AME9001 Datasheet, PDF (15/27 Pages) Analog Microelectronics – CCFL BACKLIGHT CONTROLLER

AME, Inc.
AME9001
RT2, RDELTA pin
The frequency of the drive signal at the gate of Q2 is
determined by the VCO shown in Figure1. A detail of the
VCO is shown in Figure 12. The user sets the minimum
oscillator frequency with the resistor connected to pin
RT2 (R2 in the figures). The relation is:
Frequency (Hz) = 2.8E9 / R2 (ohms)
You can see from the formula that as R2 is increased
the frequency gets smaller.
All other things being equal, as the battery voltage in-
creases the duty cycle of the driving waveform at the
gate of Q2 decreases. Often the waveform becomes
less sinusoidal as the duty cycle decreases. To avoid
this unwanted effect and to ensure that the FETs remain
in a "soft switching mode" the application described here
adjusts the oscillator frequency upwards as the battery
voltage increases. An increase in driving frequency is
desirable to minimize harmonic distortion of the output
waveform as the duty cycle of the drive signal at the gate
of Q2 decreases. Resistor R3 controls how much the
oscillator frequency increases as a function of battery
voltage. The relationship is:
Delta frequency (Hz) = 3.44E8 * (Vbatt -1.25) / R3
You can see from the formula that the frequency will
increase as the battery voltage increases. The amount
of this increase is set by R3. In the current application
EA2 (Figure 1) is connected in unity gain which will pro-
vide a constant 1.25V at the CSCOMP pin and subse-
quently at the Vco_control and Rdelta input of the VCO.
Even though Vco_control is a fixed voltage the frequency
of the VCO still modulates because the current through
R3 changes as the battery voltage increases and hence
increases the charging current into the timing capacitor
of Figure 12 thereby increasing the oscillator frequency.
Supply voltage pins, VDD and PNP
Most of the circuitry of the AME9001 works at 5V with
the exception of one output driver. That driver (OUTA)
and its power pad (VBATT) must operate up to 24V al-
though the OUTA pad may never be forced lower than 8
volts away from the VBATT pin. The OUTA pin is inter-
nally clamped to approximately 7.5 volts below the Vbatt
pin.
The AME9001 uses an external PNP device to provide
a regulated 5V supply from the battery voltage (See Fig-
ure 13). The battery voltage can range from 7V< VBATT
< 24V. The PNP pin drives the base of the external PNP
device, Q1. The VDD pin is the 5V supply into the chip.
CCFL Backlight Controller
A 4.7uF capacitor, C7, bypasses the 5V supply to ground.
If an external 5V supply is available then the external
PNP would not be necessary and the PNP pin should
float.
When the CE pin is low (<0.4V) the chip goes into a
zero current state. The chip puts the PNP pin into a high
impedance state which shuts off Q1 and lets the 5V sup-
ply collapse to zero volts. When low, the CE pin also
immediately turns PMOS transistor Q2 off, however tran-
sistors Q3-1 and Q3-2 will continue to switch until the 5V
has collapsed to 3.5V. Allowing the Q3 transistors to
continue to switch for some time after Q2 is turned off
permits the energy in the tank circuit to be dissipated
gradually without any large voltage spikes.
The VDD voltage is sensed internally so that the switch-
ing circuitry will not turn on unless the VDD voltage is
larger than 4.5V and the internal reference is valid. Once
the 4.5V threshold has been reached the switching cir-
cuitry will run until VDD is less than 3.5V (as mentioned
before).
Output drivers (OUTA, OUTAPB, OUTC)
The OUTAPB and OUTC pins are standard 5V CMOS
driver outputs (with some added circuitry to prevent shoot
through current). The OUTA driver is quite different (See
Figure 14). The OUTA driver pulls up to VBATT (max
24V) and pulls down to about 7.5 volts below VBATT. It
is internally clamped to within 7.5V of VBATT. On each
transition the OUTA pad will sink/source about 500mA
for 100nS. After the initial 100ns burst of current the
current is scaled back to 1mA(sinking) and
12mA(sourcing). This technique allows for fast edge tran-
sitions yet low overall power dissipation.
Fault Protection, the OVP and CSDET pins
The AME9001 checks for 3 different fault conditions.
When any one of the fault conditions is met then the
circuit is latched off. Only a power on reset or toggling
the CE pin will restore the circuit to normal operation.
(See Figure 15 for a schematic of the FAULT circuitry.)
The first fault condition check can be used to detect
overvoltages at the CCFL. Specifically, if the OVP pin is
above 3V then this fault condition is detected. The first
fault condition is always enabled, thus there is no blank-
ing period, or 4 succesive faults required for shutdown.
The second fault condition check can be used to en-
sure that the CCFL voltage is above some certain voltage
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