The small size and low power dissipation of cold-cathode fluorescent lamps (CCFLs) have made them popular for backlighting LCDs in various applications, and particularly those in laptop computers. A single lamp is often sufficient in such systems, but for higher brightness, the manufacturer must choose between two lamps in parallel (long, thin, straight tubes), one U-shaped tube or two L-shaped tubes. Lamps consisting of two straight tubes are less expensive.
For LCD panels that contain two adjacent CCFLs on one side of the panel, the usual arrangement is to connect the low-voltage ends of the lamps with a common return (Fig. 1). This approach is popular in automotive, industrial and avionic applications.
Though convenient, this type of configuration precludes the easy low-side current monitoring usually implemented for a single lamp. Instead, the two-lamp common-return system requires an indirect high-side current-monitoring scheme. (CCFL controllers must monitor lamp current to control lamp brightness.) This high-side current-monitoring technique can be demonstrated using Maxim Integrated Products' DS3882 dual-channel automotive CCFL controller IC.
Drive Scheme
The drive scheme shown in the DS3882 datasheet applies only for circuits that allow access to the low-voltage side of each lamp. A different configuration is necessary for driving two lamps that share a common low-voltage return. In that case, the DS3882 provides full protection for both lamps while adding only a few extra passive components.
Because the common return disallows lamp-current measurements at the low-voltage side of the lamps, the Fig. 1 drive scheme places the lamp-current feedback resistors (RFB) on the low-voltage side of the transformer secondary (instead of the low-voltage side of the lamp).
However, this approach presents a challenge: Current sensed in the RFB resistor consists not only of current flowing through the lamp, but also the current due to parasitic capacitance in the LCD panel and in the 10-pF/1-nF capacitive-divider network for overvoltage sensing.
The value of the RFB resistor must be derived empirically rather than calculated, because the amount of excess current from parasitic effects is unknown. To compensate for the effect of this current, the value of the RFB resistor must be somewhat lower than it would be when placed at the low-voltage end of the lamp.
If one assumes that 10% of the current is lost to parasitic effects, you can calculate the starting value for the RFB resistor as follows:
RFB starting value = 0.636/ILAMPRMS,
where ILAMPRMS is the nominal lamp current.
CCFLs are specified to be driven with a nominal lamp current that is usually between 4 mARMS and 7 mARMS. In the design shown here, the nominal lamp current is 5 mARMS, so the RFB is set to approximately 127 Ω.
Overvoltage-Detection Circuit
Fig. 1 contains two overvoltage-detection circuits, each comprising a 10-pF/1-nF capacitive divider (101:1), a 20-kΩ/1-kΩ resistive divider (21:1) and an RFB resistor. The capacitive divider's connection to the RFB resistor (instead of ground) removes some of its effect on the lamp-current measurement.
Given its connection to ground through the RFB resistor, the divide ratio is set lower to help mitigate the effect of a varying low-side reference voltage. To compensate for this low divide ratio, a resistive divider is then required to set a proper overvoltage limit for the controller IC. Because the IC's overvoltage threshold is 1 VPEAK, these settings yield an overvoltage limit of 2121 VPEAK, which is 1500 VRMS.
Note that nominal CCFL run voltages are between 500 VRMS and 1200 VRMS, which would correspond to 707 VPEAK and 1700 VPEAK, respectively. To strike CCFLs, the voltage must be higher. The overvoltage circuitry exists to keep the generated voltage from destroying the transformer.
Each of the IC's lamp-current monitor (LCM) and overvoltage detection (OVD) inputs includes a low-pass filter (8.2-kΩ series resistor and 120-pF shunt capacitor), which serves to block any high-frequency ripple present at the low-voltage side of the transformer secondary. No filter is necessary when lamp current is sensed at the low-voltage side of the lamps.
Two Channels in Phase
Operating both channels of the CCFL controller in phase produces a better current balance between the two lamps, because less current is lost due to capacitive coupling between the adjacent lamps. Any phase difference between the two channels causes a varying voltage potential along the linear dimension of the lamps, which in turn allows a flow of parasitic current between them.
Figs. 2 through 5 are typical waveforms, captured when the controller drives an LCD panel that contains two adjacent CCFLs sharing a common return. Fig. 2 graphs the noise across the RFB resistor, and Fig. 3 shows the effect of the low-pass filter in removing noise, which helps determine the true lamp-current signal. The waveform of Fig. 4 is taken from the middle of the capacitive divider, and the waveform in Fig. 5 is taken from the controller's OVD input.