DS3994 Maxim Integrated Products, DS3994 Datasheet - Page 16

• Manufacturer Part Number: DS3994
• Description: 4-Channel Cold Cathode Fluorescent Lamp Controller
• Manufacturer: Maxim Integrated Products

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DataSheet
4
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) in the
Detailed Register Descriptions section.
Example: Selecting the resistor values to configure a
DS3994 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
4kΩ•kHz. For the lamp frequency resistor (R
culation, K = 1600kΩ•kHz, which allows the lamp fre-
quency K value regardless of the frequency. The
formula above can now be used to calculate the resis-
tor values for R
The DS3994 monitors both the transformer’s DC supply
and its own V
els are adequate for proper operation.
The inverter’s transformer supply (V
using an external resistor-divider that is the input into a
comparator (see Figure 8) with a 2V threshold. Using
the equation below to determine the resistor values, the
supply voltage monitor (SVM) trip point (V
customized to shut off the inverter when the trans-
former’s input voltage drops below any specified value.
Operating with the transformer’s supply at too low of a
level can prevent the inverter from reaching the strike
voltage and could potentially cause numerous other
4-Channel Cold-Cathode
Fluorescent Lamp Controller
16
Setting the Lamp and Dimming Clock (DPWM)
U
.com
____________________________________________________________________
Frequencies Using External Resistors
R
R
CC
LOSC
POSC
LOSC
supply to ensure that both voltage lev-
=
R
and R
=
OSC
4
0 160
1600
k
.
Ω
POSC
=
50
•
k
kHz
kHz
Ω
kHz
f
OSC
K
•
Supply Monitoring
as follows:
kHz
=
25 0
POSC
.
=
INV
k
32
Ω
) calculation to
) is monitored
k
Ω
TRIP
,
LOSC
) can be
) cal-
problems. Proper use of the SVM can prevent these
problems. If desired, the SVM can be disabled by con-
necting the SVM pin to V
The V
lockout (UVLO) that prevents operation when the DS3994
does not have adequate voltage for its analog circuitry to
operate or to drive the external MOSFETs. The V
tor features hysteresis to prevent V
spurious operation when V
monitor cannot be disabled by any means.
The DS3994 provides extensive fault monitoring for
each channel. It can detect open-lamp, lamp overcur-
rent, failure to strike, and overvoltage conditions. The
DS3994 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) remain
disabled until either the DS3994 is power-cycled or the
inverter’s DC supply is power-cycled. The DS3994 can
also be configured to automatically attempt to clear a
detected fault (except lamp overcurrent) by restriking the
lamp, as explained in Step 4. Configuration bits for the
fault monitoring options are located in the control registers.
Figure 8. Setting the SVM Threshold Voltage
CC
EXAMPLE: R
R
R
1
2
monitor is used as a 5V supply undervoltage
V
INV
1
V
= 10kΩ, R
TRIP
SVM
2
=
= 40kΩ SETS AN SVM TRIP POINT OF 10V.
2 0
CC
.
CC
2.0V
.



R
is near the trip point. This
1
R
+
DS3994
1
CC
R
Fault Monitoring
2



noise from causing
CC
moni-