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UCC1972 Datasheet(PDF) 10 Page - Texas Instruments |
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UCC1972 Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 24 page  10 UCC1972/3 UCC2972/3 UCC3972/3 Dimming Techniques Analog Dimming: 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. () I V RR LAMP D LADJ = + é ëê ù ûú + 15 2 2 . p (12) 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 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 68k W 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. APPLICATION INFORMATION (cont.) 1.5V R FB C FB CBALLAST R L R ADJ 0V V X V X 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. |
Similar Part No. - UCC1972_13 |
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Similar Description - UCC1972_13 |
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