AME9002 Datasheet, PDF (27/37 Pages) Asahi Kasei Microsystems – CCFL Backlight Controller

AME, Inc.
AME9002
Preliminary
CCFL Backlight Controller
Application Component Description
Figure 18 shows one typical application circuit for driv-
ing 4 tubes. Similar component designations are used
on similar components both in figure 2 and Figure 18 as
well as throughout this application note.
R1 - Weak pull up for the chip enable (CE) pin. The
voltage at CE will normally rise to 5 volts for a 12V
supply. Pull down on the CE node to disable the chip
and put it into a zero Idd mode. If the user wishes to
drive node CE with 3.3 or 5.5 volt logic then R1 is not
necessary
C1 - This capacitor acts to de-bounce the CE pin and
to slow the turn on time when using R1 to pull up CE.
This can be useful when the battery power is discon-
nected from the circuit in order to turn the circuit off,
when the battery is reconnected the chip does not
immediately turn on which allows the battery voltage
to stabilize before switching starts. If the user is ac-
tively driving the CE pin then the C1 capacitor may not
be necessary.
R3 - This resistor connected to the RDELTA pin deter-
mines how much the oscillator frequency will change
with battery voltage. The relation, which is found ear-
lier in the text, is:
Delta frequency (Hz) = 3.44e8 * (5 - V(FCOMP)) / R3
C2 - This 1uF capacitor bypasses and stabilizes the
internal reference
C3 - This capacitor determines the length of the blank-
ing interval at the beginning of every dimming cycle.
At the end of every dimming cycle this capacitor is
discharged to VSS then allowed to charge up at a rate
controlled by its internal current source and C3. When
the voltage on C3 (pin SSC) crosses 3 volts the blank-
ing interval is over and all fault checks are enabled.
The charging current into C3 (out of pin SSC) is nor-
mally 140uA but for the very first cycle after the chip is
enabled the current is only 140nA, this determines
the duration of the intial start up period (nominally 1
second) and is given by the relation:
T(seconds) =( C3) * (3volts) / (140e-9amps)
And for subsequent dimming cycles the blanking in-
terval is:
T(seconds) = (C3) * (3volts) / (140e-6amps)
R2 - R2 sets the frequency of the oscillator that drives
the FETs. The relation between R2 and frequency,
that was found previously in the text, is:
Frequency (Hz) = 2.8e9/R2
R2 = 56K yields approximately 50khz
Note: that this is the frequency of the NMOS(Q3) gate drive.
The PMOS(Q2) gate drive is exactly twice this value.
R4 - This resistors pulls the base of Q1 up to Vbatt.
Coupled with Q1 and C7 it is part of the 5V regulator
that supplies the working power to the AME9002. When
the PNP pin is turned off the base of Q1 is pulled high
through R4, turning off Q1 and allowing the voltage at
the VDD node (VSUPPLY) to decay towards zero.
Q1 - This common PNP transistor (2n3906 is adequate)
forms part of the 5V linear regulator which supplies
power to most of the AME9002.
R6 - This resistor, together with adjustable resistor
R20, form a resistor divider that divides the regulated
5V down to some lower voltage. That lower voltage is
used to drive the BRIGHT pin which, in turn, deter-
mines the duty cycle of the the dimming cycles and
therefore the brightness of the lamps. If the user is
driving the BRIGHT pin with his/her own voltage source
then R6 and R20 are not necessary.
C6 - This capacitor bypasses the BRIGHT pin. A noisy
BRIGHT pin can cause unwanted flicker.
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. Limiting
COMP’s increase will limit the increase of the switch-
ing duty cycle thereby creating a “soft start” effect.
The charging 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:
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