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Maxim/Dallas > App Notes > DIGITAL POTENTIOMETERS Keywords: CCFL, cold cathode fluorescent lamp, transformer Oct 15, 2004 APPLICATION NOTE 3375 DS3984 / DS3988 Transformer Turns Ratio Selection The DS3984 and DS3988 are multi-channel Cold-Cathode Fluorescent Lamp (CCFL) controllers. The key variable in designing an optimal system with the DS3984 and DS3988 is the selection of the transformer turns ratio. This application note describes how the transformer turns ratio varies with different CCFL lamps and DC supply voltages and it provides a quick look-up table to help the user quickly select a transformer turns ratio that will work well in their design. The DS3984 and DS3988 are multi-channel Cold-Cathode Fluorescent Lamp (CCFL) controllers. These controllers use a push-pull drive to convert a DC supply voltage to the high-voltage AC waveform that is required to power the CCFL lamps. Figure 1 details this arrangement. Figure 1. DS3984 / DS3988 CCFL drive schematic. Each channel of the controller drives two logic-level N-channel MOSFETs that are tied between the ends of a step- up transformer and ground. The transformer has a center tap on the primary that is connected to the DC supply. The controller alternately turns on the two MOSFETs to create the high voltage AC waveform on the secondary. The key variable in designing a push-pull drive scheme is selecting the turns ratio for the transformer. At a given regulated lamp operating current, the transformer turns ratio is directly proportional to the CCFL lamp voltage and inversely proportional to the DC supply voltage as shown in the equation below. For a given DC supply voltage, higher operating voltage CCFL lamps require a higher transformer turns ratio. For a particular CCFL lamp, a higher DC supply voltage will require a lower transformer turns ratio. The operating voltage of CCFL lamps varies, based on a number of factors including lamp diameter and length. Lamps with smaller diameters and longer lengths operate at higher voltages. There is not an exact formula for calculating the transformer turns ratio. Table 1 lists some empirically derived transformer turns ratio for a variety of CCFL lamps. As can be seen from the table, the transformer turns ratio decreases as the DC supply voltage increases, and the transformer turns ratio decreases as the operating voltage of the CCFL lamps decreases. Table 1. Target Transformer Turns Ratios for Various CCFL Lamps Lamp 1 Lamp 2 Lamp 3 Nominal DC 2.4mm x 438mm 2.2mm x 258mm 2.0mm x 218mm Supply Voltage2 830 ~ 890 Vrms1 540 ~ 600 Vrms1 440 ~ 490 Vrms1 5V 80:1 70:1 65:1 6V 75:1 65:1 60:1 7V 70:1 60:1 55:1 8V 65:1 65:1 50:1 9V 60:1