LINEAR Application Note 81 Data Sheet Instruction Manual

Application Note 81 September 1999

Ultracompact LCD Backlight Inverters A Svelte Beast Cuts High Voltage Down to Size

Jim Williams, Linear Technology Corporation Jim Phillips, Gary Vaughn, CTS Wireless Components

INTRODUCTION mensurate adjustments in inverter characteristics. An-

other problem area is fail-safe protection due to self-

The liquid crystal display (LCD) has become ubiquitous. It

destructive transformer malfunctions. Finally, the magnetic

is in use everywhere, from personal computers of all sizes

field provided by conventional transformers can interfere

to point-of-sale terminals as well as instruments, autos

with operation of adjacent circuitry. With the exception of

and medical apparatus. The LCD utilizes a cold cathode

size, all of these problems are addressable, although

fluorescent lamp (CCFL) as a light source to back light the

incurring economic and circuit/system penalties.2 What is

display. The CCFL requires a high voltage AC supply for

really needed is high voltage generating capability that is

operation. Typically, over 1000 volts RMS is required to

inherently better suited to coming generations of backlight

initiate lamp operation, with sustaining voltages ranging

inverters. Piezoceramic transformers, an arcane and little

from 200VAC to 800VAC. To date, the high voltage section

known technology, has been tamed and made available for

of backlight “inverters” has been designed around mag-

CCFL inverter use. This publication summarizes results of

netic transformers. A great deal of effort has been directed

an extensive collaborative development effort between

towards these ends, accompanied by a large volume of

LTC and CTS Wireless Components (formerly Motorola

descriptive material.1 Unfortunately, as available circuit

Ceramic Products).

board space continues to shrink, magnetic transformer based approaches begin to encounter difficulty. In par-

Piezoelectric Transformers

ticular, it is highly desirable to fashion laptop computers

with large area screens, leaving little room for the back- Piezoelectric Transformers (PZT), like magnetic devices, light inverter board. In many cases there is so little space are basically energy converters. A magnetic transformer available that building the inverter function inside the LCD operates by converting an electrical input to magnetic panel has become attractive, although to date impractical. energy and then reconverting the magnetic energy back to an electrical output. A PZT has an analogous operating

Limitations and Problems of Magnetic CCFL mechanism. It converts an electrical input into mechanical

Transformers energy and subsequently reconverts this mechanical en- ergy back to an electrical output. The mechanical transport

Construction and high voltage breakdown characteristics

causes the PZT to vibrate, similar to quartz crystal opera-

of magnetic transformers present barriers to implement-

tion, although at acoustic frequencies. The resonance

ing them in these forthcoming space intensive designs.

associated with this acoustic activity is extraordinarily

Additionally, as refined as magnetic technology is, other

high; Q factors over 1000 are typical. This transformer

inverter problems associated with it also exist. Such

action is accomplished by utilizing properties of certain

problems include the necessity to optimize and calibrate

the inverter for best performance with a given display type. Note 1: See References 1 through 3 for examples. Note 2: Again, see References 1 through 3.

Practically, this means that the manufacturer must, via either hardware or software, adjust inverter parameters to

, LTC and LT are registered trademarks of Linear Technology Corporation.

achieve optimum performance with a given display type.

Piezoceramic inverter circuits and techniques are covered by LTC patents issued and pending. CTS Changes in display choice must be accompanied by com- Wireless Components (formerly Motorola Ceramic Products)patents, issued and pending, also apply.

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