AME9002 Datasheet, PDF (28/37 Pages) Asahi Kasei Microsystems – CCFL Backlight Controller

AME, Inc.
AME9002
Preliminary
CCFL Backlight Controller
Dim Cycle Freq(Hz) = 1 / [(4) * (R2) * (C4)]
Note that the frequency is also a function of R2. So
the frequency of the main oscillator and the frequency
of the dimming oscillator are not independent.
C7 - This capacitor is the load capacitor for the 5V
linear regulator. As such it also bypasses the 5V sup-
ply and should be laid out as close to the AME9002 as
possible.
C8 - This capacitor, in combination with resistor R7,
determines the time constant for the error amplifier (in-
tegrator) EA1. The integrator is the primary loop sta-
bilizing element of the circuit. In general this applica-
tion is tolerant of a large range of integrator time con-
stants. Increase the (C8 X R7) product to slow down
the loop response.
R7 - see C8
D6 - This diode can catch any negative going spikes
on the drain of Q2. This diode is NOT strictly neces-
sary. This is NOT a freewheeling diode such as in a
buck regulator. Since the primary windings are tightly
coupled to each other the body diodes of Q3-1 and
Q3-2 keep their own drains clamped to VSS as well as
the drain of Q2. The spikes that diode D6 may catch
are of short duration and small energy.
Q2 - This is a PMOS device. By modulating its gate
drive duty cycle the power into the transformer, and
then into the load, can be controlled. The breakdown
of this device must be higher than the highest battery
voltage that the application will use. The peak current
load is roughly twice the average current load.
Q3-1, Q3-2 - These are NMOS devices. They are
driven alternately with 50% duty cycle gate drive. The
frequency of the gate drive is one half of the gate drive
frequency of Q2. The gate drive is from 0 to 5 volts.
The breakdown voltage of these devices must be at
least twice the highest battery voltage. Peak current
is roughly twice the average supply current.
C9,R8,D2,D3 - These devices form a snubber circuit
that can dissipate ringing energy. The snubber circuit
is not strictly necessary. In fact a well designed cir-
cuit should not require these devices. (These elements
were described in more detail earlier.)
R9A, R10 - The sum of R9A and R10 sets the current
28
in one CCFL tube. As the sum of R9A and R10 de-
creases the tube current goes up, as the sum of R9A
and R10 increase the tube current goes down. The
RMS tube current is roughly:
Irms = 6V / (R9A + R10)
R9A and R10 also form a voltage divider that drives the
CSDET pin. The purpose of the voltage divider is to
keep the maximum voltage at CSDET under 5 volts
under all conditions. The CSDET pin checks to see if
there is any current in the CCFL. If the voltage at
CSDET is larger than 1.25V once every clock cycle
then the AME9002 assumes there is current in the
CCFL and allows operation to continue. CSDET is
also used to detect when the CCFL first strikes during
the initial start up period.
D4,D5 - These diodes rectify the current through the
CCFL to provide a positive voltage for regulation by the
error amplifier, EA1.
The following components are only used for multiple
tube operation:
Q4,Q5 - These bipolar devices buffer the gate of Q2.
That allows Q2 to be made much bigger without dissi-
pating more power or increasing the cost of the
AME9002. Q4 is an NPN transistor and Q5 is a PNP
transistor.
R35,R36,D16 etc. - These devices form a voltage di-
vider and rectifier combination to sense higher than
normal CCFL operating voltages. ( This operation is
explained in more detail below.) You can diode "OR"
as many of these divider/rectifier circuits as you have
different CCFLs. Each time you add another double
output transformer you must add another set of these
resistors and diode networks. ( This operation is ex-
plained in more detail in the next section.)
D20, D21, R42, R40 and C34 etc. - These devices are
not strictly necessary for single tube operation. In
single tube operation the junction of R9A and R10 can
be directly fed into the CSDET pin. However for mul-
tiple tube operation these devices are necessary to
allow for any one of the different tubes to be able to
pull CSDET below 1.25V and allow a fault to be de-
tected. Figure 1, a single tube application, has these
devices included in order to facilitate the transition to
multiple tube design as well as working quite well for
the single tube application.