DS3994Z+W Maxim Integrated Products, DS3994Z+W Datasheet - Page 10

• Manufacturer Part Number: DS3994Z+W
• Description: Display Drivers 4-Ch Cold-Cathode Fl uorescent Controller
• Manufacturer: Maxim Integrated Products

Specifications of DS3994Z+W

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The lamp-frequency MOSFET gate turn-on times are
equally phased among the four channels during the
burst period. This reduces the inrush current that would
result from all lamps switching simultaneously, and
hence eases the design requirements for the DC supply.
Figure 2 details how the four channels are phased. Note
that it is the lamp frequency signals that are phased,
NOT the DPWM signals. See the Burst Dimming Stagger
(BDS) Functionality section for details or to adjust the
DPWM signals of each channel.
The DS3994 uses a digital pulse-width modulated
(DPWM) signal (22.5Hz to 440Hz) to provide efficient
and precise lamp dimming. During the high period of
4-Channel Cold-Cathode
Fluorescent Lamp Controller
Figure 2. Channel Phasing Detail
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____________________________________________________________________
SEQUENCE
LAMP FREQUENCY
DRIVE SIGNALS AT
CHANNEL
MOSFET GATE-
GB2
GA1
GB1
GA2
GA3
GB3
GA4
GB4
Lamp Dimming Control (DPWM)
4
1
2
Channel Phasing
3
4
1
2
the DPWM cycle, the lamps are driven at the selected
lamp frequency (20kHz to 80kHz) as shown in Figure 7.
This part of the cycle is called the “burst” period
because of the lamp frequency burst that occurs dur-
ing this time. During the low period of the DPWM cycle,
the controller disables the MOSFET gate drivers so the
lamps are not driven. This causes the current to stop
flowing in the lamps, but the time is short enough to keep
the lamps from de-ionizing. Dimming is increased/
decreased by adjusting (i.e., modulating) the duty cycle
of the DPWM signal.
The DS3994 can generate its own DPWM signal internally
(set DPSS = 0 in CR1), which can then be sourced to
other DS3994s if required, or the DPWM signal can be
supplied from an external source (set DPSS = 1 in CR1).
3
4
1
2
3
DIMMING CLOCK (DPWM)
FREQUENCY
4
VARIABLE
MOSFET
GATE DUTY
CYCLE