DS3988T+T&R Maxim Integrated Products, DS3988T+T&R Datasheet - Page 14

• 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

8-Channel Cold-Cathode
Fluorescent Lamp 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 =
tions. When calculating the resistor value for the dim-
ming 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 4 in the Detailed Register
Descriptions section.
Example: Selecting the resistor values to configure a
DS3988 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 DS3988 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 7) 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
14
Setting the Lamp and Dimming Clock (DPWM)
____________________________________________________________________
Frequencies Using External Resistors
R
R
1600kΩ•kHz for lamp frequency calcula-
LOSC
POSC
CC
LOSC
supply to ensure that both voltage lev-
=
=
R
and R
4
OSC
0 160
1600
k
.
Ω
50
•
POSC
=
k
kHz
kHz
kHz
Ω
f
•
OSC
K
kHz
=
Supply Monitoring
as follows:
25 0
POSC
.
=
INV
k
32
Ω
) calculation to
k
) is monitored
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 DS3988
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 DS3988 provides extensive fault monitoring for
each channel. It can detect open-lamp, lamp overcur-
rent, failure to strike, and overvoltage conditions. The
DS3988 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 DS3988 is power-cycled or the
inverter’s DC supply is power-cycled. The DS3988 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 CR1 and CR2.
Figure 7. Setting the SVM Threshold Voltage
CC
EXAMPLE: R
R
R
1
2
monitor is used as a 5V supply undervoltage
V
INV
V
TRIP
1
= 10kΩ, R
=
SVM
2
2 0
= 40kΩ SETS AN SVM TRIP POINT OF 10V
.
CC



CC
.
2.0V
R
1
is near the trip point. This
R
+
1
R
DS3988
CC
2
Fault Monitoring



noise from causing
CC
moni-