Abstract: DS3881, DS3882, DS3984, DS3988, DS3991, DS3992 and DS3994 are cold cathode fluorescent lamp (CCFL) backlit liquid crystal display (LCD) controllers. In order to obtain satisfactory visual effects or extend the life of the lamp, brightness adjustment is required in most applications. This article first introduces two commonly used methods for CCFL dimming. It then describes how to use the DS39xx CCFL controller to achieve analog dimming.
CCFL dimming method There are two common methods of CCFL dimming: one is pulse dimming, also known as PWM dimming or digital dimming; the other is analog dimming. This article discusses the advantages and disadvantages of these two methods.
Pulse dimming can turn on and off CCFL at a specific frequency, which is called PWM dimming frequency. If the PWM dimming frequency is greater than 60 Hz, the human eye cannot perceive the CCFL on and off. During the high period of PWM, CCFL is on and working at the lamp frequency. During the low PWM period, CCFL is turned off and no current flows. By adjusting the duty cycle of the PWM pulse, the brightness of the CCFL can be increased or decreased. The main advantage of pulse dimming is that a very large dimming ratio can be achieved. However, in some applications, the PWM dimming frequency may interfere with the vertical synchronization frequency of the display signal, so that a visible interference signal will appear on the display. Pulse dimming will also introduce transformer audio noise.
CCFL is always on during analog dimming, not on and off in pulse mode. Adjust the lamp brightness by changing the lamp current amplitude. Obviously, the larger the current amplitude, the brighter the CCFL; the lower the amplitude, the darker the CCFL. The dimming range of analog dimming is very narrow, which is slightly insufficient in some applications. But since there is no PWM frequency, analog dimming will not cause transformer audio noise. In addition, analog dimming will not interfere with the vertical sync frequency.
Table 1 compares analog dimming and pulse dimming. See Application Note 3997 for more description: How to Achieve a 300: 1 Dimming RaTIo with the DS3881 / DS3882 CCFL Controllers.
Table 1. Comparison of analog dimming and pulse dimming
Realization of CCFL analog dimming The DS3881 and DS3882 CCFL controllers have built-in analog dimming control functions. Users can set the BLC register through the I²C interface to adjust the lamp current.
The DS3984, DS3988, DS3991, DS3992, and DS3994 CCFL controllers have pulse dimming. However, using the simple external circuit shown in Figure 1, the DS39xx controller can also support analog dimming.
Figure 1. The external circuit required to implement analog dimming for the DS39xx CCFL controller
In Figure 1, R4 is the lamp current feedback resistor. The peak value of the voltage across R4 is VR4. After the signal passes through the diode, the peak voltage becomes VA. The peak voltage VLCM at the input of the LCM is a linear combination of VA and the analog dimming control voltage VDIM. which is:
among them
with
The VLCM (peak) rating is 2.35V. Therefore VDIM determines VA, which will directly affect the lamp current. Note that if R1 is open and there is no VDIM voltage, the circuit is actually part of the typical multi-lamp current monitor circuit given in the DS3992 / DS3994 data sheet.
Resistance calculation When calculating the resistance value, a and b are obtained according to the system requirements and in combination with Equation 1. Then use Equations 2 and 3 to calculate the appropriate values ​​for R1, R2, and R3.
For example, if the application requires a lamp current of 7mARMS when VDIM is 0V and a lamp current of 3mARMS when VDIM is 3.3V, then VA equals 19.1Vpk when VDIM = 0V (√2 × 7mARMS × 2kΩ-0.7V), VDIM = 3.3V When VA is equal to 7.8Vpk (√2 × 3mARMS × 2kΩ-0.7V). Using these conditions and Equation 1, a = 0.422 and b = 0.123.
When a and b are known, set R3 to any value (not more than 10kΩ). Then solve equations 2 and 3 to get R1 and R2. If R3 is chosen to be 3.4kΩ, then R1 = 3.65kΩ and R2 = 12.4kΩ. Due to the harmonics in the lamp current, the crest factor of the current waveform is not always √2. Therefore, some adjustments to the resistance value are needed to achieve the desired effect. In this example, the final values ​​of R1 and R2 are 4.42kΩ and 11.5kΩ, respectively.
As mentioned above, the lamp current decreases as the dimming control voltage increases. This function is called negative slope dimming. Conversely, positive slope dimming means that the lamp current increases as the dimming control voltage increases. If you want to use positive slope dimming, you can add a reverse circuit between VDIM and R1.
Test waveforms 2 and 3 are the lamp current waveforms measured on the basis of Fig. 1. Figure 2 shows the waveform when VDIM = 0V, and Figure 3 shows the waveform when VDIM = 3.3V. The dimming ratio is 2.33: 1.
Figure 2. Lamp current waveform at VDIM = 0V
Figure 3. Lamp current waveform at VDIM = 3.3V
CCFL dimming method There are two common methods of CCFL dimming: one is pulse dimming, also known as PWM dimming or digital dimming; the other is analog dimming. This article discusses the advantages and disadvantages of these two methods.
Pulse dimming can turn on and off CCFL at a specific frequency, which is called PWM dimming frequency. If the PWM dimming frequency is greater than 60 Hz, the human eye cannot perceive the CCFL on and off. During the high period of PWM, CCFL is on and working at the lamp frequency. During the low PWM period, CCFL is turned off and no current flows. By adjusting the duty cycle of the PWM pulse, the brightness of the CCFL can be increased or decreased. The main advantage of pulse dimming is that a very large dimming ratio can be achieved. However, in some applications, the PWM dimming frequency may interfere with the vertical synchronization frequency of the display signal, so that a visible interference signal will appear on the display. Pulse dimming will also introduce transformer audio noise.
CCFL is always on during analog dimming, not on and off in pulse mode. Adjust the lamp brightness by changing the lamp current amplitude. Obviously, the larger the current amplitude, the brighter the CCFL; the lower the amplitude, the darker the CCFL. The dimming range of analog dimming is very narrow, which is slightly insufficient in some applications. But since there is no PWM frequency, analog dimming will not cause transformer audio noise. In addition, analog dimming will not interfere with the vertical sync frequency.
Table 1 compares analog dimming and pulse dimming. See Application Note 3997 for more description: How to Achieve a 300: 1 Dimming RaTIo with the DS3881 / DS3882 CCFL Controllers.
Table 1. Comparison of analog dimming and pulse dimming
| Analog Dimming | Burst Dimming |
| 1. Narrow dimming range, no more than 3: 1 2. No audible transformer noise 3. No interference with the verTIcal synchronous frequency | 1. Large dimming raTIo up to 100: 1 2. Audible transformer noise can be present. 3. The PWM dimming frequency can interfere with the verTIcal synchronous frequency. |
Realization of CCFL analog dimming The DS3881 and DS3882 CCFL controllers have built-in analog dimming control functions. Users can set the BLC register through the I²C interface to adjust the lamp current.
The DS3984, DS3988, DS3991, DS3992, and DS3994 CCFL controllers have pulse dimming. However, using the simple external circuit shown in Figure 1, the DS39xx controller can also support analog dimming.
Figure 1. The external circuit required to implement analog dimming for the DS39xx CCFL controller
In Figure 1, R4 is the lamp current feedback resistor. The peak value of the voltage across R4 is VR4. After the signal passes through the diode, the peak voltage becomes VA. The peak voltage VLCM at the input of the LCM is a linear combination of VA and the analog dimming control voltage VDIM. which is:
among them
with
The VLCM (peak) rating is 2.35V. Therefore VDIM determines VA, which will directly affect the lamp current. Note that if R1 is open and there is no VDIM voltage, the circuit is actually part of the typical multi-lamp current monitor circuit given in the DS3992 / DS3994 data sheet.
Resistance calculation When calculating the resistance value, a and b are obtained according to the system requirements and in combination with Equation 1. Then use Equations 2 and 3 to calculate the appropriate values ​​for R1, R2, and R3.
For example, if the application requires a lamp current of 7mARMS when VDIM is 0V and a lamp current of 3mARMS when VDIM is 3.3V, then VA equals 19.1Vpk when VDIM = 0V (√2 × 7mARMS × 2kΩ-0.7V), VDIM = 3.3V When VA is equal to 7.8Vpk (√2 × 3mARMS × 2kΩ-0.7V). Using these conditions and Equation 1, a = 0.422 and b = 0.123.
When a and b are known, set R3 to any value (not more than 10kΩ). Then solve equations 2 and 3 to get R1 and R2. If R3 is chosen to be 3.4kΩ, then R1 = 3.65kΩ and R2 = 12.4kΩ. Due to the harmonics in the lamp current, the crest factor of the current waveform is not always √2. Therefore, some adjustments to the resistance value are needed to achieve the desired effect. In this example, the final values ​​of R1 and R2 are 4.42kΩ and 11.5kΩ, respectively.
As mentioned above, the lamp current decreases as the dimming control voltage increases. This function is called negative slope dimming. Conversely, positive slope dimming means that the lamp current increases as the dimming control voltage increases. If you want to use positive slope dimming, you can add a reverse circuit between VDIM and R1.
Test waveforms 2 and 3 are the lamp current waveforms measured on the basis of Fig. 1. Figure 2 shows the waveform when VDIM = 0V, and Figure 3 shows the waveform when VDIM = 3.3V. The dimming ratio is 2.33: 1.
Figure 2. Lamp current waveform at VDIM = 0V
Figure 3. Lamp current waveform at VDIM = 3.3V
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