AME9002 Datasheet, PDF (15/37 Pages) Asahi Kasei Microsystems – CCFL Backlight Controller

AME, Inc.
AME9002
Preliminary
CCFL Backlight Controller
Driving the CCFL
Unlike modified Royer schemes for driving CCFLs the
secondary winding of the AME9002 method is not de-
signed to look like a voltage source to the CCFL lamp.
The circuit acts more like a current source (or a power
source). The voltage at the transformer secondary is pri-
marily determined by the operating point of the CCFL.
The circuit will increase the duty cycle of Q2 thereby
dumping more and more energy across to the secondary
tank circuit until the CCFL tube current achieves regula-
tion or one of the various fault conditions is met.
There are two major modes of operation of the AME9002.
The start up mode consists of the time from intial power
up until the tube strikes or 1 second elapses. The steady
state mode consists of operation that occurs after the
start up mode finishes.
The start up mode is useful for coaxing old or cold
tubes into striking. It is believed that as a tube ages it
becomes more and more difficult to strike an arc through
the gas. Cold temperatures make this problem even
worse. The AME9002 will allow higher than normal oper-
ating voltages across the CCFL for a period of up to one
second in order to facilitate strking. This feature should
extend the usable life of the CCFL as well as simplifying
start up for “problem” applications.
Start Up Mode
When the circuit is first powered up or the CE pin tran-
sitions from a low to a high state a special mode of op-
eration, known as the “start up mode”, is initiated that
will last for a maximum of one second. The exact dura-
tion of the start up period is determined by capacitor C3
on the SSC pin. Figure 10 shows a flow chart of the CCFL
ignition sequence described here. The start up mode
will end when one of two conditions is met:
a) The CCFL strikes and the current sense voltage at
the CSDET pin rises above 1.25V.
b) The one second time period ends without the tube
being struck, in this case the circuit will shut down.
On the first cycle after power on (or a low to high tran-
sition on CE) C3 is initially discharged and the voltage on
SSC is zero. It is charged up by a 140nA current source.
When the voltage on C3 reaches 3 volts the start up mode
has ended. A value of 0.39uF for C3 nominally yields a
one second start up period. If the one second time period
ends before the CCFL strikes then the circuit is shut-
down until the user toggles the power supply or CE tran-
sitions from low to high again. In other words, if the CCFL
successfully starts up then the start up time period will
end before the one second time period is up.
The SSC pin and C3 are also used to set the blanking
period during steady state operation. This operation is
described more completely below.
At the beginning of the start up period capacitor C32,
connected to FCOMP, is also discharged and the voltage
at FCOMP is zero. The voltage at FCOMP controls the
frequency at which the FETs are driven. When FCOMP
is zero the frequency is at its maximum value. When
FCOMP reaches 5V then the switching frequency is at
its minimum value. The exact relation between the volt-
age at FCOMP and oscillator frequency is described more
fully in the detailed description of the oscillator circuitry.
At the beginning of start up mode FCOMP is zero volts
so the switching frequency is at its maximum value. It is
intended that this maximum frequency is significantly
above the resonant frequency of the tank circuit made up
of the transformer and CCFL load. In this way the voltage
at the CCFL is lower than would be expected if the circuit
was driven nearer to its resonant frequency. At this point
in the operation of the circuit we assume that the CCFL
has not struck and therefore appears as an open circuit to
the transformer. After the tube has struck the voltage at
the transformer output is controlled by the IV relationship
of the CCFL. Without the variable frequency drive avail-
able with the AME9002 the user is unable to control the
voltage across the CCFL before the CCFL strikes and
current starts flowing in the CCFL.
Capacitor C32 is charged by a 1uA current source with
the following conditions:
a) If OVPL < 2.5V the charging current is 1uA and
the voltage at FCOMP ramps positive.
b) If OVPL > 2.5V and OVPH < 3.3V then the charg-
ing current is zero and the voltage at FCOMP re-
mains the same.
c) If OVPH > 3.3V then FCOMP is discharged to
approximately 1V, SSV is also driven to VSS.
These conditions allow the voltage across the CCFL to
be controlled during the start up period. The two thresh-
olds available at OVPL and OVPH allow the user to tailor
the start behavior for particular tubes.
In Figure 11, initially SSV=SSC=FCOMP= zero
volts. The switching duty cycle is zero, the switching
frequency is maximum and the one second time period
ramp has just started. The SSV ramps positive which
15