Datasheet

DS3881
Single-Channel Automotive CCFL Controller
26 ____________________________________________________________________
Applications Information
Addressing Multiple DS3881s
On a Common I
2
C Bus
Each DS3881 responds to one of four possible slave
addresses based on the state of the address input pins
(A0 and A1). For information about device addressing,
see the I
2
C Communication section.
Setting the RMS Lamp Current
Resistor R7 and R8 in the Typical Operating Circuit set
the lamp current. R7 and R8 = 140Ω corresponds to a
5mA
RMS
lamp current as long as the current waveform
is approximately sinusoidal. The formula to determine
the resistor value for a given sinusoidal lamp current is:
Component Selection
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 DS3881, 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 is used to strike the lamp.
Additionally, its primary/secondary resistance and induc-
tance characteristics must be considered because they
contribute significantly to determining the efficiency and
transient response of the system. Table 9 shows a trans-
former specification that has been used for a 12V invert-
er 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
G
, which is specified in
the Recommended Operating Conditions table. These
specifications are easily met by many of the dual n-
channel MOSFETs now available in 8-pin SO pack-
ages.
Table 10 lists suggested values for the external resistors
and capacitors used in the Typical Operating Circuit .
Power-Supply Decoupling
To achieve best results, it is highly recommended that
a decoupling capacitor is used on the IC power-supply
pin. Typical values of decoupling capacitors are 0.01µF
or 0.1µF. Use a high-quality, ceramic, surface-mount
capacitor, and mount it as close as possible to the V
CC
and GND pins of the IC to minimize lead inductance.
R
Ix
LAMP RMS
7
1
2
()
=
XXXXXXXX
101 0 A
0
0A
1
0
COMMUNICATIONS KEY
WRITE A SINGLE BYTE
8-BITS ADDRESS OR DATA
WHITE BOXES INDICATE THE MASTER IS
CONTROLLING SDA
NOTES
2) THE FIRST BYTE SENT AFTER A START CONDITION IS
ALWAYS THE SLAVE ADDRESS FOLLOWED BY THE
READ/WRITE BIT.
SHADED BOXES INDICATE THE SLAVE IS
CONTROLLING SDA
START ACK
NOT
ACK
S
SA
A
A
P
DATA
MEMORY ADDRESS
101 0 A
0
0A
1
0
101 0 A
0
1A
1
0
READ A SINGLE BYTE
SA
ASR
AN
P
DATA
MEMORY ADDRESS
A
PN
SR
STOP
REPEATED
START
1) ALL BYTES ARE SENT MOST SIGNIFICANT BIT FIRST.
Figure 11. I
2
C Communications Examples