AME9002 Datasheet, PDF (23/37 Pages) Asahi Kasei Microsystems – CCFL Backlight Controller

AME, Inc.
AME9002
Preliminary
snubbing circuit. The nominal voltage at the common
node is approximately twice the battery voltage. If either
of the drains of Q3 ring above that voltage then diodes D2
or D3 forward bias and allow the ringing energy to charge
capacitor C9. Resistor R8 bleeds off the extra ringing
energy preventing the voltage at the common node from
increasing substantially higher than twice the battery volt-
age. The extra power dissipation is:
P(dissipated) = Vbatt² / R8
For the example, in Figure 17, the power dissipation of
the snubber circuit with Vbatt=15V is 58mW or approxi-
mately 1% of the total input power. The value of R8 can
be optimized for a particular application in order to mini-
mize dissipated power.
Excessive ringing is usually a sign that the driving fre-
quency is not well matched to the resonant characteris-
tics of the tank circuit. In a well designed application a
snubber circuit will not be necessary.
Layout Considerations
Due to the switching nature of this circuit and the high
voltages that it produces this application can be sensitive
to board parasitics. In fact, one of the advantages, of this
design is that the circuit uses the parasitic elements of
the application as resonant components, thus eliminat-
ing the need for more added components.
Particular care must be taken with the different gounding
loops. The best performance has been obtained by us-
ing a “star” ground technique. The star technique re-
turns all significant ground paths back to the center of
the “star”. Ideally we would place the center of the star
directly on the VSS pin of the AME9002. The bypass
capacitors would, ideally, be connected as close to the
center of the star as possible. The schematic in Figure
18 attemps to show this star ground configuration by bring-
ing all the ground returns back to the same point on the
drawing. Separate ground returns back to the star are
especially important for higher current switching paths.
CCFL Backlight Controller
23