UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
Dimming Techniques
OR USE DC
CONTROL VOLTAGE
FROM D/A
Analog Dimming:
Dim
VX
A control circuit that implements analog dimming with a
potentiometer (RADJ) 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 RL and
RADJ. When the current reverses direction, D1 conducts
and the voltage on the sense node VX is clamped to the
forward drop of the diode. The resulting waveform at VX
is a half wave rectified sinusoid whose voltage is propor-
tional to lamp current.
0V
Bright
DIGITAL
PULSE
STREAM
CFB
CBALLAST
R3
R2
FB
–
SECONDARY
+
COMP
D1
R1
1.5V
1.5+ úùp
é
VD
(12)
ê
ë
2 û
Figure 3. Analog dimming control from micro- processor.
ILAMP
=
2 R + RADJ
(
)
L
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.
VX
CFB
0V
VX
C
BALLAST
RFB
–
+
RADJ
FB
1.5V
COMP
D1
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 RFB and
RLFD, CFB 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.
RL
Figure 2. Analog dimmer with potentiometer.
This voltage is averaged by the feedback components
(RFB, CFB) 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 RADJ to the appropriate value. Resistor RL 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 VX node voltage is averaged against the
digital pulse stream of the microprocessor. The averag-
ing circuit consists of R2, R3, and CFB. 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
VX
0V
CFB
ON
OFF
CBALLAST
RFB
COMP
FB
DLFD
RLFD
SECONDARY
D1
R1
1.5V
ON
ON
OFF
LFD CONTROL
SIGNAL
Figure 4. Low frequency dimming by forcing lamp
current off through the FB pin.
the pulse stream.
10
TI [ TEXAS INSTRUMENTS ]
