LT1186FIS Linear Technology, LT1186FIS Datasheet - Page 15

• Manufacturer Part Number: LT1186FIS
• Description: IC SWTCH REG CCFL DAC PROG16SOIC
• Manufacturer: Linear Technology
• Series: Bits-to-Nits™r
• Type: CCFL Controllerr

Specifications of LT1186FIS

• Frequency: 175 ~ 225 kHz
• Current - Supply: 6mA
• Voltage - Supply: 3 V ~ 6.5 V
• Operating Temperature: -40°C ~ 100°C
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -

APPLICATIONS
Maintaining closed-loop control of lamp current in a
floating lamp configuration necessitates deriving a feed-
back signal from the primary side of the Royer trans-
former. Previous solutions have used an external preci-
sion shunt and high-side sense amplifier configuration.
This approach has been integrated onto the LT1186F for
simplicity of design and ease of use. An internal 0.1
resistor monitors the Royer converter current and con-
nects between the input terminals of a high-side sense
amplifier. A 0 – 1 Amp Royer primary-side, center-tap
current is translated to a 0 A to 500 A sink current at the
CCFL V
the lamp current programmer circuit. The compensation
capacitor on the CCFL V
pensation and an averaging function to the error sink
current. Therefore, input programming current is related
to average Royer converter current. Floating lamp circuits
operate similarly to grounded lamp circuits except for the
derivation of the feedback signal.
The transfer function between lamp current and input
programming current must be empirically determined and
is dependent upon a myriad of factors including lamp
characteristics, display construction, transformer turns
ratio and the tuning of the Royer oscillator. Once again,
lamp current will be slightly higher at one end of the lamp
and input programming current should be set for this
higher level to ensure that the lamp is not overdriven.
The internal 0.1 high-side sense resistor on the LT1186F
is rated for a maximum DC current of 1A. This resistor can
be damaged by extremely high surge currents at start-up.
The Royer converter typically uses a few microfarads of
bypass capacitance at the center tap of the transformer.
This capacitor charges up when the system is first pow-
ered by the battery pack or an AC wall adapter. The amount
of current delivered at start-up can be very large if the total
impedance in this path is small and the voltage source has
high current capability. Linear Technology recommends
the use of an aluminum electrolytic for the transformer
center-tap bypass capacitor with an ESR greater than or
equal to 0.5 . This lowers the peak surge currents to an
acceptable level. In general, the wire and trace inductance
in this path also help reduce the di/dt of the surge current.
This issue only exists with floating lamp circuits as
C
pin to null against the source current provided by
U
C
INFORMATION
U
pin provides stable loop com-
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
W
U
grounded lamp circuits do not make use of the high-side
sense resistor.
Input Capacitor Type
Caution must be used in selecting the input capacitor type
for switching regulators. Aluminum electrolytics are elec-
trically rugged and the lowest cost, but are physically large
to meet required ripple current ratings, and size con-
straints (especially height) may preclude their use. Ce-
ramic capacitors are now available in larger values and
their high ripple current and voltage rating make them
ideal for input bypassing. Cost is fairly high and footprint
can be large.
Solid tantalum capacitors would be a good choice except
for a history of occasional failure when subjected to large
current surges during start-up. The input bypass capaci-
tor of regulators can see these high surges when a battery
or high capacitance source is connected. Some manufac-
turers have developed tantalum capacitor lines specially
tested for surge capability (AVX TPS series for instance),
but even these units may fail if the input voltage surge
approaches the capacitor’s maximum voltage rating. AVX
recommends derating the capacitor voltage by 2:1 for high
surge applications.
Applications Support
Linear Technology invests an enormous amount of time,
resources and technical expertise in understanding, de-
signing and evaluating backlight/LCD contrast solutions
for system designers. The design of an efficient and
compact LCD backlight system is a study of compromise
in a transduced electronic system. Every aspect of the
design is interrelated and any design change requires
complete re-evaluation for all other critical design param-
eters. Linear Technology has engineered one of the most
complete test and evaluation setups for backlight designs
and understands the issues and tradeoffs in achieving a
compact, efficient and economical customer solution.
Linear Technology welcomes the opportunity to discuss,
design, evaluate and optimize any backlight/LCD contrast
system with a customer. For further information on back-
light/LCD contrast designs, consult the References.
LT1186F
15