OZ9938
FUNCTIONAL DESCRIPTION
(Pin 6). When the voltage at VSEN reaches a
threshold of approximately 3.0V, the IC regulates
the output voltage at the transformer secondary.
If the lamps are not ignited when the voltage at
1. Power Conversion
The OZ9938 controller is designed to suit various
power conversion topologies and provides
symmetrical drive pulses to the tank circuit that
includes the transformer(s), output capacitors
and the CCFL/panel load that yields quasi-
sinusoidal CCFL voltage and current waveforms.
High efficiency operation of the OZ9938 yields
lower heat dissipation for the inverter system
resulting in higher reliability.
TIMER (pin 3) reaches
a
threshold of
approximately 3V, the IC will shutdown and latch
The approximate striking time is calculated by the
following equation.
Tstr [Sec] = CTIMER [uF]
To resume normal operation, toggle the ENA
signal or reset VDDA.
To illustrate the controller functions, refer to
Figures 1 and 2 on pages 5 and 8, respectively
for the following sections.
6. Normal Operation
2. Enable
Once the lamps are ignited and the voltage at
ISEN (pin 5) is >0.7V, the IC enters the normal
operation mode and the PWM dimming control is
activated.
The operating frequency is determined by
resistor (R9) and capacitor (C9) connected to CT
(pin 13). The control loop regulates the average
current through the lamps by adjusting the duty
cycle of the output drives. Constant frequency
operation eliminates interference with the inverter
and LCD panel that often occurs in a variable
frequency inverter system. The peak and valley
of the CT waveform are 2V and 0V respectively.
Applying a voltage level greater than 2V to ENA
(pin 10) enables the IC. A voltage less than 1V
will disable the IC.
3. Soft-Start (SST)
Utilizing a patented multi-task technique, the soft-
start function and the loop compensation function
are combined to provide
a
good start-up
characteristic. Connecting an external capacitor
to SSTCMP (pin 12) provides the functions. In
the start-up mode, current charges capacitor C13
connected to SSTCMP. The voltage at the
capacitor controls the gradual increase in power
to the transformer and subsequently to the output
load. This reduces in-rush current and provides
reliable operation to the CCFL.
The approximate operating frequency is
calculated by the following equation.
9.53 x105
fop
=
[KHz]
RCT [K ]*CCT[pF]
4. Ignition
7. Open Lamp Protection
When the VDDA voltage exceeds the under-
voltage lockout threshold, the IC is enabled an
internal striking timer is activated.
If a CCFL is removed or damaged during normal
operation, the voltage at SSTCMP (pin 12) rises
rapidly. When the voltage at SSTCMP reaches a
threshold of approximately 2.5V, a current source
charges the capacitor (C17) connected to TIMER
(pin 3). Once the voltage level at the TIMER pin
The approximate striking frequency is calculated
by the following equation.
3812/ RCT [K ] + 26
reaches
a threshold of approximately 3V, the
fst =
X1000 [KHz]
drive outputs shut down and latch.
4 *CCT[pF]
The shutdown delay feature avoids inverter
shutdown due to a VIN transient or if a lamp has
a positive impedance characteristic.
5. Aged CCFL Ignition
OZ9938 provides
a striking timer function to
ensure that any aged, slow-turn-on CCFL is
provided with sufficient voltage and time to ignite.
The transformer output voltage is sensed at
VSEN
The approximate shutdown delay time is
calculated by the following equation.
Td [Sec] = 0.33 XCTIMER [uF]
CONFIDENTIAL
OZ9938-DS-1.1
Page 6
