4-Channel Cold-Cathode

Fluorescent Lamp Controller

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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