AKM AME9002AETH Ccfl backlight controller Datasheet

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n General Description
CCFL Backlight Controller
n Pin Configuration
The AME9002 is AME’s next generation direct drive
CCFL controller. Like its cousin, the AME9001, the
AME9002 controller provides a cost efficient means to
drive single or multiple cold cathode fluorescent lamps
(CCFL), driving 3 external MOSFETs that, in turn, drive a
wirewound transformer that is coupled to the CCFL.
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AME9002
However the AME9002 includes extra circuitry that allows for a special one second start up period wherein the
voltage across the CCFL is held at a higher than normal
voltage to allow older tubes (or cold tubes) a period in
which they can “warm up”. During this one second
startup period the driving frequency is adjusted off of resonance so that the tube voltage can be controlled. As
soon as the CCFL “strikes” the special start up period
ends and the circuit operates in its normal mode.
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2
3
4
5
6
7
8
AME9002
1. VREF
2. CE
3. SSC
4. RDELTA
5. FAULTB
6. RT2
7. VSS
8. OVPH
9. OVPL
10.FCOMP
11.CSDET
12.BATTFB
The AME9002 includes features such as soft start, duty
cycle dimming control, dimming control polarity selection, undervoltage lockout and fault detection. It is designed to work with input voltages from 7V up to 24V.
When disabled the circuit goes into a zero current mode.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
OUTC
OUTAPB
OUTA
VBATT
BRPOL
VDD
CT1
FB
COMP
BRIGHT
SSV
PNP
n System Block Diagram
n Features
l Small package: 24 pins for QSOP/ SOIC/
PDIP packages
l Drives multiple tubes
l Special 1 second start up mode
l Automatically checks for common fault
conditions
l 7.0V < Vbatt < 24V
l Low component count
l Low Idd < 3.5mA
l <1uA shutdown mode
l Battery UV lockout
l Brightness polarity select
Controller
External
Components
CCFL Array
AME
9002
LIGHT
+
Resistors
+
Capacitors
N
n Applications
l Notebook computers
l LCD/TFT displays
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CCFL Backlight Controller
n Pin Description
Pin #
Pin Name
1
VREF
Reference. Compensation point for the 3.4V internal voltage reference. Must have
bypass capacitor connected here to VSS.
2
CE
Chip enable. When low (<0.4V) the chip is put into a low current (~0uA) shutdown
mode.
3
SSC
Blanking interval ramp. During the first cycle this pin sources 140nA. The first cycle
is used to define the initial start up period, often on the order of one second. During
subsequent cycles SSC sources 140mA. This is primarily used to provide a "blanking
interval" at the beginning of every dimming cycle to temporarily disable the fault
protection circuitry. The blanking interval is active when V(SSC) < 3.0 volts. (See
application notes.)
4
RDELTA
A resistor connected from this pin to VDD determines the amount that the voltage at
FCOMP modulates the switching frequency. The frequency is inversely proportional to
the voltage at FCOMP.
5
FAULTB
FAULTB pulls low when a fault is detected.
6
RT2
A resistor from this pin to VSS sets the minimum frequency of the VCO. The voltage at
this pin is 1.5V
7
VSS
Negative supply. Connect to system ground.
8
9
10
2
Pin Description
OVPH
Over voltage protection input (HIGH). Indirectly senses the voltage at the secondary of
the transformer through a resistor (or capacitor) divider. During the initial start up
period, if OVPH is > 3.3V, FCOMP is driven towards VSS (increasing the frequency)
and SSV is reset to zero (which decreases the duty cycle). After the initial start up
period is completed the circuit will shut down if OVPH is > 3.3V.
OVPL
Over voltage protection input (LOW). During the initial start up period if OVPL < 2.5
volts then FCOMP is allowed to ramp up (decreasing the oscillator frequency allowing
the circuit to get closer to resonance). If, during the initial start up period, OVPL > 2.5
volts then FCOMP is held at its original value (not allowed to increase so the
oscillator frequency stays constant). This action is designed to hold the voltage
across the CCFL constant while the CCFL "warms up".
FCOMP
Frequency control point. Initially this pin is at VSS which yields a maximum switching
frequency. Depending on the voltage at OVPL and OVPH the pin FCOMP will normally
ramp upwards lowering the switching frequency towards the circuit’s resonant
frequency.
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CCFL Backlight Controller
n Pin Description
Pin #
Pin Name
Pin Description
11
CSDET
Current sense detect. Connect this pin to the CCFL current sense resistor divider.
During the initial startup period this pin senses that the CCFL has struck when
V(CSDET) > 1.25 volts. If, after the initial start up period, this pin is below 1.25V for 4
consecutive clock cycles after SSC > 3V then the circuit will shutdown.
12
BATTFB
UVLO feedback pin. If this pin is above 1.5V then the OUTA pin is allowed to switch, if
below 1.25V then OUTA is disabled.
13
OUTC
14
OUTAPB
15
OUTA
Drives the high side PFET.
16
VBATT
Battery input. This is the positive supply for the OUTA driver.
17
BRPOL
Brightness polarity control. When this pin is low the CCFL brightness increases as
the voltage at the BRIGHT pin increases. When this pin is high the CCFL brightness
decreases as the voltage at the BRIGHT pin increases.
18
VDD
Regulated 5V supply input.
19
CT1
Sets the dimming cycle frequency. Usually about 100Hz.
20
FB
Negative input of the voltage control loop error amplifier.
21
COMP
22
BRIGHT
23
SSV
Soft start ramp for the voltage control loop. (20uA source current.) The voltage at SSV
clamps the voltage at COMP to be no greater than SSV thereby limiting the increase of
the switching duty cycle.
24
PNP
Drives the base of an external PNP transistor used for the 5V LDO.
Drives one of the external NFETs, opposite phase of OUTAPB.
Drives one of the external NFETs, opposite phase of OUTC.
Output of the voltage control loop error amplifier.
Brightness control input. A DC voltage on this controls the duty cycle of the dimming
cycle. This pin is compared to a 3V ramp at the CT1 pin.
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CCFL Backlight Controller
n Ordering Information
AME9002 x x x x x
Special Feature
Number of Pins
Package Type
Operating Temperature Range
Pin Configuration
Pin Configuration
A: 1 .
2 .
3 .
4 .
5 .
6 .
7 .
8 .
9 .
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
4
VREF
CE
SSC
RDELTA
FAULTB
RT2
VSS
OVPH
OVPL
FCOMP
CSDET
BATTFB
OUTC
OUTAPB
OUTA
VBATT
BRPOL
VDD
CT1
FB
COMP
BRIGHT
SSV
PNP
Operating
Temperature Range
E: -40OC to 85OC
Package Type
J: SOIC (300 mil)
P: Plastic DIP
T: QSOP
Number of
Pins
H: 24
Special Feature
Z: Lead free
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AME9002
CCFL Backlight Controller
n Ordering Information (contd.)
Part Number
Marking
Output Voltage
Package
Operating Temp. Range
AME9002AETH
AME9002AETH
xxxxxxxx
yyww
N/A
QSOP-24
- 40oC to + 85oC
AME9002AEPH
AME9002AEPH
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yyww
N/A
PDIP-24
- 40oC to + 85oC
AME9002AEJH
AME9002AEJH
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yyww
N/A
SOIC-24
- 40oC to + 85oC
n Absolute Maximum Ratings
Parameter
Maximum
Unit
Battery Voltage (VBATT)
25
V
ESD Classification
B
Caution: Stress above the listed absolute maximum rating may cause permanent damage to the device
n Recommended Operating Conditions
Parameter
Battery Voltage (VBATT)
Rating
Unit
7 - 24
V
Ambient Temperature Range
- 40 to + 85
o
Junction Temperature
- 40 to + 125
o
C
C
n Thermal Information
Parameter
Maximum
Unit
o
Thermal Resistance (QSOP - 24)
325
Maximum Junction Temperature
150
o
C
Maximum Lead Temperature (10 Sec)
300
o
C
C/W
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AME9002
CCFL Backlight Controller
n Electrical Specifications
TA= 25OC unless otherwise noted, VBATT = 15V, CT1 = 0.047uF, RT2 = 56K
Parameter
Symbol
Test Condition
Min
Typ
Max
Units
4.9
5.15
5.35
V
5V supply (VSUPPLY)
Output voltage
VDD
Line regulation
VDDLINE
7<Vbatt<24
-0.5
0.5
%
Load regulation
V DDLOAD
Vbatt=7V, 0mA < Iload < 25mA
-0.2
0.2
%
VDDTC
-10C < Ta < 70C
VREF
Vbatt = 15V, Iref = 0
3.25
V REFLINE
7< Vbatt < 24V
-0.1
V REFTC
-10C <Ta < 70C
Temperature drift
0.5
%
3.4V reference (VREF)
Initial voltage
Line regulation
Temperature drift
3.4
3.525
V
0.1
%
100
ppm/C
3
V
Brightness oscillator (CT1, BRIGHT)
Ramp amplitude
VCT1
Frequency
FCT1
Line regulation
LINECT1
7< Vbatt < 24V
Temperature drift
TCCT1
-10C < Ta < 70C
Comparator offset
VOSCT1
70
130
Hz
-1
1
%
=+-3
%
10
mV
Vco oscillator (RT2, RDELTA)
Initial frequency
FVCO(OUTA)
RT2 = 56k
47
52
kHz
Line regulation
LINEVCO
7< Vbatt < 24V
-0.8
0.8
%
Temperature drift
TCVCO
-10C < Ta < 70C
VCO pullin range
PULLVCO
+-0.5
%
RT2/(RDELTA X 5)
%
Error amplifiers (FB, COMP)
Offset voltage, WRT Vref
-40
40
mV
Input bias current
IB
1
nA
Input offset current
IOS
1
nA
Open loop gain
AOL
70
dB
FT
1
Mhz
Unity gain frequency
6
VOS
Output high voltage (comp)
VOH
ISOURCE = 50uA
Output low voltage
VOL
ISINK = 500uA
3.39
V
0.4
V
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AME9002
CCFL Backlight Controller
n Electrical Specifications(contd.)
TA= 25OC unless otherwise noted, VBATT = 15V, CT1=0.047uF, RT2 = 56K
Parameter
Symbol
Test Condition
Min
Typ
Max
Units
Output A (OUTA)
Peak current
1
IPEAKA
Output Low Voltage
VOL
0.2mA
Output High Voltage
VOH
-5mA
Amp
10.55
14.4
V
V
Other outputs (OUTAPB, OUTC)
Peak current
IPEAKBC
1
Output Low Voltage
VOL
ISINK = 10mA
Output High Voltage
VOH
ISOURCE = 10mA
Amp
0.25
VDD - .7
V
V
Soft start clamps (SSC, SSV)
Initial SSC current
ISSCINIT
100
500
nA
Normal SSC current
ISSC
100
500
uA
SSV current
ISSV
10
25
uA
CE high threshold
CE HIGH
1.5
CE low threshold
CELOW
OVPH threshold
OVPHI
OVPL threshold
Other parameters
V
0.4
V
3.2
3.55
V
OVPLO
2.3
2.7
V
CSDET threshold
VTHCS
1.1
1.4
V
FCOMP charging current
IFCOMP
0.8
1.2
uA
BATTFB high threshold
V THBATHI
1.4
1.6
V
BATTFB low threshold
VTHBATLO
1.15
1.35
V
Average supply current
IBATT
No FET gate current
6
mA
Average off current
IOFF
In the application
10
uA
1
2.5
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AME9002
CCFL Backlight Controller
n Block Diagram
VBATT
REF
3.4V
REF
1
C2
1µ
µ
24
Q1
2VOK
To Chip Enable
Logic
CE
2
SSV
CE
23
140
µA
140
nA
SSC
1ST
C3
0.039µ
µ
BRIGHT
BLANK
RES_SSC
C14
1000pF
RES_SSV
Later
3
22
BRIGHTNESS
Control VOLT.
3V
RDELTA
R3
VSupply
R4
2K
PNP
5V
LDO
COMP
4
21
C8
47nF
FAULTB
FB
5
20
RAMP
CLK
VCO
B
2.5V
RT2
CT1
1.5V
6
19
R2
56K
Dimming
RAMP
CEN
VSS
A
7
3.3V
RES_SSC
RES_SSV
OVPH
C
8
R35
D16
R7
30K
EA1
2.5V
RES_FCOMP
C7
4.7µ
µF
Fault
Logic
BRPOL
HI=Reverse
LO=Normal
µA
1µ
OVPL
VBATT
9
R36
VDD
18
17
2.5V
C4
0.047µ
µF
16
STRIKE
C7
7.5V
FCOMP
R10
604
RES_FCOMP
C32
µF
1µ
CLK
1.25V
CSDET
15
B
14
R41
10K
2
BATTFB
12
Q2
A
OUTAPB
VBATT
R40
60K
8
OUTA
NORM
2bit
Count
11
R9
249
HS
Driver
PWM
10
Q3-1
C
OUTC
1.5V
1.25V
BAITOK
13
Q3-2
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AME9002
CCFL Backlight Controller
n Application Schematic
Figure 2. Double Tube Application Schematic (7V < Vbatt < 24V)
R1
2K
1
VREF
PNP
24
2
CE
SSV
23
3
SSC
BRIGHT
22
Q1
C2
1µ
µF
C14
1000p
0.1µ
µF
VDD
4
RDELTA
COMP
21
FB
20
C8
47nF
R3
15k
5
FAULTB
C3
0.047µ
µF
R7
30K
6
RT2
CT1
19
C4
0.047µ
µF
R2
40K
7
VSS
VDD
18
HI=Reverse
LO=Normal
D17
C33
100p
8
OVPH
BRPOL
17
9
OVPL
VBATT
16
10
FCOMP
OUTA
15
C7
4.7µ
µF
D16
R36
R45
1M
R40
60k
Q2
C32
2.2nF
T1
11
CSDET
12
BATTFB
OUTAPB
14
OUTC
13
Q3-2
Q3-1
R41
10k
R37
2meg
R35
2meg
R36
3k
R38
3k
R10
680
D23 D22
R40
7.5k
C34
0.01u
R9B
221
D21 D20
R42,43
10k
R9A
221
VDD
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n Application Notes
Overview
The AME9002 application circuit drives a CCFL (cold
cathode fluorescent lamp) with a high voltage sine wave
in order to produce an efficient and cost effective light
source. The most common application for this will be as
the backlight of either a notebook computer display, flat
panel display, or personal digital assistant (PDA).
The CCFL tubes used in these applications are usually
glass rods that can range from several cm to over 30cm
and 2.5mm to 6mm in diameter. Typically they require a
sine wave of 600V and they run at a current of several
milliamperes. However, the starting (or striking) voltage
can be as high as 2000V. At start up the tube looks like
an open circuit, after the plasma has been created the
impedance drops and current starts to flow. The starting
voltage is also known as the striking voltage because
that is the voltage at which an arc “strikes” through the
plasma. The IV characteristic of these tubes is highly
non-linear.
Traditionally the high voltage required for CCFL operation has been developed using some sort of transformer LC tank circuit combination driven by several small power
mosfets. The AME9002 application uses one external
PMOS, 2 external NMOS and a high turns ratio transformer with a centertapped primary. Lamp dimming is
achieved by turning the lamp on and off at a rate faster
than the human eye can detect, sometimes called “duty
cycle dimming”. These "on-off" cycles are known as
dimming cycles. Alternate dimming schemes are also
available.
Steady State Circuit Operation
Figure 1 shows a block diagram of the AME9002.
Throughout this datasheet like components have been
given the same designations even if they are on a different figure. The block diagram shows PMOS Q2 driving
the center tap primary of T1. The gate drive of Q2 is a
pulse width modulated (PWM) signal that controls the
current into the transformer primary and by extension,
controls the current in the CCFL. The gate drive signal of
Q2 drives all the way up to the battery voltage and down
to 7.5 volts below Vbatt so that logic level transistors
may be used without their gates being damaged. An
internal clamp prevents the Q2 gate drive (OUTA) from
driving lower than Vbatt-7.5V.
NMOS transistors Q3-1 and Q3-2 alternately connect
the outside nodes of the transformer primary to VSS.
These transistors are driven by a 50% duty cycle square
wave at one-half the frequency of the drive signal applied
to the gate of Q2.
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CCFL Backlight Controller
Figure 3 illustrates some ideal gate drive waveforms for
the CCFL application. Figure 4 and 5 are detailed views
of the power section from Figures 1 and 2. Figure 5 has
the transformer parasitic elements added while Figure 4
does not. Referring to Figures 4 and 5, NMOS transistors Q3-1 and Q3-2 are driven out of phase with a 50%
duty cycle signal as indicated by waveforms in Figure 3.
The frequency of the NMOS drive signals will be the frequency at which the CCFL is driven. PMOS transistor,
Q2, is driven with a pulse width modulated signal (PWM)
at twice the frequency of the NMOS drive signals. In
other words, the PMOS transistor is turned on and off
once for every time each NMOS transistor is on. In this
case, when NMOS transistor Q3-1 and PMOS transistor
Q2 are both on then NMOS transistor Q3-2 is off, the
side of the primary coil connected to NMOS transistor
Q3-1 is driven to ground and the centertap of the transformer primary is driven to the battery voltage. The other
side of the primary coil connected to NMOS transistor
Q3-2 (now “off”) is driven to twice the battery voltage
(because each winding of the primary has an equal number of turns).
Current ramps up in the side of the primary connected
to Q3-1 (the “on” transistor), transferring power to the
secondary coil of transformer. The energy transferred from
the primary excites the tank circuit formed by the transformer leakage inductance and parasitic capacitances that
exist at the transformer secondary. The parasitic capacitances come from the capacitance of the transformer secondary itself, wiring capacitances, as well as the parasitic
capacitance of the CCFL. Some applications may actually add a small amount of parallel capacitance (~10pF)
on the output of the transformer in order to dominate the
parasitic capacitive elements.
When the PMOS, Q2, is turned off, the voltage of the
transformer centertap returns to ground as does the drain
of NMOS transistor Q3-2 (the drain of Q3-2 was at twice
the battery voltage). Halfway through one cycle, NMOS
transistor Q3-1 (that was on) turns off and NMOS transistor Q3-2 (that was off) turns on. At this point, PMOS
transistor Q2 turns on again, allowing current to ramp up
in the side of the primary that previously had no current.
Energy in the primary winding is transferred to the secondary winding and stored again in the leakage inductance Lleak, but this time with the opposite polarity. The
current alternately goes through one primary winding then
the other.
The duty cycle of PMOS transistor Q2 controls the
amount of power transferred from the primary winding to
the secondary winding in the transformer. Note that the
CCFL circuit can work with PMOS transistor Q2 on con-
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AME9002
CCFL Backlight Controller
Figure 3. Idealized Gate Drive Waveforms
Q2 Gate
VBATT
(OUTA)
VBATT- 7.5V
5V
Q3-1 Gate
(OUTAPB)
0V
5V
0V
Q3-2 Gate
(OUTC)
Figure 4.
Power Stage Single Tube Components
Figure 5.
Power Stage Single Tube Components
with parasitic elements
(Same component designations used throughout)
(Same component designations used throughout)
VBATT
OUTA
D = 0-100%
Vhi = Vbatt
Vlo = Vbatt-7.5V
F = fosc
Q2
VBATT
OUTA
D = 0-100%
Vhi = Vbatt
Vlo = Vbatt-7.5V
F = fosc
Q2
T1
Lp
Lp
OUTAPB
D=50%
Vhi = 5V
Vlo = 0V
F = fosc/2
Q3-1
Q3-2
OUTC
D=50%
Vhi = 5V
Vlo = 0V
F = fosc/2
T1
OUTAPB
D=50%
Vhi = 5V
Vlo = 0V
F = fosc/2
Q3-1
1:N
Q3-2
OUTC
D=50%
Vhi = 5V
Vlo = 0V
F = fosc/2
L leak
Cparasitic
Signals OUTC
and OUTAPB
are the inverse
of each other.
Signals OUTC
and OUTAPB
are the inverse
of each other.
CCFL
D5
D4
CCFL
D5
D4
To Control
Circuitry
R9+R10
To Control
Circuitry
R9+R10
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stantly (i.e. a duty cycle of 100%), although the power
would be unregulated in this case.
Figures 6,7 illustrates various oscilloscope waveforms
generated by the CCFL circuit in operation. These figures show that the duty cycle of the gate drive at Q2
decreases as the battery voltage increases from 9 V to
21 V (as one would expect in order to maintain the same
output power).
The first three traces in Figures 6 and 7 show the gate
drive waveforms for transistors Q2, Q3-1, and Q3-2, respectively. As mentioned before, the gate drive waveform for transistor Q2 drives up to the battery voltage but
down only to approximately 7.5 V below the battery voltage. The fourth trace (in Figures 6,7) shows the voltage
at centertap of the primary winding (it is also the drain of
PMOS transistor, Q2). This waveform is essentially a
ground to a battery voltage pulse of varying duty cycle.
When the centertap of the primary is driven high, current
increases through PMOS transistor, Q2 as indicated by
the sixth trace down from the top. In region I the drain
current of Q2 is equal and opposite to the drain current of
Q3-1 since the gate of Q3-1 is high and Q3-1 is on. In
region III the drain current of Q2 will be equal and opposite to the drain current of Q3-2 (not shown). In region II
when PMOS transistor Q2 is switched off, the current
through this transistor, after an initial sharp drop, ramps
back down towards zero.
In Figures 6 and 7 the fifth trace down from the top
shows the drain voltage of Q3-1. (The trace for NMOS
transistor Q3-2, not shown, would be identical, but shifted
in time by half a period.) The seventh trace down from
the top shows the current through the NMOS transistor
Q3-1, which is equal to the current in PMOS transistor
Q2 for the portion of time that PMOS transistor Q2 is
conducting (see region I, for example). As the current
ramps up in the primary winding, energy is transferred to
the secondary winding and stored in the leakage inductance Lleak (and any parasitic capacitance on the secondary winding). If the current in the NMOS transistor is
close to zero when that NMOS transistor is turned off
that means that the CCFL circuit is being driven close to
its resonant frequency. If the circuit is being driven too
far from its resonant point then there will be large residual
currents in the transistors when they are turned off causing large ringing, lower efficiency and more stress on the
components. So called "soft switching" is achieved when
the MOS drain current is zero while the MOS is being
turned off. The driving frequency and transformer parameters should be chosen so that soft switching occurs.
Once PMOS transistor Q2 completes one on/off cycle,
12
CCFL Backlight Controller
it is repeated again with the alternate NMOS transistor
conducting. This complementary operation produces a
symmetric, approximately sinusoidal waveform at the input to the CCFL load, as shown by the bottom trace in
Figures 6 and 7.
The operation of the CCFL circuit can be divided into 4
regions (I, II, III, and IV) as shown in Figures 6 and 7.
Figure 8-1 shows the equivalent transformer and load circuit model for region I. During region I, one of the primary windings is connected across the battery, the current in that winding increases and energy is coupled
across to the secondary. No current flows in the other
winding because its NMOS is turned off and its body
diode is reverse biased. The drain of that NMOS stays
at twice the battery voltage because both primary windings have the same number of turns and the battery voltage is forced across the other primary winding.
Figure 8-2 shows the equivalent transformer and load
circuit model for region II. During region II, the battery is
disconnected from the primary winding. In this configuration, current flows through both of the primary windings. The current decreases very quickly at first then
ramps down to zero at a rate that is slower than the
current ramped up. The initial drop is due to the almost
instantaneous change in inductance when current flow
shifts from one portion of the primary winding to both
portions of the primary.
Figure 8-3 shows the equivalent transformer and load
circuit model for region III. During region III, the primary
winding opposite from the one used in region I is connected across the battery, increasing current in that primary winding but in a direction opposite to that of region
I. Energy is coupled across to the secondary as in region I but with opposite polarity. No current flows in the
undriven winding because its NMOS is turned off and its
body diode is reverse biased. The drain of that NMOS
stays at twice the battery voltage because both primary
windings have the same number of turns and the battery
voltage is forced on the other primary. Region III is, effectively, the inverse of region I.
Figure 8-4 shows the equivalent transformer and load
circuit model for region IV. During region IV, the battery
is disconnected from the primary winding. In this configuration, current flows through both of the primary windings with opposite polarity to that in region II. The current decreases very quickly at first then ramps down to
zero at a rate that is slower than the current ramped up.
Once again, the initial drop is due to the effective change
in inductance when current flow shifts from one portion of
the primary winding to both portions of the primary. Region IV is effectively the inverse of region II.
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AME9002
CCFL Backlight Controller
Figure 6. Typical Waveforms VBATT=9V
VBATT=9V
1.5V
Q2 Gate
(OUTA)
5V
0V
Q3-1 Gate
(OUTAPB)
5V
0V
Q3-2 Gate
(OUTC)
VBATT
Center tap
(Q2 DRAIN)
0V
VBATT x 2
0
0
IMAX
Q3-1 Drain
IDQ2
0
IDQ3-1
IMAX
ILAMP
Region
I
Region
II
Region
III
Region
IV
Figure 7. Typical Waveforms VBATT=21V
VBATT=21V
Q2 Gate
13.5V
5V
0V
(OUTA)
Q3-1 Gate
(OUTAPB)
Q3-2 Gate
5V
0V
(OUTC)
Center tap
VBATT
(Q2 DRAIN)
0V
2 x VBATT
0V
Q3-1 Drain
0A
IDQ2
IMAX
0A
IDQ3-1
IMAX
ILAMP
Region
I
Region
II
Region
III
Region
IV
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AME9002
CCFL Backlight Controller
Figure 8-2. Region II
Figure 8-1. Region I
ILEAK
ILEAK
Cparasitic
Cparasitic
Load
I
Load
I
Figure 8-3. Region III
Figure 8-4. Region IV
ILEAK
ILEAK
I
Load
Cparasitic
Cparasitic
I
Load
Figure 9. Steady State Dimming Waveforms (after initial start up period)
3V
BRIGHT
0.5V
CT1
5V
SSV
OV
5V
3V
3V
SSC
OV
"Blanked" Faults
CSDET < 1.25V
OVPL > 2.5V
"Unblanked" Faults
OVPH > 3.3V
Ignore
Respond
Always Respond to Over voltage Faults
Tube Current
(time not
to scale)
~ 6ms
14
Ignore
Respond
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Driving the CCFL
Unlike modified Royer schemes for driving CCFLs the
secondary winding of the AME9002 method is not designed 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 primarily 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 regulation 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 operating 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 transitions from a low to a high state a special mode of operation, known as the “start up mode”, is initiated that
will last for a maximum of one second. The exact duration 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 transition 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 shutdown until the user toggles the power supply or CE transitions from low to high again. In other words, if the CCFL
CCFL Backlight Controller
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 voltage 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 available 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 charging current is zero and the voltage at FCOMP remains 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 thresholds 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
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AME9002
CCFL Backlight Controller
Figure 10. Ignition Flow Chart
START
Start 1 second
timer
F=Fmax
Set SSV = 0V
Yes
V(OVPH)>
3.3V
No
No
V(OVPL) > 2.5V
Yes
Timer
End?
V(OVPL) > 2.5V
Yes
Yes
No
Shutdown
Yes
V(CSDET) <
1.25V
Yes
Timer
End?
No
Yes
V(CSDET) < 1.25V
(after normal blanking
and for 4 clk cycles)
No
No
No
No
16
F > Fmin?
F > Fmin?
Yes
Yes
F(new)=F(old) - delta
F(new)=F(old) - delta
Start Up Side ------- | ------ Steady State
Operation Side
No
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AME9002
CCFL Backlight Controller
Figure 11. Start Up and Steady State Waveform
3V
BRIGHT *
CT1
**
1.25V
CSDET
5V
VBATTOK
VDDOK
} VALID
<3V
SSC
BLANK
BLANK
OVPH>3.3V
F = fMIN
F = fMIN
FCOMP
<1 Sec
OVPL<2.5V
OVPH<3.3V
OVPL>2.5V
OVPH < 3.3V
OVPL OVPL>2.5V
<2.5V OVPH < 3.3V
OVPL<2.5V
OVPH<3.3V
~ 6mS
SSV
< Initial Start Up Period>
<Two full scale brightness cycles after strike>
<Steady state op. with duty cycle dimming>
<After CCFL strikes>
IF:
THEN:
IF:
THEN:
OVPL<2.5V
OVPH<3.3V
FCOMP ramps up
Frequency decreases
OVPH > 3.3V
OVPL>2.5V
OVPH < 3.3V
FCOMP constant
Frequency constant
OVPL > 2.5V
during blanking period: nothing
after blanking period: shutdown
OVPH>3.3V
F COMP = V
Frequency = FMAX
SSV=0V
COMP=0V
CSDET < 1.25V for
4 consecutive clock cycles
during blanking period: nothing
after blanking period: shutdown
CSDET > 1.25V
Tube has struck and initial
start period has ended.
Immediate shutdown
* BRPOL is Low
** Time axis is not to scale
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allows the switching duty cycle to increase which, in turn,
increases the voltage across the CCFL.
At some point later SSV=5 volts, SSC and FCOMP
are still ramping up. The tube voltage continues to increase, the switching duty cycle is no longer limited by
SSV and is able to go to 100%, if indicated by the error
amp loop. The switching frequency continues to decrease
forcing the tube voltage higher. If the CCFL voltage is
high enough so that OVPL > 2.5V (OVPL senses the
CCFL voltage through a resistor or capacitor divider) then
FCOMP stops increasing and the frequency remains constant. The frequency will remain constant until:
OVPL < 2.5V
OR....
OVPH > 3.3V (see below)
OR......
The one second time period runs out and the
circuit shuts down.
If the voltage across the tube increases enough so that
OVPH > 3.3V (as sensed through a resistor or capacitor
divider) then FCOMP is pulled low (~1V), the switching
frequency is increased, SSV is pulled low and the switching duty cycle goes to zero. It will remain in this state
until:
OVPH < 3.3V
OR....
The one second time period runs out and the circuit
shuts down.
Ideally, during one of these states, the CCFL will strike,
current will flow in the CCFL and the circuit will move
from the start up mode into the steady state mode. Once
an arc has struck, as sensed by CSDET > 1.25 volts,
then the circuit will drive the CCFL at 100% brightness
for approximately two dimming cycles (dimming cycles
are on the order of 6mS as determined by the capacitor
on CT1) in order to ensure that the CCFL is really “on”.
After those two full brightness dimming cycles the normal duty brightness control takes over, alternately turning the CCFL on and off at a duty cycle determined by
the voltage at the BRIGHT pin.
Remember, the circuit will only “try” to turn on for one
second, after that point it gives up and shuts down.
18
CCFL Backlight Controller
Steady State Mode
At the beginning of each dimming cycle (after the start
up mode) there is initially no arc struck in the CCFL. The
CCFL load looks like an open circuit. (However an arc
has been struck successfully in the start up mode so we
assume the gas has “warmed up” and is ready to strike
an arc again.) SSV is pulled to zero volts then ramps to 5
volts allowing the duty cycle of the switches to slowly
increase to its steady state value. The voltage across
the CCFL will increase with each successive clock cycle.
Two events may then happen:
1) The gas inside the CCFL will ionize, the voltage across
the CCFL will drop, the current through the CCFL will
increase, and a stable steady state operating point
will be reached.
OR....
2) One of the three fault conditions will be met that shut
down the circuit (see Figure 11):
a) The CCFL tube voltage continues to rise until the
OVPH pin is higher than 3.3V at which point the
circuit will shut down (immediately).
b) The CCFL tube voltage continues to rise until the
OVPL pin is higher than 2.5V at which point the
circuit will shut down (except during the blanking
interval).
c) The CCFL current fails to rise high enough to keep
the undercurrent threshold at the CSDET pin from
tripping (for 4 consecutive clock cycles).
Note that condition a) can be met at any time while the
AME9002 is in steady state operation (after the start up
mode). Condition b) can only be met after the SSC pin
has risen above 3V (after blanking interval). Condition c)
can only be met after the SSC pin has crossed 3V (after
blanking interval) AND four successive undercurrent events
occur in a row (CSDET < 1.25V).
The SSC pin is pulled to VSS everytime the lamp is
turned off, whether for a dimming cycle, user shutdown
or fault occurrence. It ramps up slowly depending on the
size of capacitor C3 connected to the SSC pin. The period of time when the b) and c) fault checks are disabled
is called the "blanking" time. The blanking time occurs
from the time SSC is pulled to VSS until it reaches 3V.
See Figure 9 for some idealized waveforms illustrating
the behavior just described.
Control Algorithm
There are 2 major control blocks (loops) within the IC.
The first loop controls the duty cycle of the driving waveform. It senses the CCFL current (Figure 1 or 2, resistor
R9 and R10) rectifies it, integrates it against an internal
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reference and adjusts the duty cycle to obtain the desired power. This loop uses error amplifier EA1 whose
negative input is pin FB and whose output is COMP. The
positive input of EA1 is connected to a 2.5V reference.
External components, R7 and C8, set the time constant
of the integrator, EA1. In order to slow the response of
the integrator increase the value of the product:
(R7 X C8).
The second control block adjusts the brightness by
turning the lamp on and off at varying duty cycles. Each
time the lamp turns on and off is referred to as a “dimming cycle”. At the end of each dimming cycle the SSV
pin is pulled low, this forces COMP low as well due to the
clamping action of Clamp1 shown in Figure 1. At the
beginning of a new dimming cycle COMP tries to increase
quickly but it is clamped to the voltage at the SSV(softstart voltage) pin. A capacitor on the SSV pin (C8, Figure
1), which is discharged at the end of every dimming cycle,
sets the slew rate of the voltage at the SSV pin, and
hence also the maximum positive slew rate of the COMP
pin. [“Dimming cycle” is explained more fully below]
CCFL Backlight Controller
means that harmonic distortion can be minimized by optimizing the frequency and transformer characteristics for
a particular duty cycle rather than a large range of duty
cycle.
The BRIGHT, CT1 and BRPOL pins
A user-provided voltage at the BRIGHT pin is compared
with the ramp voltage at the CT1 pin (See Figure 12). If
BRPOL is tied to VSS then as the voltage at BRIGHT
increases the duty cycle of the dimming cycle and the
brightness of the CCFL increase. If BRPOL is tied to
VDD then the brightness of the CCFL diminishes as the
BRIGHT voltage increases. The frequency of the dimming cycles is set by the value of the capacitor at pin
CT1 (C4 in Figure 1 and 2) and it is also proportional to
the current set by resistor R2. Setting C4 equal to
0.047uF and R2 equal to 47.5k yields a dimming cycle
frequency of approximately 125Hz. The frequency should
vary inversely with the value of C4 according to the relation:
Frequency(Hz) = 1/[4 X R2 X C4]
The brightness may also be controlled by using a variable resistor in place of R10 (See Figure 13). In this case
the BRIGHT pin should be pulled to VDD so that the CCFL
remains on constantly. This method can lead to flicker at
low intensities but it is easy to implement. Harmonic
distortion may also increase since the duty cycle of the
waveform at the gate of Q2 will vary greatly with brightness. When using burst brightness control the duty cycle
of the driving waveforms should not vary because the
CCFL is running at 100% power or it is turned off. As
long as the battery voltage does not change the duty cycle
of the driving waveform also does not change greatly. This
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AME9002
CCFL Backlight Controller
Figure 12. Duty Cycle Dimming
Outside Chip
Inside Chip
BRPOL
BRIGHT
CHOP
+
-
Brightness
control
voltage
CHOP causes the CCFL
to turn on and off
periodically.
CT1
+
-
3V
C4
S
Q
-
R
+
50mV
Figure 13. Alternative Brightness Control
Inside Chip
Outside Chip
This method disables
duty cycle dimming
BRPOL
T1
BRIGHT
+
CHOP
Always Hi
-
5V
CT
K
Maximum current=
R1//(2R+R)
COMP
+
K
To PWM
Comparator
R2
FB
EA1
-
RF
R1
2.5V
2R
+
To Fault
Control Logic
20
CSDET
-
R-C-D
optional
network
R
1.25V
Minimum current=
(R1+R2)//(2R+R)
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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 14. 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.
Resistor R3 controls how much the oscillator frequency
increases as a function of the voltage at FCOMP. The
relationship is:
Delta frequency (Hz) = 3.44E8 * (5 - V(FCOMP)) / R3
You can see from the formula that the frequency will
decrease as the FCOMP voltage increases. The amount
of this increase is set by R3. The current in R3 decreases
as the voltage at FCOMP increases and hence decreases
the charging current into the timing capacitor of Figure
14 thereby decreasing the oscillator frequency.
Supply voltage pins, VDD and PNP
Most of the circuitry of the AME9002 works at 5V with
the exception of one output driver. That driver (OUTA)
and its power pad (VBATT) must operate up to 24V although the OUTA pad may never be forced lower than 8
volts away from the VBATT pin. The OUTA pin is internally clamped to approximately 7.5 volts below the Vbatt
pin.
The AME9002 uses an external PNP device to provide
a regulated 5V supply from the battery voltage (See Figure 15). 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.
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 supply collapse to zero volts. When low, the CE pin also
immediately turns PMOS transistor Q2 off, however transistors Q3-1 and Q3-2 will continue to switch until the 5V
has collapsed to 3.5V. By allowing the Q3 transistors to
continue to switch for some time after Q2 is turned off
the energy in the tank circuit is dissipated gradually without any large voltage spikes.
CCFL Backlight Controller
The VDD voltage is sensed internally so that the switching 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 circuitry 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 16). 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 transitions yet low overall
power dissipation.
Fault Protection, the OVPH, OVPL and CSDET pins
During the startup mode the AME9002 does not actually sense for fault conditions, instead it uses the voltages at OVPL and OVPH to adjust the operating frequency for a smooth start up. The startup itself (or
“strike”) is detected when the voltage at CSDET rises
above 1.25V. There are no voltages at OVPL, OVPH or
CSDET that can cause a fault during the start up mode.
During steady state operation the AME9002 checks
for 3 different fault conditions. There are two overvoltage
conditions and one undercurrent condition that can cause
a fault. 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 17 for a schematic of the FAULT circuitry.)
The first fault condition check can be used to detect
overvoltages at the CCFL. Specifically, if the OVPH pin
is above 3V then this fault condition is detected. The
first fault condition is always enabled, there is no blanking period (except, of course, during the start up period
when fault detection is disabled).
The second fault condition checks that the voltage at
OVPL is below 2.5V. This protection is disabled while
the SSC ramp is below 3V such as during the beginning
of every dimming cycle. Again, this check is disabled
during the start up period like all the fault checks.
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In order to enable the first two fault condition checks
then the OVP pin must, indirectly, sense the high voltage at the input of the CCFL. The actual CCFL voltage
must be reduced by using either a resistor or capacitor
divider such that in normal operation the voltage at OVPL
is lower than 2.5V and the voltage at OVPH is lower than
3.3V.
The third fault condition check can be used to monitor
the CCFL current. Specifically, it checks whether the
voltage at the CSDET pin is higher than 1.25V. If CSDET
does not cross its 1.25V threshold once during 4 successive clock cycles then this fault will be triggered. This
protection is disabled while the SSC ramp is below 3V,
such as at the beginning of every dimming cycle. This
fault check is disabled during the start up mode, as are
all the fault checks. This fault condition is used to check
that a reasonable minimum amount of current is flowing
in the tube.
Figure 17 is a simplified schematic of the fault protection circuitry used in the AME9002. Most of the signals
have been previously defined however some need a little
explanation. The VDDOK signal is a power OK signal
that goes high when the 5V supply (VDD) is valid. The
CHOP signal stops the operation of the switching circuitry once every dimming cycle for burst mode brightness control. The output signal, FIRST, is high during
the start up mode then is low during subsequent cycles.
It causes the SSC pin to initially source 1000 times less
current than on subsequent dimming cycles in order to
provide the 1 second initial start up period. The NORM
signal is an enable signal to the switching circuitry. When
it is high the circuit works normally. When it is low the
switching circuitry stops.
SSC and SSV pins
Besides defining the initial 1 second start up period
the SSC pin’s primary role is to define a time period in
which the 2nd and 3rd fault condition (previously described) are disabled. This period of time is called the
blanking interval. During the initial start up period after a
power on reset or just after a low to high transition on the
CE pin the SSC pin sources 140nA into an external capacitor, C3. For subsequent dimming cycles the SSC
pin sources 140uA. During steady state operation the
blanking interval is defined as the time during which
V(SSC) < 3V. Once the voltage at SSC crosses 3V the
blanking interval is finished and all three fault condition
checks are enabled. (The OVPH > 3.3V fault check is
always enabled after the initial start up period.) At the
22
CCFL Backlight Controller
beginning of the next dimming cycle the SSC pin is pulled
to VSS then allowed to ramp upwards again.
During steady state operation the SSV pin (like the
SSC pin) is pulled to ground at the beginning of every
dimming cycle then sources 20uA into an external capacitor. This creates a 0 to 5 volt ramp at the SSV pin.
This ramp is used to limit the duty cycle of the PWM
gate drive signal available at the OUTA pin. The SSV pin
accomplishes duty cycle limiting by clamping the COMP
voltage to no higher than the SSV voltage. Because the
magnitude of the COMP voltage is proportional to the
duty cycle of the PWM signal at OUTA the duty cycle
starts each dimming cycle at zero and slowly increases
to its steady state value as the voltage at SSV increases.
(Figure 9 shows this operation.)
During the initial start up mode the SSV pin starts at
zero volts and ramps up to 5V just as in steady state
operation. However, during the start up mode, if OVPH >
3.3V then SSV is pulled to VSS and only allowed to ramp
up when OVPH < 3.3V. This action sets the duty cycle
back to 0 volts then allows the duty cycle to increase as
the SSV voltage increases.
This type of duty cycle limiting is commonly called
“soft-start” operation. Soft start operation lessens overshoot on start up because the power increases gradually
rather than immediately.
Unlike the SSC pin the current sourced by the SSV pin
remains approximately 20uA during ALL dimming cycles.
BATTFB
The BATTFB pin is designed to sense the battery voltage and enable the pin OUTA. When the voltage at
BATTFB is below 1.25 volts then OUTA is disabled, when
the voltage at BATTFB is larger than 1.5V then OUTA is
enabled. There is 250mV of hysteresis between the turn
on and the turnoff thresholds. This pin does not disable
any other portion of the circuit except the OUTA pin.
Notably, the other two drivers, OUTAPB and OUTC continue to switch when the voltage at BATTFB is below
1.25V.
Ringing
Due to the leakage inductances of transformer T1 voltages at the drains of Q3 can potentially ring to values
substantially higher than the ideal value (which is twice
the battery voltage). The application schematic in Figure
17 uses a snubbing circuit to limit the extent of the ringing voltage. Components C9,R8,D2 and D3 make up the
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CCFL Backlight Controller
snubbing circuit. The nominal voltage at the common
node is approximately twice the battery voltage. If either
of the drains of Q3 ring above that voltage then diodes D2
or D3 forward bias and allow the ringing energy to charge
capacitor C9. Resistor R8 bleeds off the extra ringing
energy preventing the voltage at the common node from
increasing substantially higher than twice the battery voltage. The extra power dissipation is:
P(dissipated) = Vbatt2 / R8
For the example, in Figure 17, the power dissipation of
the snubber circuit with Vbatt=15V is 58mW or approximately 1% of the total input power. The value of R8 can
be optimized for a particular application in order to minimize dissipated power.
Excessive ringing is usually a sign that the driving frequency is not well matched to the resonant characteristics of the tank circuit. In a well designed application a
snubber circuit will not be necessary.
Layout Considerations
Due to the switching nature of this circuit and the high
voltages that it produces this application can be sensitive
to board parasitics. In fact, one of the advantages, of this
design is that the circuit uses the parasitic elements of
the application as resonant components, thus eliminating the need for more added components.
Particular care must be taken with the different gounding
loops. The best performance has been obtained by using a “star” ground technique. The star technique returns all significant ground paths back to the center of
the “star”. Ideally we would place the center of the star
directly on the VSS pin of the AME9002. The bypass
capacitors would, ideally, be connected as close to the
center of the star as possible. The schematic in Figure
18 attemps to show this star ground configuration by bringing all the ground returns back to the same point on the
drawing. Separate ground returns back to the star are
especially important for higher current switching paths.
23
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AME9002
CCFL Backlight Controller
Figure 14. VCO Detail
1µ
µA
R3
VDD
Vco_Control
RDELTA
0
VDD
2.5V
I_in
50:1
curent
divider
OVPL
I_out
1.5V
0
+
-
RAMP
RT2
+
-
3.0V
FCOMP
C32
1µ
µF
CLK
SSV
OVPH
R2
Inside chip
VSS
3.3V
Outside chip
Figure 15. LDO Detail
Outside Chip
Inside Chip
R4
PNP
VBATT
1
Q1
VDDOK
2
-
Start
UP
+
-
24
CE
To user
enable circuitry
C7
EN
2.5V
27 < VBATT < 24
VDD
To Fault
Logic
µF
4.7µ
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AME9002
CCFL Backlight Controller
Figure 16. OUTA Driver Circuitry
Inside Chip
Outside Chip
Vbatt
BV=5V
BV=4V
BV=7.5V
OUTA
PWM
SIGNAL
External
PMOS, Q2
100nS
100nS
1mA
25
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AME9002
CCFL Backlight Controller
Figure 17. Fault Logic
CE
VDD
VDDOK
POR
BATTFB
1.25V
+
CLK
-
FAULTB
1.25V
CSDET
+
-
L1
Q
RES
S
Q
NORM
2Bit
Counter
L2
R
OVPL
2.5V
+
-
BLK_CS
SSC
OVPH
3.3V
S
+
-
CHOPOUT
3.0V
+
-
BLNK
L3
Q
SSV
R
S
FIRST
R
VDDOK
CT1
C4
Q
RES
EN
2 Bit
Shift
Dimming
Oscillator
D
VDD
BRIGHT
BRPOL
FCOMP
26
+
CHOPIN
Q
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Application Component Description
Figure 18 shows one typical application circuit for driving 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 disconnected 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 actively driving the CE pin then the C1 capacitor may not
be necessary.
R3 - This resistor connected to the RDELTA pin determines how much the oscillator frequency will change
with battery voltage. The relation, which is found earlier 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 blanking 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 blanking interval is over and all fault checks are enabled.
The charging current into C3 (out of pin SSC) is normally 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 interval is:
CCFL Backlight Controller
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, determines 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 switching 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:
T(seconds) = (C3) * (3volts) / (140e-6amps)
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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 supply 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 (integrator) EA1. The integrator is the primary loop stabilizing element of the circuit. In general this application 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
in one CCFL tube. As the sum of R9A and R10 decreases 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.
D6 - This diode can catch any negative going spikes
on the drain of Q2. This diode is NOT strictly necessary. 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.
The following components are only used for multiple
tube operation:
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.
R35,R36,D16 etc. - These devices form a voltage divider 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 explained in more detail in the next section.)
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 circuit 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
CCFL Backlight Controller
Q4,Q5 - These bipolar devices buffer the gate of Q2.
That allows Q2 to be made much bigger without dissipating more power or increasing the cost of the
AME9002. Q4 is an NPN transistor and Q5 is a PNP
transistor.
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 multiple 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 detected. 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.
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Multiple Tube Operation
The AME9002 is particularly well suited for multiple
tube applications. Figure19 shows the power section of
a two tube application. The major difference between
this application and the single tube application is the addition of another secondary winding on the transformer.
The primary side of the transformer and its associated
FETs are exactly the same as the single tube case although the FETs may need to be resized due to the increased current in two tube applications.
The secondaries are wound so that the outputs to the
CCFL are of opposite phase (see Figure 20) although
this is not strictly necessary. When the voltage at one
secondary output is high (+600 volts) the other secondary output should be low (-600 volts). The other secondary terminals are connected to each other. In a balanced
circuit the voltage at the connection of the two secondaries will, ideally, be zero. Of course in a real application
the voltage at the connection of the two secondaries will
deviate somewhat from zero.
The multi-tube configuration is modular. Since each
double transformer can drive two CCFLs it is possible to
construct 2, 4, 6..... tube solutions using the basic architecture. Of course the FETs must be properly sized to
handle the increased current. Figure 21 shows a 4 tube
application. In this configuration the common secondary
connection (the node NOT connected to the lamp) is made
with the opposite transformer. In this way the secondary
current from the winding on the first transformer should
be equal to the secondary current of its companion winding on the second transformer. In the case of 4 lamps
driven by two transformers there are two sets of common
secondary nodes.
Sensing the current in the multiple tube case requires
some extra circuitry. Normally the CSDET pin checks
for the existence (or absence) of current in the CCFL. If
current is detected then the initial start mode terminates
and steady state operation begins. During steady state
operation if no current is detected for 8 consecutive clock
cycles then the circuit is shutdown. Since there is only
one CSDET pin yet there are multiple tubes extra circuitry is required.
Take the two tube case of Figure 19 for example. The
current through the tube on the right hand side is regulated by the integrator made of R7, C8 and EA1. However, for purposes of fault detection and strike detection it
is beneficial to monitor the current through both tubes.
In this case R9B senses the current in the left tube in the
same way R9A senses the current in the right hand tube.
If the current through either tube is zero then R9A or R9B
CCFL Backlight Controller
will try to pull node A or B to zero. Resistors R42 and
R43 attempt to pull node A and B up but the value of R42
and R43 (nominally 10K) is much larger than the values
of resistors R9A and R9B (nominally 221ohms) allowing
node A and B to pull close to VSS when there is zero
current in their respective CCFL tubes. The absence of
current in either tube essentially pulls node A or B to
VSS.
In normal operation the voltage at nodes A and B should
look like alternating, positive half sinusoids. (See figure
22.) If, however, there is no current flowing in one of the
tubes then one half of the sinusoids would be missing
and the voltage at CSDET would drop compared to its
normal value. The values of the RC network made up of
R4 and C34 are chosen so that the voltage at CSDET is
always larger than 1.25 volts when both half sinusoids
are present but is less than 1.25V when only one sinusoid is present. The concept can be applied to any even
multiple of tubes. The tube without the current will dominate the voltage at CSDET so a failure in any single tube
will cause the circuit to shutdown. In a similar manner,
during start up all tubes must have current flowing in
them before CSDET will rise above 1.25V and signal that
the tubes have struck and that the initial start up mode is
over.
For every 2 extra tubes that need to be added the
user must add one more transformer, and two resistor
divider networks plus two diodes (R35, R36, R37, R38,
D16, D17) to sense the CCFL voltage as well as two
more diodes and two more resistors to sense the tube
current (R9A, R9B, D20, D22). Resistors R42, R43, R40,
diodes D21, D23 and capacitor C34 do not need to be
replicated every time more CCFLs are added because
they are shared in common on the CSDET node.
Figure 18 shows a complete four tube schematic. Figure 21 shows a detail of the current and voltage sensing
circuitry for the four tube application. Analogous components have been given the same numbers as in the single
tube schematic. There is really very little difference between the the single tube configuration and the multitube version. Transistors Q4 and Q5 are added to buffer
the high side drive OUTA. This may be necessary because the PMOS devices for larger current applications
have larger gate drive requirements.
The MOS transistors are sized bigger for the 4 tube
application as would be expected. The peak currents
are much higher so the Vbatt bypassing capacitor must
be increased as well. The schematic shows C5 as a
100uF capacitor but higher values such as 220uF are not
uncommon in order to minimize ripple on Vbatt.
29
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AME9002
CCFL Backlight Controller
Figure 18. Four Tube Application Schematic
BATT
Q4
R4
2K
Q1
R8
NPN
2N3904
R1
3
1
1Meg
3.9k
C9
1uF
PNP
2N3906
Q2
5602
SNUB
Q5
PNP
2N3906
D2
IN914
2, 4
D3
IN914
R6
BRIGHT
9
4
3
10
4
9
3
10
51k
T2
7
5
2
12
2XTRANS
7
5
2
12
T1
2XTRANS
R3
15k
C2
1uF
U1
Vref
3
C3
4
0.047uF
5
6
R2
7
40k
C1
8
0.1u
9
10
R40
60K
AME9002
1
2
CE
LX
C32
2200p
11
12
Vref
PNP
CE
SSV
SSC
BRIGHT
RDELTA
COMP
FAULTB
FB
RT2
VSS
CT1
VDD
OVPH
VDD1
OVPL
VBATT
FCOMP
OUTA
CSDET
BATTFB
OUTAPB
OUTC
24
23
22
21
C8
20
47nF
19
D4
R7
18
OUT-1
VDD
17
30.1k
Q3-1
R20
16
C14
1000p
15
IRFR3303
IN914
Q3-2
100k
R10
680
R35
14
13
R37
R39
R38
R50
IRFR3303
R51
D19
D18
D17
D16
D5
IN914
R41
10K
C4
C5
+
0.047uF 100uF
R36
C6
C7
0.1uF
4.7uF
303
R9A
221
D6
1N5819
R52
R9B
221
R9C
221
R9D
221
VDD
D20
D21
D22
D23
R40
7.5k
30
C34
0.01u
D24
R42
D25
10k
D26
R43
10k
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AME9002
CCFL Backlight Controller
Figure 19. Double CCFL Power Section
VBatt
OUTA
Q2
T1
OUTB
OVPL/OVPH
Q3-2
Q3-1
R37
R35
R38
R36
OUTC
OVPL/OVPH
C8
Outside Chip
D4
D5
(B)
R9B
R10
(A)
(C)
D22 D23
R42
R7
D21 D20 R9A
FB
2.5V
COMP
EA1
Inside Chip
To PWM Comparator
CSDET
R43
To Fault Logic
1.25V
VDD
R40
C34
Figure 20. Double transformer construction detail
Low voltages
Secondary
Primaries
Large Positive
(Negative) Voltage
Secondary
Large Negative
(Positive) Voltage
Common Core
Low voltages in
the center
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CCFL Backlight Controller
Figure 21. Four Tube Power Section
VDD
R42
OUTA
T2
R35
R36
R37
R38
R39
R50
R51
R52
R43
D26
R9D
D24 D25
Q2
VBatt
R9C
D23
D22
R9B
OUTAPB
Q3-1
D4
T1
D5
To R7 and C8 integrator
OUTC
Q3-2
C34
R9A
To OVPL
OVPH
32
R40
R10
D20 D21
To CSDET
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CCFL Backlight Controller
Figure 22.
Normal Operation
(Filtered Voltage > 1.25V è No Fault)
NODE A
NODE B
unfiltered
1.25
NODE C
filtered
One Tube Missing Operation
(Filtered Voltage < 1.25V è Fault)
NODE A
NODE B
No Current in TUBE B
unfiltered
NODE C
1.25
filtered
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AME9002
CCFL Backlight Controller
n Package Dimension
QSOP24
Top View
SYMBOLS
MAX
MIN
MAX
A
1.524
1.752
0.060
0.069
A1
0.101
0.228
0.004
0.009
A2
Bottom View
K
Side View
ZD
A2 A
e
b
See Detail A
End View
Detail A
b1
R
L1
c
34
£ c
1
£ c
2
L
c
£
c1
(c)
(b)
0.058REF
0.203
0.304
0.008
0.012
b1
0.203
0.279
0.008
0.011
c
0.177
0.254
0.007
0.010
c1
0.177
0.228
0.007
0.009
D
8.559
8.737
0.337
0.344
0.838REF
0.033REF
E
5.791
6.197
0.228
0.244
E1
3.810
3.987
0.150
0.157
L
0.406
1.270
0.016
0.050
L1
0.254BSC
0.010BSC
e
0.635BSC
0.025BSC
J
1.27REF
0.050REF
K
1.27REF
0.050REF
è
0o
8o
0o
8o
è1
5o
15 o
5o
15o
è2
0o
-
0o
-
R
A1
1.473REF
b
ZD
J
INCHES
MIN
D
E1 E
MILLIMETERS
0.33 x 45
o
0.013 x 45 o
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AME9002
CCFL Backlight Controller
n Package Dimension
SOIC24
Top View
SYMBOLS
E
Bottom View
INCHES
MIN
MAX
MIN
MAX
A
2.35
2.65
0.092
0.104
A1
0.10
0.30
0.004
0.012
A2
2.25
2.31
0.089
0.091
B
0.33
0.51
0.013
0.020
C
0.23
0.32
0.009
0.013
D
15.20
15.60
0.598
0.614
E
7.40
7.60
0.291
0.299
H
Pin No.1 Indentifier
MILLIMETERS
1.27BSC
e
0.050BSC
H
10.00
10.65
0.394
0.419
L
0.40
1.27
0.016
0.050
θ
0
o
8
o
0
o
8o
Side View
D
A2
e
A1
B
Detail A
End View
C
See Detail A
A
c
£
L
35
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AME9002
CCFL Backlight Controller
n Package Dimension
PDIP24
Top View
SYMBOLS
R1.524 x 0.762 DP
(R0.060 x 0.030") DP
CL
E1
Side View
D
£ c
1
D1
A2
A
Base Plane
Seating Plane
L
A1
b2
End View
(Outer to Outer)
E
£ c
2
c
eB
36
b
INCHES
MIN
MAX
MIN
MAX
A
-
6.35
-
0.250
A1
0.39
-
0.015
-
A2
3.18
4.95
0.125
0.195
b
0.35
0.56
0.014
0.022
b2
0.77
1.77
0.030
0.070
C
0.20
0.39
0.008
0.015
D
29.30
32.70
1.154
1.287
D1
0.13
-
0.005
-
E
15.24
15.87
0.600
0.625
E1
12.32
14.73
0.485
0.580
e
e
MILLIMETERS
2.54BSC
eB
15.24
L
2.93
θ1
θ2
0.100BSC
17.78
0.600
5.08
0.115
0.700
0.200
7
o
o
7
7
o
7o
www.ame.com.tw
E-Mail: [email protected]
Life Support Policy:
These products of AME, Inc. are not authorized for use as critical components in life-support devices or systems, without the express written approval of the president
of AME, Inc.
AME, Inc. reserves the right to make changes in the circuitry and specifications of its devices and
advises its customers to obtain the latest version of relevant information.
 AME, Inc. , May 2004
Document: 2023-DS9002-B
Corporate Headquarter
U.S.A.(Subsidiary)
AME, Inc.
Analog Microelectronics, Inc.
2F, 302 Rui-Guang Road, Nei-Hu District
Taipei 114, Taiwan, R.O.C.
3100 De La Cruz Blvd., Suite 201
Santa Clara, CA. 95054-2046
Tel: 886 2 2627-8687
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