AME9001 Datasheet, PDF (22/27 Pages) Analog Microelectronics – CCFL BACKLIGHT CONTROLLER

AME, Inc.
AME9001
transformer operating voltages. ( This operation is ex-
plained in more detail below.)
R38,R39,D17 - See R35,R36,D16 description above.
You can diode "OR" as many of these divider/rectifier
circuits as you have different transformers. Each time
you add another double output transformer you must
add another set of these resistors and diode networks.
( This operation is explained in more detail in the next
section.)
Multiple Tube Operation
The AME9001 is particularly well suited for multiple
tube applications. Figure17 shows the power section of
a two tube application. The major difference between
this application and the single tube application is the ad-
dition of another secondary winding on the transformer.
The primary side of the transformer and its associated
FETs are exactly the same as the single tube case al-
though the FETs may need to be resized due to the in-
creased current in two tube applications.
The secondaries are wound so that the outputs to the
CCFL are of opposite phase (see Figure 18). When the
voltage at one secondary output is high (+600 volts) the
other secondary output should be low (-600 volts). The
other secondary terminals are connected to each other.
In a balanced circuit the voltage at the connection of the
two secondaries will, ideally, be zero. Of course in a real
application the voltage at the connection of the two sec-
ondaries will deviate somewhat from zero.
The common connection of the secondaries offers us
an excellent method to check for high voltage fault condi-
tions. As previously mentioned, when the CCFL loads
are balanced then the voltage at the common connection
of the secondaries will remain relatively low compared to
the high voltages available at the other terminals of the
secondaries. However, when the loads become unbal-
anced, as would be the case if a tube was broken or
there was a bad connection, then the voltage at the com-
mon point of the secondaries will be much higher than its
normal value. It is simple to size resistors R35 and R36
in Figure17 such that in normal operation the OVP volt-
age remains below 3 volts while during abnormal opera-
tion the OVP voltage goes above 3 volts causing a sys-
tem fault.
CCFL Backlight Controller
The multi-tube configuration is modular. Since each
double transformer can drive two CCFLs it is possible to
construct 2, 4, 6..... tube solutions using the basic archi-
tecture. Of course the FETs must be properly sized to
handle the increased current. Figure19 shows a 4 tube
application. In this configuration the common secondary
connection (the node NOT connected to the lamp) is
made with the opposite transformer. In this way the sec-
ondary current from the winding on the first transformer
should be equal to the secondary current of its compan-
ion winding on the second transformer. In the case of 4
lamps driven by two transformers there are two sets of
common secondary nodes. Each set drives a resistor
divider ( in this case R35/R36 and R38/R39) whose out-
puts are diode "OR'd" together at the OVP node. That
way either transformer that experiences an overvoltage
fault condition will be able to pull up the OVP node and
cause the system to shut down.
The concept can be expanded for more than four tubes.
For every 2 extra tubes that need to be added the user
must add one more transformer, a resistor divider, and a
small diode such as a 1N914.
Figure 16 shows a complete multi-tube architecture
schematic. Analogous components have been given the
same numbers as in the single tube schematic. There is
really very little difference between the the single tube
configuration and the multi-tube version. Transistors Q4
and Q5 are added to buffer the high side drive OUTA.
This may be necessary because the PMOS devices for
larger current applications have larger gate drive require-
ments. Capacitor C30 is added to the OVP node so that
unwanted high frequency signals do not couple to the
OVP node and cause an undesired shutdown.
The MOS transistors are sized bigger for the 4 tube
application as would be expected. The peak currents
are much higher so the Vbatt bypassing capacitor must
be increased as well. The schematic shows C5 as a
100uF capacitor but higher values such as 220uF are not
uncommon in order to minimize ripple on Vbatt.
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