DS3988T+T&R Maxim Integrated Products, DS3988T+T&R Datasheet - Page 21

• 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

Each DS3988 responds to one of two possible slave
addresses based on the state of the address input (A
For information about device addressing see the I
Communications section.
To achieve best results, it is recommended that each
V
to GND. Use high-quality, ceramic, surface-mount capac-
itors, and mount the capacitors as close as possible to
the V
Resistor R8 in the typical operating circuit (Figure 12)
sets the lamp current. R8 = 140Ω corresponds to a
5mA
is approximately sinusoidal. The formula to determine
the resistor value for a given sinusoidal lamp current is:
Figure 11. I
CC
COMMUNICATIONS KEY
READ A SINGLE BYTE
WRITE A SINGLE BYTE
RMS
pin is decoupled with a 0.01µF or a 0.1µF capacitor
CC
SR
S
S
P
S
and GND pins to minimize trace inductance.
1 0 1
1 0 1
START
STOP
REPEATED
START
lamp current as long as the current waveform
2
C Communications Examples
Addressing Multiple DS3988s On a
Applications Information
R
0
0
Setting the RMS Lamp Current
8
A
N
0
0
=
0
X X X X X X X X
0
NOT
ACK
ACK
Power-Supply Decoupling
A
A
0
0
2
0
0
×
A
I
A
LAMP RMS
1
SHADED BOXES INDICATE THE SLAVE IS
CONTROLLING SDA
WHITE BOXES INDICATE THE MASTER IS
CONTROLLING SDA
Common I
(
MEMORY ADDRESS
MEMORY ADDRESS
____________________________________________________________________
)
8-BITS ADDRESS OR DATA
Fluorescent Lamp Controller
2
C Bus
A
A
2
0
C
).
SR
8-Channel Cold-Cathode
1 0 1
External component selection has a large impact on the
overall system performance and cost. The two most
important external components are the transformers
and n-channel MOSFETs.
The transformer should be able to operate in the 40kHz
to 80kHz frequency range of the DS3988, and the turns
ratio should be selected so the MOSFET drivers run at
28% to 35% duty cycle during steady state operation.
The transformer must be able to withstand the high
open-circuit voltage that will be used to strike the lamp.
Additionally, its primary/secondary resistance and
inductance characteristics must be considered
because they contribute significantly to determining the
efficiency and transient response of the system. Table 5
shows a transformer specification that has been utilized
for a 12V inverter supply, 438mm x 2.2mm lamp design.
The n-channel MOSFET must have a threshold voltage
that is low enough to work with logic-level signals, a low
on-resistance to maximize efficiency and limit the n-
channel MOSFET’s power dissipation, and a break-
down voltage high enough to handle the transient. The
breakdown voltage should be a minimum of 3x the
inverter voltage supply. Additionally, the total gate
charge must be less than Q
Recommended DC Operating Conditions table. These
specifications are easily met by many of the dual n-
channel MOSFETs now available in SO-8 packages.
Table 6 lists suggested values for the external resistors
and capacitors used in the typical operating circuit.
DATA
0
0
0
A
NOTES
0
1) ALL BYTES ARE SENT MOST SIGNIFICANT BIT FIRST.
2) THE FIRST BYTE SENT AFTER A START CONDITION IS
A
ALWAYS THE SLAVE ADDRESS FOLLOWED BY THE
READ/WRITE BIT.
0
P
A
Component Selection
G
, which is specified in the
DATA
N
P
21