LT1186FCS Linear Technology, LT1186FCS Datasheet - Page 11

• Manufacturer Part Number: LT1186FCS
• Description: IC SWTCH REG CCFL DAC PROG16SOIC
• Manufacturer: Linear Technology
• Series: Bits-to-Nits™r
• Type: CCFL Controllerr

Specifications of LT1186FCS

• Frequency: 175 ~ 225 kHz
• Current - Supply: 6mA
• Voltage - Supply: 3 V ~ 6.5 V
• Operating Temperature: 0°C ~ 100°C
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -

APPLICATIONS
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
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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 A 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
LT1186F
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