參數(shù)資料
型號(hào)： DS3994
廠商： Maxim Integrated Products, Inc.
英文描述： 4-Channel Cold-Cathode Fluorescent Lamp Controller
中文描述： 4通道冷陰極熒光燈控制器
文件頁(yè)數(shù)： 16/28頁(yè)
文件大小： 557K
代理商： DS3994
D
4-Channel Cold-Cathode
Fluorescent Lamp Controller
16
____________________________________________________________________
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 = 1600k
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
POSC
) calculation to
4k
kHz. For the lamp frequency resistor (R
LOSC
) cal-
culation, K = 1600k
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
LOSC
and R
POSC
as follows:
Supply Monitoring
The DS3994 monitors both the transformer
’
s DC supply
and its own V
CC
supply to ensure that both voltage lev-
els are adequate for proper operation.
The inverter
’
s transformer supply (V
INV
) is monitored
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
TRIP
) can be
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
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
CC
.
The V
CC
monitor is used as a 5V supply undervoltage
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
CC
moni-
tor 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 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.
V
R
R
R
TRIP
.
2 0
=
+
1
2
1
R
k
kHz
kHz
kHz
kHz
k
R
k
.
k
LOSC
POSC
,
.
=
=
=
=
1600
50
32
4
0 160
25 0
R
K
f
OSC
OSC
=
V
INV
2.0V
SVM
EXAMPLE: R
1
= 10k
, R
2
= 40k
SETS AN SVM TRIP POINT OF 10V.
R
2
R
1
DS3994
Figure 8. Setting the SVM Threshold Voltage
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