Setting the Lamp and Dimming
Clock (DPWM) Frequencies
Using External Resistors
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 7
in the
Detailed Register Descriptions
section.
Example: Selecting the resistor values to configure a
DS3882 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
POSC
) calculation to 4kΩ • kHz. For the
lamp frequency resistor (R
LOSC
) calculation, K =
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
LOSC
and R
POSC
as follows:
Supply Monitoring
The DS3882 has supply voltage monitors (SVMs) for
both the inverter’s transformer DC supply (V
INV
) and its
own V
CC
supply to ensure that both voltage levels are
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
TRIP
) can be
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
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
CC
.
The V
CC
monitor is used as a 5V supply undervoltage
lockout (UVLO) that prevents operation when the
DS3882 does not have adequate voltage for its analog
circuitry to operate or to drive the external MOSFETs.
The V
CC
monitor features hysteresis to prevent V
CC
noise from causing spurious operation when V
CC
is
near the trip point. This monitor cannot be disabled by
any means.
Fault Monitoring
The DS3882 provides extensive fault monitoring for
each channel. It can detect open-lamp, lamp overcur-
rent, failure to strike, and overvoltage conditions. The
DS3882 can be configured to disable all channels if
one or more channels enter a fault state or it can be
configured to disable only the channel where the fault
occurred. Once a fault state has been entered, the
FAULT output is asserted and the channel(s) remains
disabled until it is reset by a user or host control event.
See
Step 4, Fault Handling
for more detail. The DS3882
can also be configured to automatically attempt to clear
a detected fault (except lamp overcurrent) by re-striking
the lamp. Configuration bits for the fault monitoring
options are located in CR1 and CR2. The DS3882 also
has real-time status indicators bits located in the SR1
and SR2 register (SRAM) that assert whenever a corre-
sponding fault occurs.
.
