DS3988T+T&R Maxim Integrated Products, DS3988T+T&R Datasheet - Page 10

• Manufacturer Part Number: DS3988T+T&R
• Description: Display Drivers 8-Ch CCFL Controller All N-FET Drive, La
• Manufacturer: Maxim Integrated Products

Specifications of DS3988T+T&R

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

The lamp-frequency MOSFET gate turn-on times are
equally phased among the eight 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 sup-
ply. Figure 2 details how the eight channels are
phased. Note that it is the lamp-frequency signals that
are phased, NOT the DPWM signals.
The DS3988 uses a digital pulse-width modulated
(DPWM) signal (22.5Hz to 440Hz) to provide efficient
and precise lamp dimming. During the high period of
the DPWM cycle, the lamps are driven at the selected
8-Channel Cold-Cathode
Fluorescent Lamp Controller
Figure 2. Channel Phasing Detail
10
____________________________________________________________________
Lamp Dimming Control (DPWM)
SEQUENCE
LAMP FREQUENCY
DRIVE SIGNALS AT
CHANNEL
MOSFET GATE-
GB3
GA4
GA1
GB1
GA2
GB2
GA3
GB4
GA5
GB5
GA6
GB6
GA7
GB7
GA8
GB8
8
1
2
Channel Phasing
3
4
5
6
7
8
1
2
3
4
lamp frequency (40kHz to 80kHz) as shown in Figure 6.
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 DS3988 can generate its own DPWM signal internally
(set DPSS = 0 in CR1), which can then be sourced to
other DS3988s if required, or the DPWM signal can be
supplied from an external source (set DPSS = 1 in CR1).
5
6
7
8
1
2
3
4
5
DIMMING CLOCK (DPWM)
6
FREQUENCY
7
8
VARIABLE
MOSFET
GATE DUTY
CYCLE