Datasheet

11
UCC1972/3
UCC2972/3
UCC3972/3
Referring to Fig. 5, at time t0 the control signal is
brought low and the voltage in the resonant tank begins
to build. At time t1 there is sufficient voltage for the
lamp to strike and the feedback loop controls the lamp
at rated current using a fixed current sense resistor.
When the LFD signal is brought low at time T2, the
COMP output is low and the OUT pin stops switching.
The resonant tank voltage decays until the lamp extin
-
guishes. If the on time were extended to t3 the average
lamp intensity would be increased accordingly, the next
low frequency cycle begins at time t4.
The time relationship between the resonant and gating
frequency has been exaggerated so that the sinusoidal
waveforms can be depicted. In order to avoid visible
lamp flicker, the low frequency gating rate (t0-t4) should
be greater than 100Hz. To prevent “beat” frequency in
-
terference, it may be advantageous to synchronize the
gating frequency to a multiple of the monitor scan rate
of the LCD display. This can be accomplished by con
-
trolling the duty cycle with a timer routine within the LCD’s
software program.
LFD waveforms at 200Hz and 50% duty cycle are shown
in Fig. 6a. Fig. 6b show a time expanded photo of the
same waveforms. Channel 1 is lamp voltage at 500V /div,
Channel 2 is lamp current at 20mA / div, and Channel 3 is
the LFD control voltage. Since the photos are from a digi
-
tal oscilloscope, alias exists in the waveforms.
Lamp Current Control Loop
The current control loop for the CCFL circuit is discussed
in detail in Unitrode Application Note U-148 and is briefly
repeated here for completeness. A block diagram for the
current control loop is shown in Fig. 7.
The PWM modulator small signal gain is inversely propor
-
tional to the internal saw tooth ramp and proportional to
the input voltage (the inductor’s current slope increases as
VBAT increases). The resonant tank and buck inductor
form a RLC filter at the center point of the push pull trans-
former. The effective L of the filter is dominated by buck in-
ductor and the effective C is approximately 8 times the
resonant capacitor (C
RES
) value. This occurs because the
reflected ballast capacitance is equal to C
RES
and the
equivalent capacitance at the push-pull center point is four
times the capacitance across the tank. The equivalent re-
sistance at the push-pull center point is equal to ¼ the
tank voltage squared divided by the lamp power. The cor-
ner frequency and Q of the filter are:
F
LC
CORNER
BUCK RES
=
··
1
28
p
(13)
Q
FL
R
FILTER BUCK
FILTER
=
2p
(14)
APPLICATION INFORMATION (cont.)
LAMP
VOLTAGE
LAMP
CURRENT
LFD
CONTROL
SIGNAL
t0 t1 t2 t3 t4
ON
OFF
5V
0V
Figure 5. Low frequency dimming timing waveforms.
Figure 6a. LFD at 50% duty cycle.
Figure 6b. Time expanded showing lamp strike and
feedback delay.