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DS3994Z+ Datasheet(PDF) 11 Page - Maxim Integrated Products |
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DS3994Z+ Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 28 page 4-Channel Cold-Cathode Fluorescent Lamp Controller ______________________________________________________________________________________________________ 11 To generate the DPWM signal internally, the DS3994 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 DS3994’s clock can be generated by the DS3994’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 POSCR1 and POSCR2 bits in CR2 must be set to match the desired dimming clock frequency. Lamp Dimming Control (DPWM) 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. Users can select a positive or negative slope for the bright pin’s dimming input as well as the voltage range. If SLOPE = 0 in CR3, then the slope is positive. This means that a BRIGHT voltage less than the minimum voltage causes the DS3994 to operate with the minimum burst duty cycle, providing the lowest brightness set- ting, while any voltage greater than the maximum volt- age causes a 100% burst duty cycle (i.e., lamps always being driven), which provides the maximum brightness. For voltages between the minimum voltage and the maximum voltage, the duty cycle varies 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 DS3994s, if any, in the circuit. This allows all DS3994s in the system to be synchronized to the same DPWM signal. The DS3994 that is generating the DPWM signal for other DS3994s in the system is referred to as the DPWM source. BRIGHT PSYNC (OUTPUT) POSC 0.5V OR 0.0V 2.0V OR 3.3V 22.5Hz TO 440Hz RESISTOR TO SET THE DIMMING FREQUENCY DPWM SIGNAL ANALOG DIMMING CONTROL VOLTAGE RESISTOR-SET DIMMING CLOCK BRIGHT PSYNC (OUTPUT) POSC 22.5Hz TO 440Hz 22.5Hz TO 440Hz DPWM SIGNAL EXTERNAL DIMMING CLOCK ANALOG DIMMING CONTROL VOLTAGE EXTERNAL DIMMING CLOCK 0.5V OR 0.0V 2.0V OR 3.3V Figure 3. DPWM Source Configuration Options BRIGHT PSYNC (INPUT) POSC 22.5Hz TO 440Hz DPWM SIGNAL Figure 4. DPWM Receiver Configuration Table 1. BRIGHT Analog Dimming Input Slope and Voltage Range Configuration CR2.7 CR3.0 RANGE SLOPE MINIMUM BRIGHTNESS MAXIMUM BRIGHTNESS 0 0 0.5 to 2V Positive 0.5V 2.0V 0 1 0.5 to 2V Negative 2.0V 0.5V 1 0 0 to 3.3V Positive 0V 3.3V 1 1 0 to 3.3V Negative 3.3V 0V |
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