ds3881et-r-c Maxim Integrated Products, Inc., ds3881et-r-c Datasheet - Page 14

• Manufacturer Part Number: ds3881et-r-c
• Description: Ds3881 Single-channel, Automotive Ccfl Controller
• Manufacturer: Maxim Integrated Products, Inc.

Single-Channel Automotive CCFL Controller
Both the lamp and dimming clock frequencies can be
set using external resistors. The resistance required for
either frequency can be determined using the following
formula:
where K = 1600kΩ • kHz for lamp frequency calculations.
When calculating the resistor value for the dimming clock
frequency, K will be one of four values as determined by
the desired frequency and the POSCR0 and POSCR1 bit
settings as shown in the Control Register 2 (CR2) Table 6
in the Detailed Register Descriptions section.
Example: Selecting the resistor values to configure a
DS3881 to have a 50kHz lamp frequency and a 160Hz
dimming clock frequency: For this configuration,
POSCR0 and POSCR1 must be programmed to 1 and
0, respectively, to select 90Hz to 220Hz as the dimming
clock frequency range. This sets K for the dimming
clock resistor (R
lamp frequency resistor (R
1600kΩ • kHz, which sets the lamp frequency K value
regardless of the frequency. The formula above can
now be used to calculate the resistor values for R
and R
The DS3881 has supply voltage monitors (SVMs) for
both the inverter’s transformer DC supply (V
own V
adequate for proper operation. The transformer supply
is monitored for overvoltage conditions at the SVMH pin
and undervoltage conditions at the SVML pin. External
resistor-dividers at each SVM input feed into two com-
parators (see Figure 7), both having 2V thresholds.
Using the equation below to determine the resistor val-
ues, the SVMH and SVML trip points (V
customized to shut off the inverter when the trans-
former’s supply voltage rises above or drops below
specified values. Operating with the transformer’s sup-
ply at too low of a level can prevent the inverter from
14
____________________________________________________________________
POSC
CC
supply to ensure that both voltage levels are
R
LOSC
as follows:
R
POSC
Setting the Lamp and Dimming
POSC
=
R
OSC
=
Clock (DPWM) Frequencies
1600
) calculation to 4kΩ • kHz. For the
4
Using External Resistors
0 160
k
.
50
Ω
=
k
Ω
kHz
•
f
OSC
kHz
•
K
kHz
LOSC
kHz
Supply Monitoring
=
) calculation, K =
=
25 0
32 0
.
.
k
TRIP
Ω
k
INV
Ω
) can be
) and its
LOSC
reaching the strike voltage and could potentially cause
numerous other problems. Operating with the trans-
former voltage at too high of a level can be damaging
to the inverter components. Proper use of the SVMs
can prevent these problems. If desired, the high and/or
low SVMs can be disabled by connecting the SVMH
pin to GND and the SVML pin to V
The V
lockout (UVLO) that prevents operation when the
DS3881 does not have adequate voltage for its analog
circuitry to operate or to drive the external MOSFETs.
The V
noise from causing spurious operation when V
near the trip point. This monitor cannot be disabled by
any means.
The DS3881 provides extensive fault monitoring. It can
detect open-lamp, lamp overcurrent, failure to strike,
and overvoltage conditions. The DS3881 can be con-
figured to disable the output if the channel enters a
fault state. Once a fault state has been entered, the
FAULT output is asserted and the channel remains dis-
abled until it is reset by a user or host control event.
See Step 4, Fault Handling for more detail. The DS3881
can also be configured to automatically attempt to clear
a detected fault (except lamp overcurrent) by re-strik-
ing the lamp. Configuration bits for the fault monitoring
options are located in CR1 and CR2. The DS3881 also
has real-time status indicator bits located in the SR1
and SR2 register (SRAM) that assert whenever a corre-
sponding fault occurs.
Figure 7. Setting the SVM Threshold Voltage
V
TRIP
R
R
2
1
V
CC
INV
CC
monitor is used as a 5V supply undervoltage
monitor features hysteresis to prevent V
SVML
2.0V
V
TRIP
=
DS3881
2 0
.
⎛
⎜
⎝
R
1
R
+
1
CC
Fault Monitoring
R
2
.
⎞
⎟
⎠
SVMH
2.0V
V
INV
V
R
R
CC
TRIP
2
1
CC
is