OZ9936 Просмотр технического описания (PDF) - O2Micro International
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OZ9936 Datasheet PDF : 11 Pages
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FUNCTIONAL DESCRIPTION
1. Power Conversion
The power train can be selected from the
standard half-bridge, full-bridge, push-pull or
Class-D topologies to provide symmetrical drive
pulses to the tank circuit that includes the
transformer(s), output capacitors and the
CCFL/panel load, to yield quasi-sinusoidal CCFL
voltage and current waveforms. High-efficiency
operation of the OZ9936 yields lower heat
dissipation for the inverter system resulting in
higher reliability.
To illustrate the controller functions, refer to
Figures 1 and 2 on pages 4 and 7, respectively
for the following sections. Other topologies can
be found in the OZ9936 Application Note.
2. Enable
The OZ9936 is enabled when the voltage on Pin
1 is greater than 0.55V.
3. Soft Start (SST)
Connecting an external capacitor to Pin 1
provides the SST function. A charging current is
provided to capacitor C11. At Start-up, as
capacitor C11 charges, the voltage level controls
the gradual increase in power to the transformer.
This reduces in-rush current and provides reliable
operation to the CCFL.
4. Ignition
The ignition process requires a higher striking
frequency to strike the CCFL. The striking
frequency is approximately 1.3 times the normal
operating frequency. The striking frequency is
determined by external resistor R13 connected to
RT (Pin 2).
5. Normal Operation
OZ9936 operates in a constant frequency mode.
This eliminates any undesired interference
between the controller and LCD panel, as the
interference is usually associated with variable-
frequency design. Interference may result in a
poor user experience because of “waterfall”
display distortion and other poor display
appearance.
Once the CCFL is ignited and current is sensed
at Pin 8 (ISEN), the control loop regulates the
CCFL current. An external resistor, R13,
OZ9936
connected to RT (Pin 2) determines the operating
frequency. The approximate operating frequency
is calculated by the following equation.
fop =
5.0 x103
RT [KΩ]
[KHz]
6. Over-Voltage Protection
The control logic protects the transformer from an
abnormally high voltage at the secondary output.
The transformer output voltage is sensed through
Rectifier Diodes (CR1, CR3, CR5, CR8) and
transistor Q5. When the voltage at the emitter of
Q5 is less than –VBE drop, Q5 will conduct and
regulate the output voltage to the transformer
secondary.
7. Open Lamp Protection
When a CCFL is removed or damaged during
normal operation, the OZ9936 shuts off the
output drives after the shutdown delay timer
expires. When the damaged lamp is replaced,
toggling the enable pin from low to high resumes
normal operation.
OZ9936 provides a shutdown delay feature if the
lamp current is not regulated. The inverter
module will remain in normal operation for
approximately 2 to 3 seconds if the input voltage
suddenly drops and subsequently resumes to
normal level.
8. Aged CCFL Ignition
During the ignition process, the controller senses
the voltage at the CCFL. For an open-circuit
condition, the power train delivers a regulated
voltage at the CCFL for approximately 2.0
seconds. If no current is sensed after
approximately 2.0 seconds, the OZ9936 shuts
off. Toggling the enable signal from low to
floating will resume normal operation. This is to
ensure that any aged, slow-turn-on CCFL is
provided with sufficient voltage and time to ignite.
9. Dimming Control
The OZ9936 provides an analog dimming control
function. The input to Pin 7 (DIM) is an analog
voltage of 0.5V to 1.25V that corresponds to
approximately 40% to 100% of the LCD panel
brightness.
CONFIDENTIAL
OZ9936-DS-1.1
Page 5
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