LT1184F_15 Datasheet, PDF (11/24 Pages) Linear Technology – CCFL/LCD Contrast Switching Regulators

LT1182/LT1183/LT1184/LT1184F
PIN FUNCTIONS
Bulb (Pin 15): This pin connects to the low side of a 7V
threshold comparator between the BAT and Bulb pins.
This circuit sets the maximum voltage level across the
primary side of the Royer converter under all operating
conditions and limits the maximum secondary output
under start-up conditions or open lamp conditions. This
eases transformer voltage rating requirements. Set the
voltage limit to insure lamp start-up with worst-case, lamp
start voltages and cold-temperature system operating
conditions. The Bulb pin connects to the junction of an
external divider network. The divider network connects
from the center tap of the Royer transformer or the actual
battery supply voltage to the top side of the current source
“tail inductor”. A capacitor across the top of the divider
network filters switching ripple and sets a time constant
that determines how quickly the clamp activates. When
the comparator activates, sink current is generated to pull
the CCFL V_C pin down. This action transfers the entire
regulator loop from current mode operation into voltage
mode operation.
BAT (Pin 14): This pin connects to the battery or battery
charger voltage from which the CCFL Royer converter and
LCD contrast converter operate. This voltage is typically
higher than the V_IN supply voltage but can be equal or less
than V_IN. However, the BAT voltage must be at least 2.1V
greater than the internal 2.4V regulator or 4.5V minimum
up to 30V maximum. This pin provides biasing for the
lamp current programming block, is used with the Royer
pin for floating lamp configurations, and connects to one
input for the open lamp protection circuitry. For floating
lamp configurations, this pin is the noninverting terminal
of a high-side current sense amplifier. The typical quies-
cent current is 50µA into the pin. The BAT and Royer pins
monitor the primary side Royer converter current through
an internal 0.1Ω top side current sense resistor. A 0A to 1A
primary side, center tap converter current is translated to
an input signal range of 0mV to 100mV for the current
sense amplifier. This input range translates to a 0µA to
500µA sink current at the CCFL V_C pin that nulls against the
source current provided by the programmer circuit. The
BAT pin also connects to the top side of an internal clamp
between the BAT and Bulb pins.
Royer (Pin 13): This pin connects to the center-tapped
primary of the Royer converter and is used with the BAT
pin in a floating lamp configuration where lamp current is
controlled by sensing Royer primary side converter cur-
rent. This pin is the inverting terminal of a high-side
current sense amplifier. The typical quiescent current is
50µA into the pin. If the CCFL regulator is not used in a
floating lamp configuration, tie the Royer and BAT pins
together. This pin is only available on the LT1182/LT1183/
LT1184F.
V_IN (Pin 12): This pin is the supply pin for the LT1182/
LT1183/LT1184/LT1184F. The ICs accept an input voltage
range of 3V minimum to 30V maximum with little change
in quiescent current (zero switch current). An internal,
low dropout regulator provides a 2.4V supply for most of
the internal circuitry. Supply current increases as switch
current increases at a rate approximately 1/50 of switch
current. This corresponds to a forced Beta of 50 for each
switch. The ICs incorporate undervoltage lockout by sens-
ing regulator dropout and lockout switching for input
voltages below 2.5V. Hysteresis is not used to maximize
the useful range of input voltage. The typical input voltage
is a 3.3V or 5V logic supply.
LT1182/LT1183
LCD V_C (Pin 7): This pin is the output of the LCD contrast
error amplifier and the input of the current comparator for
the LCD contrast regulator. Its uses include frequency
compensation and current limiting. The voltage on the
LCD 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). The LCD V_C pin has a high impedance
output and permits external voltage clamping to adjust
current limit. A series R/C network to ground provides
stable loop compensation.
LCD PGND (Pin 8): This pin is the emitter of an internal
NPN power switch. LCD contrast 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 recom-
mends star-ground layout techniques.
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