Datasheet

10
UCC1972/3
UCC2972/3
UCC3972/3
Dimming Techniques
Analog Dimming:
A control circuit that implements analog dimming with a
potentiometer (R
ADJ
) is shown in Fig. 2. When the sec
-
ondary has a positive polarity current, D1 is reversed bi
-
ased and lamp current is sensed directly through R
L
and
R
ADJ
. When the current reverses direction, D1 conducts
and the voltage on the sense node V
X
is clamped to the
forward drop of the diode. The resulting waveform at V
X
is a half wave rectified sinusoid whose voltage is propor
-
tional to lamp current.
()
I
V
RR
LAMP
D
LADJ
=
+
é
ë
ê
ù
û
ú
+
15
2
2
.
p
(12)
This voltage is averaged by the feedback components
(R
FB
,C
FB
) and held to 1.5V by the error amplifier when
the control loop is active. The resulting voltage at the out
-
put of the error amplifier (COMP) sets the duty cycle of
PWM stage. Average lamp current is controlled by ad
-
justing R
ADJ
to the appropriate value. Resistor R
L
sets
the high current level of the lamp.
Analog Dimming by PWM or D/A Control Signal:
Analog dimming control of the lamp can be achieved by
providing a digital pulse stream (or DC control voltage)
from the system microprocessor as shown in Fig. 3. For
this technique, the lamp current sense resistor (R1) is
fixed and the V
X
node voltage is averaged against the
digital pulse stream of the microprocessor. The averag
-
ing circuit consists of R2, R3, and C
FB
. A higher average
value from the pulse stream will result in less average
lamp current. The frequency of the digital pulse stream
should be high enough to maintain a constant DC value
across the feedback capacitor. If a D/A converter is avail
-
able in the system, a DC output can be used in place of
the pulse stream.
Low Frequency Dimming (LFD):
Analog dimming techniques described previously can
provide excellent dimming over a 10:1 range, depending
upon the physical layout and the amount of stray capaci-
tance in the backlight's secondary circuitry. Beyond this
level the lamp may begin to exhibit the "thermometer ef-
fect" causing uneven illumination across the tube.
Low frequency dimming (LFD) is accomplished by oper-
ating the lamp at rated current and gating the lamp on
and off at a low frequency. Since the lamp is operated at
full intensity when on, the system layout has little effect
on dimming performance. The average lamp intensity is
a function of the duty cycle and period of the gating sig
-
nal. The duty cycle can be controlled to a low minimum
value, allowing a very wide dimming range. Low fre
-
quency dimming can be implemented by summing a
PWM signal into the feedback node to turn the lamp off
as shown in Fig. 4. A 68kW resistor is used for R
FB
and
R
LFD
,C
FB
is reduced to 6.8nF to speed up the lamp
re-strike. The repetition rate of the signal should be
greater than 120Hz to avoid visible flicker.
APPLICATION INFORMATION (cont.)
1.5V
R
FB
C
FB
C
BALLAST
R
L
R
ADJ
0V
V
X
V
X
SECONDARY
D1
+
–
COMP
FB
Figure 2. Analog dimmer with potentiometer.
C
BALLAST
R1
SECONDARY
D1
1.5V
FB
C
FB
0V
V
X
R3
Dim
Bright
R2
COMP
OR USE DC
CONTROL VOLTAGE
FROM D/A
DIGITAL
PULSE
STREAM
+
–
Figure 3. Analog dimming control from micro- processor.
C
BALLAST
R1
SECONDARY
D1
1.5V
FB
C
FB
0V
V
X
RLFD
RFB
COMP
DLFD
ON
OFF
ON
ON
OFF
LFD CONTROL
SIGNAL
Figure 4. Low frequency dimming by forcing lamp
current off through the FB pin.