9

LT1786F

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

50

µA 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

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 50

µA into the pin. The BAT and Royer

pins monitor the primary-side Royer converter current

through an internal 0.1

Ω 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

0

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

regulator loop from current mode operation into voltage

mode operation.

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.