LT1184F_15 Datasheet, PDF (10/24 Pages) Linear Technology – CCFL/LCD Contrast Switching Regulators

LT1182/LT1183/LT1184/LT1184F
PIN FUNCTIONS
LT1182/LT1183/LT1184/LT1184F
CCFL PGND (Pin 1): This pin is the emitter of an internal
NPN power switch. CCFL switch current flows through
this pin and permits internal, switch-current sensing. The
regulators provide a separate analog ground and power
ground(s) to isolate high current ground paths from low
current signal paths. Linear Technology recommends the
use of star-ground layout techniques.
I_CCFL (Pin 2): This pin is the input to the CCFL lamp current
programming circuit. This pin internally regulates to 450mV
(LT1182/LT1183) or 465mV (LT1184/LT1184F). The pin
accepts a DC input current signal of 0µA to 100µA full
scale. This input signal is converted to a 0µA to 500µA
source current at the CCFL V_C pin. By shunt regulating the
I_CCFL pin, the input programming current can be set with
DAC, PWM or potentiometer control. As input program-
ming current increases, the regulated lamp current in-
creases. For a typical 6mA lamp, the range of input
programming current is about 0µA to 50µA.
DIO (Pin 3): This pin is the common connection between
the cathode and anode of two internal diodes. The remain-
ing terminals of the two diodes connect to ground. In a
grounded lamp configuration, DIO connects to the low
voltage side of the lamp. Bidirectional lamp current flows
in the DIO pin and thus the diodes conduct alternately on
half cycles. Lamp current 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_C pin. This current nulls against the source
current provided by the lamp-current programmer circuit.
A single capacitor on the CCFL V_C pin provides both stable
loop compensation and an averaging function to the half-
wave-rectified sinusoidal lamp current. Therefore, input
programming current relates to one-half of average lamp
current. This scheme reduces the number of loop com-
pensation components and permits faster loop transient
response in comparison to previously published circuits.
If a floating-lamp configuration is used, ground the DIO
pin.
CCFL V_C (Pin 4): This pin is the output of the lamp current
programmer circuit and the input of the current compara-
tor for the CCFL regulator. Its uses include frequency
compensation, lamp-current averaging for grounded lamp
circuits, and current limiting. The voltage on the CCFL V_C
pin determines the current trip level for switch turnoff.
During normal operation this pin sits at a voltage between
0.95V (zero switch current) and 2.0V (maximum switch
current) with respect to analog ground (AGND). This pin
has a high impedance output and permits external voltage
clamping to adjust current limit. A single capacitor to
ground provides stable loop compensation. This simpli-
fied loop compensation method permits the CCFL regula-
tor to exhibit single-pole transient response behavior and
virtually eliminates transformer output overshoot.
AGND (Pin 5): This pin is the low current analog ground.
It is the negative sense terminal for the internal 1.24V
reference and the I_CCFL summing voltage in the LT1182/
LT1183/LT1184/LT1184F. It is also a sense terminal for
the LCD dual input error amplifier in the LT1182/LT1183.
Connect external feedback divider networks that terminate
to ground and frequency compensation components that
terminate to ground directly to this pin for best regulation
and performance.
SHUTDOWN (Pin 6): Pulling this pin low causes complete
regulator shutdown with quiescent current typically re-
duced to 35µA. The nominal threshold voltage for this pin
is 0.85V. If the pin is not used, it can float high or be pulled
to a logic high level (maximum of 6V). Carefully evaluate
active operation when allowing the pin to float high.
Capacitive coupling into the pin from switching transients
could cause erratic operation.
CCFL V_SW (Pin 16): This pin is the collector of the internal
NPN power switch for the CCFL regulator. The power
switch provides a minimum of 1.25A. Maximum switch
current is a function of duty cycle as internal slope com-
pensation ensures stability with duty cycles greater than
50%. Using a driver loop to automatically adapt base drive
current to the minimum required to keep the switch in a
quasi-saturation state yields fast switching times and high
efficiency operation. The ratio of switch current to driver
current is about 50:1.
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