參數(shù)資料
型號(hào)： UCC3977TRPW
廠商： Texas Instruments, Inc.
英文描述： BALLAST/BACKLIGHT CONTROLLER/DRIVER|TSSOP|8PIN|PLASTIC
中文描述： 鎮(zhèn)流器/背光控制器/驅(qū)動(dòng)器| TSSOP封裝| 8引腳|塑料
文件頁(yè)數(shù)： 12/28頁(yè)
文件大?。?/td> 433K
代理商： UCC3977TRPW
SLUS499A
–
NOVEMBER 2001
–
REVISED JANUARY 2002
12
www.ti.com
APPLICATION INFORMATION
lamp characteristics
A cold cathode fluorescent lamp has non-linear V-I characteristics as shown in Figure 10. The lamp
’
s intensity
(lumens) is roughly proportional to lamp current where lamp voltage remains somewhat constant over the
operating range. Lamp voltage is dependant on the diameter and length of the lamp used in the application. This
results in increased impedance when the lamp is dimmed. The impedance of the lamp will influence the gain
of the piezoelectric transformer (see Figure 9) and thus the operating frequency of the system.
start
Figure 9
60
65
70
75
80
50
0
150
100
200
300
250
ROUT = 500 k
ROUT = 250 k
ROUT = 100 k
ROUT = 750 k
f
–
Frequency
–
kHz
V
–
–
PIEZOELECTRIC GAIN
vs
FREQUENCY
strike
B
A
C
operate
Figure 10
550
500
650
600
700
800
750
500
0
1500
1000
2000
3000
2500
0
1
2
3
4
5
6
I
–
V
–
–
CCFL CURRENT
vs
LAMP VOLTAGE AND IMPEDANCE
ILAMP
–
Lamp Current
–
mA
LAMP
VOLTAGE
LAMP
IMPEDANCE
variable frequency control system
A simplified block diagram of a PZT based backlight converter is shown in Figure 11. The PZT is driven by a
resonant power stage whose amplitude is proportional to input voltage. The PZT then provides the voltage gain
necessary to drive the lamp. A control loop is formed around the error amplifier that compares average lamp
current to a reference signal (REF) allowing the intensity of the lamp to be regulated. The resulting control
voltage V
C
drives a VCO that determines the operating frequency of the resonant power stage.
The frequency range of the VCO must include the strike and operating frequencies of the PZT with some
tolerance included for component variation. Minimizing the programmable range improves the control response
of the feedback loop. For example, a frequency range of 67 kHz to 77 kHz might be used for the PZT in Figure 9.
The gain of the PZT must guarantee sufficient lamp voltage at minimum input voltage to ensure that the control
loop will always operate on the right side of resonance.
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