AME9002 Datasheet, PDF (29/37 Pages) Asahi Kasei Microsystems – CCFL Backlight Controller

AME, Inc.
AME9002
Preliminary
CCFL Backlight Controller
Multiple Tube Operation
The AME9002 is particularly well suited for multiple
tube applications. Figure19 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 20) although
this is not strictly necessary. When the voltage at one
secondary output is high (+600 volts) the other second-
ary output should be low (-600 volts). The other second-
ary terminals are connected to each other. In a balanced
circuit the voltage at the connection of the two secondar-
ies will, ideally, be zero. Of course in a real application
the voltage at the connection of the two secondaries will
deviate somewhat from zero.
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. Figure 21 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 secondary
current from the winding on the first transformer should
be equal to the secondary current of its companion wind-
ing on the second transformer. In the case of 4 lamps
driven by two transformers there are two sets of common
secondary nodes.
Sensing the current in the multiple tube case requires
some extra circuitry. Normally the CSDET pin checks
for the existence (or absence) of current in the CCFL. If
current is detected then the initial start mode terminates
and steady state operation begins. During steady state
operation if no current is detected for 8 consecutive clock
cycles then the circuit is shutdown. Since there is only
one CSDET pin yet there are multiple tubes extra cir-
cuitry is required.
Take the two tube case of Figure 19 for example. The
current through the tube on the right hand side is regu-
lated by the integrator made of R7, C8 and EA1. How-
ever, for purposes of fault detection and strike detection it
is beneficial to monitor the current through both tubes.
In this case R9B senses the current in the left tube in the
same way R9A senses the current in the right hand tube.
If the current through either tube is zero then R9A or R9B
will try to pull node A or B to zero. Resistors R42 and
R43 attempt to pull node A and B up but the value of R42
and R43 (nominally 10K) is much larger than the values
of resistors R9A and R9B (nominally 221ohms) allowing
node A and B to pull close to VSS when there is zero
current in their respective CCFL tubes. The absence of
current in either tube essentially pulls node A or B to
VSS.
In normal operation the voltage at nodes A and B should
look like alternating, positive half sinusoids. (See figure
22.) If, however, there is no current flowing in one of the
tubes then one half of the sinusoids would be missing
and the voltage at CSDET would drop compared to its
normal value. The values of the RC network made up of
R4 and C34 are chosen so that the voltage at CSDET is
always larger than 1.25 volts when both half sinusoids
are present but is less than 1.25V when only one sinu-
soid is present. The concept can be applied to any even
multiple of tubes. The tube without the current will domi-
nate the voltage at CSDET so a failure in any single tube
will cause the circuit to shutdown. In a similar manner,
during start up all tubes must have current flowing in
them before CSDET will rise above 1.25V and signal that
the tubes have struck and that the initial start up mode is
over.
For every 2 extra tubes that need to be added the
user must add one more transformer, and two resistor
divider networks plus two diodes (R35, R36, R37, R38,
D16, D17) to sense the CCFL voltage as well as two
more diodes and two more resistors to sense the tube
current (R9A, R9B, D20, D22). Resistors R42, R43, R40,
diodes D21, D23 and capacitor C34 do not need to be
replicated every time more CCFLs are added because
they are shared in common on the CSDET node.
Figure 18 shows a complete four tube schematic. Fig-
ure 21 shows a detail of the current and voltage sensing
circuitry for the four tube application. Analogous compo-
nents have been given the same numbers as in the single
tube schematic. There is really very little difference be-
tween 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 be-
cause the PMOS devices for larger current applications
have larger gate drive requirements.
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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