AME9001 Datasheet, PDF (21/27 Pages) Analog Microelectronics – CCFL BACKLIGHT CONTROLLER

AME, Inc.
AME9001
R20 - see description of R6
C14 - This capacitor sets the slope of the soft-start
ramp on pin SSV. The voltage at SSV limits the duty
cycle of the Q2 gate drive signal available at pin OUTA.
The voltage at the COMP node is internally clamped
to the SSV node. Therefore the C14 cap limits how
fast SSV, and hence, COMP can increase. The charg-
ing current out of SSV is approximately 20uA so the
rate of change of the SSV voltage is:
SSV(Volts/sec) = (20e-6amps) / C14
C5 - This is the main battery bypass capacitor.
C4 - This capacitor sets the frequency of the dimming
cycles according to the relation:
Dim Cycle Freq(Hz) = 1 / [(17) * (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 AME9001
as possible.
C8 - This capacitor, in combination with resistor R7,
determines the time constant for the error amplifier
(integrator) EA1. The integrator is the primary loop
stabilizing element of the circuit. In general this appli-
cation is tolerant of a large range of integrator time
constants. 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
CCFL Backlight Controller
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.)
R9, R10 - The sum of R9 and R10 sets the current in
one CCFL tube. As the sum of R9 and R10 decreases
the tube current goes up, as the sum of R9 and R10
increase the tube current goes down. The RMS tube
current is roughly:
Irms = 6V / (R9 + R10)
R9 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 250mV once every clock cycle
then the AME9001 assumes there is current in the
CCFL and allows operation to continue.
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
AME9001. Q4 is an NPN transistor and Q5 is a PNP
transistor.
R35,R36,D16 - These devices form a voltage divider
and rectifier combination to sense higher than normal
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