UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
frequencies, the UCC3972/3 synchronizes the buck fre-
quency to the frequency of the push-pull stage. The tra-
ditional buck topology is inverted to take advantage of
the lower R
DS(on)
characteristics of an N-Channel
MOSFET switch (S
BUCK
). With a sinusoidal voltage
across the tank, the resulting output of the buck stage
(V
BUCK
) becomes a full-wave rectified voltage referenced
to VBAT as shown in Fig. 1.
Lamp current is sensed directly with R
S
and a parallel di-
ode on each half cycle. The resulting voltage across the
sense resistor R
S
is kept at a 1.5V average by the error
amplifier, which in turn controls the duty cycle of S
BUCK
.
The buck converter typically operates in continuous cur-
rent mode but can operate with discontinuous current as
the CCFL is dimmed.
Design Procedure
A notebook computer backlight circuit will be presented
here to illustrate a design based on the UCC3972/3 con-
troller. The converter will be designed to drive a single
cold cathode fluorescent lamp (CCFL) with the following
specifications:
Table 1. Lamp Specifications
Lamp Length
Lamp Diameter
Striking Voltage (20°C)
Operating Voltage (5mA)
Full Rated Current
Full Rated Power
250mm (10”)
6mm
1000V (PEAK)
375V (RMS)
5mA
1.9W
lamp, providing a high impedance sinusoidal current
source with which to drive the CCFL. This approach im-
proves the optical efficiency of the system, as capacitive
leakage effects are minimized due to reduced harmonic
content in the voltage waveforms. Unfortunately, from an
electrical efficiency standpoint, an increased tank voltage
produces increased flux losses in the transformer and in-
creased circulating currents in the tank. In practice, the
voltage drop across the ballast capacitor is selected to
be approximately twice the lamp voltage (750V in our
case) at rated lamp current. Assuming a 50kHz resonant
frequency and 5mA operating current, a ballast capaci-
tance of 22pF is selected. Since the lamp and ballast ca-
pacitor impedance are 90 degrees out of phase, the
vector sum of lamp and capacitor voltages determine the
secondary voltage on the transformer.
V
SEC
=
(
V
CB
)
2
+
(
V
LAMP
)
2
(2)
The resulting secondary voltage at rated lamp current is
820V. Since the capacitor dominates the secondary im-
pedance, the lamp current maintains a sinusiodal shape
despite the non-linear behavior of the lamp. As the CCFL
is dimmed, lamp voltage begins to dominate the second-
ary impedance and current becomes less sinusiodal.
Transformer secondary voltage is reduced, however, so
high frequency capacitive losses are less pronounced.
The value of ballast capacitor has no effect on current
regulation since the average lamp current is sensed di-
rectly by the controller.
Once the ballast capacitor is selected, the resonant fre-
quency of the push-pull stage can be determined from
the transformer’s inductance (L), turns ratio (N), and the
selection of resonating capacitor (C
RES
).
F
RESONANT
=
1
2
p
ç
÷
L
PRIMARY
æ
C
RES
+
N
2
·
C
BALLAST
ö
è
ø
(3)
Input Voltage Range:
The notebook computer will be powered by a 4 cell Lith-
ium-Ion battery pack with an operational voltage range of
10V to 16.8V. When the pack is being charged, the back
light converter is powered from an AC adapter whose DC
output voltage can be as high as 22V.
Resonant Tank and Output Circuit
The selection of components to be used in the resonant
tank of the converter is critical in trading off the electrical
and optical efficiencies of the system. The value of the
output circuit’s ballast capacitor plays a key role in this
trade-off. The voltage across the ballast capacitor is a
function of the resonant frequency and secondary lamp
current:
V
CB
=
I
LAMP
2
· p ·
C
BALLAST
·
F
RESONANT
(1)
(
)
Output distortion is minimized by keeping the independ-
ent resonant frequencies of the primary and secondary
circuits equal. This is achieved by making the resonant
capacitor equal to the ballast capacitance times the turns
ratio squared:
C
RES
=
N
2
·
C
BALLAST
=
(
67
)
·
22
pF
=
0. 1
m
F
2
(4)
A voltage drop across C
BALLAST
many times the lamp
voltage will make the secondary current insensitive to
distortions caused by the non-linear behavior of the
7
The resulting resonant frequency is about 50kHz, this
frequency will vary depending upon the lamp load and
amount of stray capacitance in the system. Since the
UCC3972/3 has an internal oscillator, it is important that
TI [ TEXAS INSTRUMENTS ]
