DS3984 datasheet(11/24 Pages) DALLAS

11 / 24 page

4-Channel Cold-Cathode

Fluorescent Lamp Controller

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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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