LT1786F_15 Datasheet, PDF (9/20 Pages) Linear Technology – SMBus Programmable CCFL Switching Regulator

LT1786F
PIN FUNCTIONS
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.
BAT (Pin 14): This pin connects to the battery or AC wall
adapter voltage from which the CCFL Royer converter
operates. This voltage is typically higher than the V_CC
supply voltage but can equal V_CC if V_CC is a 5V logic supply.
The BAT voltage must be at least 2.1V greater than the
internal 2.4V regulator or 4.5V. 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
quiescent current is 50µA into the pin. The BAT and Royer
pins monitor the primary-side Royer converter current
through an internal 0.1Ω topside 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 topside of the
internal clamp between the BAT and BULB pins that is used
for open-lamp protection.
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 ensure lamp start-up with worst-case,
lamp start voltages and cold temperature, system operat-
ing 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 topside 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.
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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