LTC1697

5

1697f

APPLICATIO S I FOR ATIO

Background

Current generation handheld computers and instruments

typically use backlit liquid crystal displays (LCDs). Cold

cathode fluorescent lamps (CCFLs) provide the highest

available efficiency for backlighting the display, where

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 converter. The lamps operate from DC, but

migration effects damage the lamp and shorten its life-

time. Lamp drive should ideally contain zero DC compo-

nent. 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, which can interfere with

other devices and degrade overall operating efficiency.

Sinusoidal CCFL drive also maximizes current-to-light

conversion in the lamp. The circuit also permits 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 usually a priority. Handheld portable

computers offer an excellent example. The CCFL and its

power supply can be responsible for almost 50% of the

total battery drain.

The CCFL regulator drives an inductor that acts as a

switch-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. This constant current and the resulting lamp inten-

sity is programmable. Lamp intensity is further controlled

by modulating the current to the Royer converter at 150Hz

to 500Hz.

Operation

The LTC1697 is a fixed frequency, current mode regulator.

Such a switcher controls switch duty cycle directly by

switch current rather than by output voltage. Referring to

the block diagram for the LTC1697, the NMOS switch

turns ON at the start of each oscillator cycle. The NMOS

switch turns back OFF when switch current reaches a

predetermined level.

Current Sensing

Lossless current sensing converts the peak current signal

to a voltage which is summed with the internal slope

provide a peak current control command for the PWM.

Current Limit

The current limit amplifier will shut the NMOS switch off

once the current exceeds the current limit threshold. The

current amplifier delay to the output is typically 50ns.

Synchronous Rectifier

The LTC1697 operates as a synchronous converter. When

the NMOS switch turns OFF as mentioned above, the

PMOS switch turns ON. This gives a low resistance current

Dimming PWM

An on-chip PWM dimming circuit enables and disables the

current mode regulator for each dimming cycle. It also

disconnects the feedback network from the compensation

oscillator for the dimming PWM produces a triangle wave

whose frequency is determined by an external capacitor