11

LT1186F

Introduction

Current generation portable computers and instruments

use backlit Liquid Crystal Displays (LCDs). Cold Cathode

Fluorescent Lamps (CCFLs) provide the highest available

efficiency in back lighting the display. Providing the most

light out for the least amount of input power is the most

important goal. These lamps require high voltage AC to

operate, mandating an efficient high voltage DC/AC con-

verter. The lamps operate from DC, but migration effects

damage the lamp and shorten its lifetime. Lamp drive

should contain zero DC component. In addition to good

efficiency, the converter should deliver the lamp drive in

the form of a sine wave. This minimizes EMI and RF

emissions. Such emissions can interfere with other de-

vices and can also degrade overall operating efficiency.

Sinusoidal CCFL drive maximizes current-to-light conver-

sion in the lamp. The circuit should also permit lamp

intensity control from zero to full brightness with no

hysteresis or “pop-on.”

The small size and battery-powered operation associated

with LCD equipped apparatus dictate low component

count and high efficiency for these circuits. Size con-

straints place severe limitations on circuit architecture and

long battery life is a priority. Laptop and handheld portable

computers offer an excellent example. The CCFL and its

power supply are responsible for almost 50% of the

battery drain. Additionally, all components, including PC

board and hardware, usually must fit within the LCD

enclosure with a height restriction of 5mm to 10mm.

The CCFL regulator drives an inductor that acts as a

switched-mode current source for a current-driven Royer-

class converter with efficiencies as high as 90%. The

control loop forces the CCFL PWM to modulate the aver-

age inductor current to maintain constant current in the

lamp. The constant current value, and thus lamp intensity,

is programmable. This drive technique provides a wide

range of intensity control. A unique lamp-current pro-

gramming block permits either grounded lamp or floating

lamp configurations. Grounded lamp circuits directly sense

one-half of average lamp current. Floating lamp circuits

directly sense the Royer’s primary-side converter current.

Floating-lamp circuits provide symmetric differential drive

to the lamp and reduce the parasitic loss from stray lamp-

to-frame capacitance, extending illumination range.

Block Diagram Operation

The LT1186F is a fixed frequency, current mode switching

regulator. A fixed frequency, current mode switcher con-

trols switch duty cycle directly by switch current rather

than by output voltage. Referring to the block diagram for

the LT1186F, the switch turns ON at the start of each

oscillator cycle. The switch turns OFF when switch current

reaches a predetermined level. The control of output lamp

current is obtained by using the output of a unique

programming block to set current trip level. The current

mode switching technique has several advantages. First,

it provides excellent rejection of input voltage variations.

Second, it reduces the 90

° phase shift at mid-frequencies

in the energy storage inductor. This simplifies closed-loop

frequency compensation under widely varying input volt-

age or output load conditions. Finally, it allows simple

pulse-by-pulse current limiting to provide maximum switch

protection under output overload or short-circuit condi-

tions.

The LT1186F incorporates a low dropout internal regula-

tor that provides a 2.4V supply for most of the internal

circuitry. This low dropout design allows input voltage to

vary from 3V to 6.5V with little change in quiescent

current. An active low shutdown pin typically reduces total

supply current to 35

µA by shutting off the 2.4V regulator

and locks out switching action for standby operation. The

IC incorporates undervoltage lockout by sensing regulator

dropout and locking out switching below about 2.5V. The

regulator also provides thermal shutdown protection that

locks out switching in the presence of excessive junction

temperatures.

A 200kHz oscillator is the basic clock for all internal timing.

The oscillator turns on the output switch via its own logic

and driver circuitry. Adaptive anti-sat circuitry detects the

onset of saturation in the power switch and adjusts base

drive current instantaneously to limit switch saturation.

This minimizes driver dissipation and provides rapid turn-

off of the switch. The CCFL power switch is guaranteed to

provide a minimum of 1.25A in the LT1186F. The anti-sat