LTC1697
5
1697f
APPLICATIO S I FOR ATIO
WUUU
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
compensation. This summed signal is compared to V
C
to
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
path for the inductor current back to V
IN
.
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
node (V
C
) to reduce slew time at the next enable time. The
oscillator for the dimming PWM produces a triangle wave
whose frequency is determined by an external capacitor
on the C
DIM
pin. The dimming PWM frequency is equal to
5Hz/C
DIM
(µF) with its duty cycle set by the voltage on the
