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DS3984Z+ Datasheet(PDF) 11 Page - Dallas Semiconductor |
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DS3984Z+ Datasheet(HTML) 11 Page - Dallas Semiconductor |
11 / 24 page 4-Channel Cold-Cathode Fluorescent Lamp Controller ____________________________________________________________________ 11 To generate the DPWM signal internally, the DS3984 requires a clock (referred to as the dimming clock) to set the DPWM frequency. The user can supply the dim- ming clock by setting POSCS = 1 in CR1 and applying an external 22.5Hz to 440Hz signal at the POSC pin, or DS3984’s clock can be generated by the DS3984’s oscillator (set POSCS = 0 in CR1), in which case the frequency is set by an external resistor at the POSC pin. These two dimming clock options are shown in Figure 3. Regardless of whether the dimming clock is generated internally or sourced externally, the POSC1 and POSC2 bits in CR2 must be set to match the desired dimming clock frequency. When the DPWM signal is generated internally, its duty cycle (and, thus, the lamp brightness) is controlled by a user-applied analog voltage at the BRIGHT input. A BRIGHT voltage less than 0.5V will cause the DS3984 to operate with the minimum burst duty cycle, providing the lowest brightness setting, while any voltage greater than 2.0V will cause a 100% burst duty cycle (i.e., lamps always being driven), which provides the maximum brightness. For voltages between 0.5V and 2V the duty cycle will vary linearly between the minimum and 100%. The internally generated DPWM signal is available at the PSYNC I/O pin (set RGSO = 0 in CR1) for sourcing to other DS3984s, if any, in the circuit. This allows all DS3984s in the system to be synchronized to the same DPWM signal. The DS3984 that is generating the DPWM signal for other DS3984s in the system is referred to as the DPWM source. When the DPWM signal is provided by an external source, either from the PSYNC pin of another DS3984 or from some other user-generated source, it is input into the PSYNC I/O pin of the DS3984. In this mode, the BRIGHT and POSC inputs are disabled and should be grounded (see Figure 4). When multiple DS3984s are used in a design, DS3984s configured to use externally generated DPWM signals are referred to as DPWM receivers. Lamp Frequency Configuration The DS3984 can generate its own lamp frequency clock internally (set LFSS = 0 in CR1), which can then be sourced to other DS3984s if required, or the lamp clock can be supplied from an external source (set LFSS = 1 in CR1). When the lamp clock is internally generated, the frequency (40kHz to 80kHz) is set by an external resistor at the LOSC. In this case, the DS3984 can act as a lamp frequency source because the lamp clock is output at the LSYNC I/O pin for synchronizing any other DS3984s configured as lamp frequency receivers. The DS3984 acts as a lamp frequency receiver when the lamp clock is supplied externally. In this case, a 40kHz to 80kHz clock must be supplied at the LSYNC I/O. The external clock can originate from the LSYNC I/O of a DS3984 configured as a lamp frequency source or from some other source. BRIGHT PSYNC (OUTPUT) POSC 2.0V 0.5V 22.5Hz TO 440Hz RESISTOR TO SET THE DIMMING FREQUENCY DPWM SIGNAL ANALOG DIMMING CONTROL VOLTAGE RESISTOR-SET DIMMING CLOCK BRIGHT PSYNC (OUTPUT) POSC 2.0V 0.5V 22.5Hz TO 440Hz 22.5Hz to 440Hz DPWM SIGNAL EXTERNAL DIMMING CLOCK ANALOG DIMMING CONTROL VOLTAGE EXTERNAL DIMMING CLOCK Figure 3. DPWM Source Configuration Options BRIGHT PSYNC (OUTPUT) POSC 22.5Hz TO 440Hz DPWM SIGNAL Figure 4. The DPWM Receiver Configuration |
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