oscillate with capacitance greater than 10pF. For example,
loading the I
oscillation and erratic CCFL regulator operation because
CCFL
pin is sensitive to capacitive loading and will
CCFL
CCFL
pin with a 1 or 10 scope probe causes
U
pin. This pin is the input of a shunt
INFORMATION
U
W
C
U
pin. The
of the probe’s respective input capacitance. A current
meter in series with the I
lation due to its shunt capacitance. Use a decoupling
resistor of several kilohms between the I
I
mally, this resistor is not required.
In some applications, the maximum programming current
required at the I
less than the full-scale output current of the DAC, which is
100 A. The system designer can either limit the maximum
programming current through software built into the system,
or use a current splitter which shunts a percentage of the full-
scale current from the I
in Figure 1. A divider string is used from a reference voltage
to set up a voltage level equal to the I
or 465mV. The main current flowing in the divider string
should be chosen to swamp out the effects of the shunted
current into the divider string.
Grounded Lamp Configuration
In a grounded lamp configuration, the low voltage side of
the lamp connects directly to the LT1786F DIO pin. This
pin is the common connection between the cathode and
anode of two internal diodes. In previous grounded lamp
solutions, these diodes were discrete units and are now
integrated onto the IC, saving cost and board space.
Bidirectional lamp current flows in the DIO pin and thus,
the diodes conduct alternately on half cycles. Lamp cur-
rent is controlled by monitoring one-half of the average
lamp current. The diode conducting on negative half
cycles has one-tenth of its current diverted to the CCFL V
pin and nulls against the source current provided by the
I
OUT
OUT
FULL-SCALE
100 A
pin if excessive trace stray capacitance exists. Nor-
I
V1
(1 – X)I
R1
R2
XI
CCFL
pin for a maximum lamp current will be
I1
V(I
465mV
V
CCFL
CCFL
REF
R3
V(I
R4
Figure 1
CCFL
)
pin. A splitter circuit is illustrated
CCFL
)
pin will also produce oscil-
I = 100 A
0 < X < 1
SELECT V1 WITHIN THE DAC I
COMPLIANCE RANGE
(EX. V1 = 2V FOR V
CHOOSE I1 >> (1 – X)I
R1 = (V1 – 0.465)/[(X)(100 A)]
R2 = (V1 – 0.465)/[(1 – X)(100 A)]
R3 = (V
R4 = 0.465R3/[(1 – X)(100 A)(R3)
CCFL
+ (V
REF
REF
– 0.465)/I1
summing voltage,
– 0.465)]
CCFL
LT1786F
CC
pin and the
= 3.3V OR 5V)
15
OUT
1786 F01
C