OZ960
FUNCTIONAL INFORMATION
regulated through a voltage feedback loop where
output voltage is monitored. The signal, being
sent to the OVP pin, commands the output drives
to provide the desired output voltage. This design
provides high degree of flexibility while
1. Steady-State Operation
Refer to the schematic shown in Figure 1, the
OZ960 drives a full-bridge power train where the
transformer couples the energy from the power
source to the secondary CCFL load. The
switches in the bridge denoted as QA, QB, QC
and QD are configured such that QA and QB, QC
and QD are turned on complementarily. The
duration of QA and QD, QB and QC turn on
simultaneously determines an amount of energy
put into the transformer which in turn delivers to
the CCFL. The current in CCFL is sensed via
resistor R9 and regulated through the adjustment
of the turn-on time for both diagonal switches.
This is accomplished through an error amplifier in
the current feedback loop. A voltage loop is also
established to monitor the output voltage so that
a programmable striking voltage is achieved. The
OVP represents the peak-detect signal of the
voltage on the output of the transformer. A soft-
start circuit ensures a gradual increase in the
input and output power. The soft-start capacitor
determines the rate of rise of the voltage on SST
pin where the voltage level determines the on-
time duration of QA and QD, QB and QC
diagonal switches. This minimizes the surge
impacts in circuit designs.
maintaining OZ960
device.
a very high integration
One protection feature needed is removing the
lamp during normal operation. The OZ960
senses the missing current signal through current
amplifier, it shuts off the output drives and stay in
the latched mode. This is differentiated
intelligently with turning on the inverter while
CCFL is not connected. Recycle of the IC power
is necessary to resume normal operation.
Dimming control: dimming control of the inverter
is implemented by adjusting the amount of
energy processed and delivered to the CCFL. A
PWM burst-mode scheme is internally generated
which provides 0% to 100% wide dimming
control. An input analog voltage signal is fed into
DIM pin and determines the dimming level of the
CCFL.
The
burst-mode
frequency
is
programmable through a capacitor C10 as shown
in the schematic.
The OZ960 inverter operates in a constant
frequency mode. This eliminates any undesired
interference between inverter and LCD panels
where the interference is usually associated with
variable-frequency designs.
Apply enable signal to the ENA pin of the IC after
the bias voltage applied to VDDA initiates the
operation of the circuit. The output drives, include
PDR_A, NDR_B, PDR_C and NDR_D put out a
complementary square pulse. The frequency is
determined by R4 and C5 where they are
connected to RT and CT pins respectively.
Initially, the energy converted from the power
source to the CCFL is low due to the soft start
function. It increases as soft start capacitor
voltage increases linearly with time. The voltage
at the secondary side of the transformer T1
Symmetrical drive to the power transformer gives
a very dynamic choice of selecting transformers.
This vulnerable design offers flexibility to the
system designers to choose transformer sources.
There is no limitation to the gap-less transformer.
2. CCFL Ignition Time
increases
correspondingly.
This
process
continues until the CCFL current is detected and
reaches a regulated value. The output of the
error amplifier, CMP, follows the feedback signal,
commands a proper switching among the four
output drives to maintain current regulation. The
operations of the four switches are implemented
with zero-voltage-switching to provide a high-
efficiency power conversion.
Ignition time for CCFLs varies with CCFL length,
diameter, module package and temperature. The
OZ960 provides a flexible design where a
capacitor is connected to CTIMR pin to determine
the necessary striking time. An approximate of
the timing calculation is:
T[second] = C[uF]
In the case of open-lamp condition, the OZ960
provides
a
programmable striking-frequency
This capacitor remains reset at no charge if lamp
is connected and at normal operation.
intelligence to optimize the ignition scheme. This
is implemented through resistor R5. Effectively,
R5 is in parallel with R4 to yield a required
striking frequency. In addition, the striking time is
also programmable through the capacitor C8.
Striking voltage, or the open-lamp voltage, is
OZ960-DS-1.6
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