LT1786F
SMBus Programmable
CCFL Switching Regulator
FEATURES
■
■
■
■
■
■
■
Grounded Lamp or Floating Lamp Configurations
Precision 100µA Full-Scale DAC
Programming Current
2-Wire SMBus Interface
DAC Setting Is Retained in Shutdown
Wide Battery Input Range: 4.5V to 30V
Open Lamp Protection
Two Selectable SMBus Addresses
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APPLICATIO S
■
■
■
Notebook and Palmtop Computers
Portable Instruments
Personal Digital Assistants
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DESCRIPTIO
The LT ®1786F is a fixed frequency, current mode, switching regulator that provides the control function for Cold
Cathode Fluorescent Lighting (CCFL). The IC includes an
efficient high current switch, an oscillator, output drive
logic, control circuitry and a micropower 6-bit 100µA fullscale current output DAC. The DAC provides simple “bits-
to-lamp-current control” and communicates using the
2-wire SMBus serial interface. The LT1786F acts as an
SMBus slave device using one of two selectable SMBus
addresses set by the address pin ADR.
On Power-up, the DAC output current assumes midrange
or zero scale, depending on the logic state of the ADR
pin.The entire IC can be shut down through the SMBSUS
pin or by setting the SHDN bit = 1 in the SMBus command
byte. Digital data for the DAC output current is retained
internally and the supply current drops to 40µA for standby
operation. The active low SHDN pin disables the CCFL
control circuitry, but keeps the DAC alive. Supply current
in this operating mode drops to 150µA.
The LT1786F control circuitry operates from a logic supply
voltage of 3.3V or 5V. The IC also has a battery supply pin
that operates from 4.5V to 30V. The LT1786F draws 6mA
typical quiescent current. A 200kHz switching frequency
minimizes magnetic component size. Current mode switching techniques with cycle-by-cycle limiting gives high
reliability and simple loop frequency compensation. The
LT1786F is available in a 16-pin narrow SO package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
90% Efficient Floating CCFL with 2-Wire SMBus Control of Lamp Current
CCFL BACKLIGHT APPLICATION CIRCUITS
CONTAINED IN THIS DATA SHEET ARE COVERED
BY U.S. PATENT NUMBER 5408162
AND OTHER PATENTS PENDING
D1
BAT85
1
2
3
C7, 1µF
4
5
SHUTDOWN
6
7
8
CCFL
PGND
CCFL VSW
ICCFL
BULB
16
SHDN
IOUT
3
SMBSUS
SCL
ADR
SDA
R2
220k
+
C4
2.2µF
3V ≤ VCC
≤ 6.5V
10
9
2
+
12
11
6
L2 = COILTRONICS CTX100-4
*DO NOT SUBSTITUTE COMPONENTS
L1
15
14
BAT
LT1786F
13
CCFL VC
ROYER
VCC
10
C2
27pF
3kV
C5
1000pF
DIO
AGND
L1 = COILTRONICS CTX210605
LAMP
4
COILTRONICS (561) 241-7876
5
+
C3B
2.2µF
35V
C3A
2.2µF
35V
BAT
8V TO 28V
R1
750Ω
C1*
0.068µF
R3
100k
Q2*
TO
SMBus
HOST
ALUMINUM ELECTROLYTIC IS RECOMMENDED FOR C3A AND C3B.
MAKE 3CB ESR ≥ 0.5Ω TO PREVENT DAMAGE TO THE LT1786F HIGH-SIDE
SENSE RESISTOR DUE TO SURGE CURRENTS AT TURN-ON
C1 MUST BE A LOW LOSS CAPACITOR, C1 = WIMA MKI OR MKP-20
= PANASONIC ECH-U
1
Q1*
L2
100µH
D1
1N5818
1786F TA01
0µA TO 50µA ICCFL CURRENT GIVES
0mA TO 6mA LAMP CURRENT
FOR A TYPICAL DISPLAY.
FOR ADDITIONAL CCFL/LCD CONTRAST APPLICATION CIRCUITS,
REFER TO THE LT1182/83/84/84F DATA SHEET
Q1, Q2 = ZETEX ZTX849 OR ROHM 2SC5001
1
LT1786F
W W
W
AXI U
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ABSOLUTE
RATI GS
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W
PACKAGE/ORDER I FOR ATIO
(Note 1)
VCC ........................................................................... 7V
BAT, Royer, BULB .................................................. 30V
CCFL VSW ............................................................... 60V
Shutdown ................................................................. 6V
ICCFL Input Current .............................................. 10mA
DIO Input Current (Peak, < 100ms).................... 100mA
Digital Inputs .............................. – 0.3V to (VCC + 0.3V)
Digital Outputs ............................ – 0.3V to (VCC + 0.3V)
DAC Output Voltage ..................... – 15V to (VCC + 0.3V)
Junction Temperature (Note 2) ............................ 100°C
Operating Ambient Temperature Range ..... 0°C to 70°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
CCFL PGND 1
ICCFL 2
DIO 3
CCFL VC 4
15 BULB
14 BAT
LT1786FCS
13 ROYER
AGND 5
12 VCC
SHDN 6
11 IOUT
SMBSUS 7
10 SCL
ADR 8
ORDER PART
NUMBER
16 CCFL VSW
9
SDA
S PACKAGE
16-LEAD PLASTIC SO
TJMAX = 100°C, θJA = 100°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = SHUTDOWN = SMBSUS = SCL = SDA = 3.3V,
BAT = Royer = BULB = 12V, I CCFL = CCFL VSW = Open, DIO = IOUT = GND, CCFL VC = 0.5V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
TYP
MAX
UNITS
IQ
Supply Current
3V ≤ VCC ≤ 6.5V, IOUT = 0µA
●
6
9.5
mA
ISUS
SMBSUS Supply Current
SMBSUS = 0V or Command Code Bit 7 = 1,
CCFL VC = Open (Note 3)
●
40
100
µA
ISHDN
SHUTDOWN Supply Current
SHUTDOWN = 0V, CCFL VC = Open (Note 3)
●
150
300
µA
SHUTDOWN Input Bias Current
SHUTDOWN = 0V, CCFL VC = Open
5
10
µA
SHUTDOWN Threshold Voltage
f
DC(MAX)
BV
Switching Frequency
Maximum Switch Duty Cycle
Measured at CCFL VSW, ISW = 50mA,
ICCFL = 100µA, CCFL VC = Open
MIN
●
0.45
0.85
1.2
V
●
175
160
200
200
225
240
kHz
kHz
●
80
75
85
85
%
%
60
70
V
Measured at CCFL VSW
Switch Breakdown Voltage
Measured at CCFL VSW
Switch Leakage Current
VSW = 12V, Measured at CCFL VSW
VSW = 30V, Measured at CCFL VSW
ICCFL Summing Voltage
3V ≤ VCC ≤ 6.5V
●
2
20
40
µA
µA
0.465
0.465
0.505
0.555
V
V
5
15
mV
–5
5
15
µA
0.425
0.385
∆ICCFL Summing Voltage for
∆Input Programming Current
ICCFL = 0µA to 100µA
CCFL VC Offset Sink Current
CCFL VC = 1.5V, Positive Current Measured into Pin
∆CCFL VC Source Current for
∆ICCFL Programming Current
ICCFL = 25µA, 50µA, 75µA, 100µA,
CCFL VC = 1.5V
●
4.70
4.95
5.20
µA/µA
CCFL VC to DIO Current Servo Ratio
DIO = 5mA out of Pin, Measure I(VC) at CCFL VC = 1.5V
●
94
99
104
µA/mA
CCFL VC Low Clamp Voltage
VBAT – VBULB = BULB Protect Servo Voltage
●
0.1
0.3
V
CCFL VC High Clamp Voltage
ICCFL = 100µA
●
1.7
2.1
2.4
V
CCFL VC Switching Threshold
CCFL VSW DC = 0%
●
0.6
0.95
1.3
V
CCFL High-Side Sense Servo Current
ICCFL = 100µA, I(VC) = 0µA at CCFL VC = 1.5V
●
0.93
1.00
1.07
A
LT1786F
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = SHUTDOWN = SMBSUS = SCL = SDA = 3.3V,
BAT = Royer = BULB = 12V, I CCFL = CCFL VSW = Open, DIO = IOUT = GND, CCFL VC = 0.5V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
CCFL High-Side Sense Servo Current
Line Regulation
BAT = 5V to 30V, ICCFL = 100µA,
I(VC) = 0µA at CCFL VC = 1.5V
TYP
MAX
UNIT
0.1
0.16
%/V
CCFL High-Side Sense Supply Current Current Measured into BAT and Royer Pins
●
50
100
150
µA
BULB Protect Servo Voltage
ICCFL = 100µA, I(VC) = 0µA at CCFL VC = 1.5V,
Servo Voltage Measured between BAT and BULB Pins
●
6.5
7.0
7.5
V
BULB Input Bias Current
ICCFL = 100µA, I(VC) = 0µA at CCFL VC = 1.5V
5
9
µA
ILIM
CCFL Switch Current Limit
Duty Cycle = 50%
Duty Cycle = 75% (Note 4)
●
●
1.9
1.6
3.0
2.6
A
A
VSAT
CCFL Switch On Resistance
CCFL ISW = 1A
●
0.6
1.0
Ω
∆IQ
∆ISW
Supply Current Increase During
CCFL Switch On Time
CCFL ISW = 1A
20
30
mA/A
1.25
0.9
DAC Resolution
DAC Full-Scale Current
6
V(IOUT) = 0.465V
●
DAC Zero Scale Current
V(IOUT) = 0.465V
98
96
100
100
●
DAC Differential Nonlinearity
●
±0.1
0.2
Bits
102
104
µA
µA
±200
nA
±1
LSB
2
LSB
±1
µA
DAC Supply Voltage Rejection
3V ≤ VCC ≤ 6.5V, IOUT = Full Scale, V(IOUT) = 0.465V
●
IIN
Logic Input Current
0V ≤ VIN ≤ VCC
●
VIH
High Level Input Voltage
ADR
SMBSUS
SCL, SDA
●
●
●
VIL
Low Level Input Voltage
SMBSUS, ADR
SCL, SDA
●
●
0.8
0.6
V
V
VOL
Low Level Output Voltage
IOUT = 3mA, SDA Only
IOUT = 1.6mA, SMBSUS = 0V, Measured at SHDN Pin
●
●
0.4
0.4
V
V
100
kHz
VCC – 0.3
2.4
1.4
V
V
V
SMBus Timing (Notes 5, 6)
fSMB
SMB Operating Frequency
●
10
tBUF
Bus Free Time Between Stop and Start Condition
●
4.7
µs
tHD:STA
Hold Time After (Repeated) Start Condition
●
4.0
µs
tSU:STA
Repeated Start Condition Setup Time
●
4.7
µs
tSU:STO
Stop Condition Setup Time
●
4.0
µs
tHD:DAT
Data Hold Time
●
300
ns
tSU:DAT
Data Setup Time
●
250
ns
tLOW
Clock Low Period
●
4.7
µs
tHIGH
Clock High Period
●
4.0
tf
Clock/Data Fall Time
tr
Clock/Data Rise Time
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LT1786FCS: TJ = TA + (PD)(100°C/W)
Note 3: Does not include switch leakage.
50
µs
●
300
ns
●
1000
ns
Note 4: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by ILIM = 1.4(1.393 – DC) for the
LT1786F due to internal slope compensation circuitry.
Note 5: Timings for all signals are referenced to VIH and VIL signals.
Note 6: These parameters are guaranteed by design and are not tested in
production. Refer to the Timing Diagrams for additional information.
3
LT1786F
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TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current
vs Temperature
8
8O
7
70
6
60
5
4
VCC = 5V
40
3
30
20
1
10
VCC = 3V
–5
1.1
VCC = 3V
150
1
VCC = 3V
120
90
60
0
– 55 – 25
25 50 75 100 125 150
TEMPERATURE (°C)
–5
25 50 75 100 125 150
TEMPERATURE (°C)
1786 G03
Frequency vs Temperature
240
230
CCFL FREQUENCY (kHz)
5
SHDN THRESHOLD VOLTAGE (V)
1.0
0.9
0.8
220
210
200
190
180
0.7
170
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
0.6
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G04
1786 G06
ICCFL Summing Voltage
vs Temperature
95
ICCFL SUMMING VOLTAGE (V)
93
91
89
87
85
83
81
79
77
75
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G07
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G05
Maximum Duty Cycle
vs Temperature
4
160
–75 –50 –25
ICCFL Summing Voltage
Load Regulation
0.53
0.52
0.51
0.50
0.49
0.48
0.47
0.46
0.45
0.44
0.43
0.42
0.41
0.40
0.39
0.38
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G08
∆ICCFL SUMMING VOLTAGE (mV)
SHDN INPUT BIAS CURRENT (µA)
1.2
2
VCC = 5V
180
SHDN Threshold Voltage
vs Temperature
6
VCC = 5V
210
1786 G02
SHDN Input Bias Current
vs Temperature
3
240
30
0
–55 –25
1786 G01
4
SHDN = 0V
270
50
2
300
SMBSUS = 0V
SHDN SUPPLY CURRENT (µA)
90
ISUS (µA)
100
9
SUPPLY CURRENT (mA)
10
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
CCFL MAXIMUM DUTY CYCLE (%)
SHDN Supply Current
vs Temperature
ISUS Current vs Temperature
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
T = –55°C
T = 25°C
T = 125°C
0
20 40 60 80 100 120 140 160 180 200
ICCFL PROGRAMMING CURRENT (µA)
1786 G09
LT1786F
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TYPICAL PERFORMANCE CHARACTERISTICS
∆CCFL VC Source Current for
∆ICCFL Programming Current
vs Temperature
VC Sink Offset Current
vs Temperature
CCFL VC = 1.0V
4
3
2
1
CCFL VC = 0.5V
0
–1
–2
–3
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1.2
5.05
1.0
ICCFL = 100µA
5.00
4.95
ICCFL = 50µA
ICCFL = 10µA
4.90
4.85
4.80
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
I(DIO) = 10mA
1.2
I(DIO) = 5mA
I(DIO) = 1mA
0.6
0.4
0.2
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
VC Low Clamp Voltage
vs Temperature
0.30
102
101
100
I(DIO) = 10mA
I(DIO) = 1mA
99
I(DIO) = 5mA
98
97
96
95
–75 – 50 –25
0.25
0.20
0.15
0.10
0.05
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G15
1786 G14
VC Switching Threshold
vs Temperature
2.3
2.2
2.1
2.0
1.9
1.8
1.7
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G16
CCFL VC SWITCHING THRESHOLD VOLTAGE (V)
VC High Clamp Voltage
vs Temperature
CCFL VC HIGH CLAMP VOLTAGE (V)
0.4
1786 G12
103
1786 G13
2.4
I(DIO) = 1mA
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
CCFL VC LOW CLAMP VOLTAGE (V)
CCFL VC DIO CURRENT SERVO RATIO (µA/mA)
NEGATIVE DIO VOLTAGE (V)
1.4
0
–75 –50 –25
0.6
VC to DIO Current Servo Ratio
vs Temperature
1.6
I(DIO) = 5mA
1786 G11
Negative DIO Voltage
vs Temperature
0.8
I(DIO) = 10mA
0.8
0.2
1786 G10
1.0
POSITIVE DIO VOLTAGE (V)
CCFL VC = 1.5V
5.10
BULB Protect Servo Voltage
vs Temperature
7.5
1.3
BULB PROTECT SERVO VOLTAGE (V)
9
8
7
6
5
∆CCFL VC SOURCE CURRENT FOR
∆ICCFL PROGRAMMING CURRENT (µA/µA)
CCFL VC SINK OFFSET CURRENT (µA)
10
Positive DIO Voltage
vs Temperature
1.2
1.1
1.0
0.9
0.8
0.7
0.6
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G17
7.4
7.3
7.2
7.1
ICCFL = 100µA
7.0
6.9
6.8
ICCFL = 50µA
6.7
6.6
ICCFL = 10µA
6.5
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G18
5
LT1786F
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TYPICAL PERFORMANCE CHARACTERISTICS
High-Side Sense Supply Current
vs Temperature
6
4
2
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
150
CCFL HIGH-SIDE SENSE NULL CURRENT (A)
8
140
130
120
110
100
90
80
70
60
50
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1787 G19
0.140
0.9
CCFL VSW SAT VOLTAGE (V)
CCFL HIGH-SIDE SENSE LINE REGULATI0N (%V)
1.0
0.080
0.060
0.040
0.020
1.020
1.000
0.980
0.960
0.940
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G21
VSW Current Limit vs Duty Cycle
2.5
0.8
T = 25°C
0.7
T = 125°C
0.6
T = –5°C
0.5
0.4
0.3
0.2
2.0
T = 0°C
T = 25°C
1.5
T = 125°C
MINIMUM
1.0
0.5
0.1
0.000
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
0
0
0.3
0.9
1.2
0.6
SWITCH CURRENT (A)
0
10
20
30 40 50 60
DUTY CYCLE (%)
104
110
FULL-SCALE OUTPUT CURRENT (µA)
100
90
80
70
60
50
40
30
20
10
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
CCFL ISW (A)
1786 G25
V(IOUT) = 0.465V
103
102
101
100
99
98
97
96
–75 –50 –25
70
80
90
1786 G24
Full-Scale Output Current
vs Temperature
Forced Beta vs ISW on VSW
FORCED BETA
0
1.5
1787 G23
1786 G22
6
1.040
VSW Sat Voltage
vs Switch Current
0.160
0.100
1.060
1786 G20
High-Side Sense Null Current Line
Regulation vs Temperature
0.120
High-Side Sense Null Current
vs Temperature
CCFL VSW CURRENT LIMIT (A)
BULB INPUT BIAS CURRENT (µA)
10
CCFL HIGH-SIDE SENSE SUPPLY CURRENT (µA)
BULB Input Bias Current
vs Temperature
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G26
LT1786F
U W
TYPICAL PERFORMANCE CHARACTERISTICS
INL vs Code
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
INL (LSB)
DNL (LSB)
DNL vs Code
1.0
0
–0.2
0
–0.2
–0.4
–0.4
–0.6
–0.6
–0.8
–0.8
–1.0
–1.0
16
0
32
CODE
48
64
1786 G27
0
16
32
CODE
48
64
1786 G28
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U
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PIN FUNCTIONS
CCFL PGND (Pin 1): This pin is the emitter of an internal
NPN power switch. CCFL switch current flows through
this pin and permits internal, switch-current sensing. The
regulator provides a separate analog ground and power
ground to isolate high current ground paths from low
current signal paths. Linear Technology recommends the
use of star-ground layout techniques.
ICCFL (Pin 2): This pin is the input to the CCFL lamp current
programming circuit. This pin internally regulates to
465mV. The pin accepts a DC input current signal of 0µA
to 100µA full scale from the DAC. This input signal is
converted to a 0µA to 500µA source current at the CCFL VC
pin. As input programming current increases, the regulated lamp current increases.
DIO (Pin 3): This pin is the common connection between
the cathode and anode of two internal diodes. The remaining terminals of the two diodes connect to ground. In a
grounded-lamp configuration, DIO connects to the low
voltage side of the lamp. Bidirectional lamp current flows
in the DIO pin and thus the diodes conduct alternately on
half cycles. Lamp current is controlled by monitoring onehalf of the average lamp current. The diode conducting on
negative half cycles has one-tenth of its current diverted to
the CCFL VC pin. This current nulls against the source
current provided by the lamp-current programmer circuit.
A single capacitor on the CCFL VC pin provides both stable
loop compensation and an averaging function to the halfwave-rectified sinusoidal lamp current. Therefore, input
programming current relates to one-half of average lamp
current. This scheme reduces the number of loop compensation components and permits faster loop transient
response in comparison to previously published circuits.
If a floating lamp configuration is used, ground the DIO
pin.
CCFL VC (Pin 4): This pin is the output of the lamp current
programmer circuit and the input of the current comparator for the CCFL regulator. Its uses include frequency
compensation, lamp-current averaging for grounded-lamp
circuits and current limiting. The voltage on the CCFL VC
pin determines the current trip level for switch turn-off.
During normal operation this pin sits at a voltage between
0.95V (zero switch current) and 2.1V (maximum switch
current) with respect to analog ground (AGND). This pin
has a high impedance output and permits external voltage
clamping to adjust current limit. A single capacitor to
ground provides stable loop compensation. This simplified loop compensation method permits the CCFL regulator to exhibit single-pole transient response behavior and
virtually eliminates transformer output overshoot.
7
LT1786F
U
U
U
PIN FUNCTIONS
AGND (Pin 5): This is the low current analog ground. It is
the negative sense terminal for the internal 1.24V reference and the ICCFL summing voltage in the LT1786F.
Connect low current signal paths that terminate to ground
and frequency compensation components that terminate
to ground directly to this pin for best regulation and
performance.
pin is tied to GND, the SMBus address is set to 5A (HEX)
and the DAC IOUT powers up to zero scale. If the ADR pin
is tied to VCC, the SMBus address is set to 58 (HEX) and
the DAC IOUT powers up to half scale. If a different value is
required for the DAC IOUT on power-up, use the SHDN pin
to keep the CCFL regulator off until the required value has
been programmed for the DAC via the SMBus.
SHDN (Pin 6): Pulling this pin low causes regulator
shutdown with quiescent current typically reduced to
150µA. In this condition, the DAC circuitry remains alive
and the DAC IOUT level is maintained. If this pin is not used,
use a pull-up resistor to force a logic high level (maximum
of 6V). The pin can be floated and an internal current
source will pull the pin to a logic high level. However, poor
PCB layout techniques can permit switching noise to inject
into this pin and cause erratic operation. LTC recommends
the use of a pull-up resistor. If the SMBSUS pin is pulled
low or Bit 7 = 1 in the Command Byte, complete IC
shutdown is enabled. An internal open drain N-channel
device turns on and pulls the SHDN pin low. The N-channel
can sink up to 1.6mA.
SDA (Pin 9): This is the SMBus bidirectional data input and
digital output pin. Data is shifted into the SDA pin and
acknowledged by the SDA pin. SDA is a high impedance
pin while data is shifted into the pin and an open-drain
N-channel output during acknowledges. SDA requires a
pull-up resistor or current source to VCC.
SMBSUS (Pin 7): Pulling this pin low causes complete
shutdown for the IC with quiescent current typically
reduced to 40µA. In this SMBus suspend condition, the
DAC retains its last output current setting and returns to
this level when the logic low signal at this pin is removed.
If this pin is not used, use a pull-up resistor to force a
logic high level or tie it directly to VCC. Poor PCB layout
techniques can permit switching noise to inject into this
pin and cause erratic operation. A small value capacitor
may be required to filter out this noise. Setting Bit 7 = 1
in the Command Byte also enables an SMBus suspend
condition. Enabling an SMBus suspend condition turns
on an internal open drain N-channel device which pulls
the SHDN pin low. The N-channel device sinks up to
1.6mA at the SHDN pin.
ADR (Pin 8): This is the SMBus address select pin. Tie this
pin to either VCC or GND to select one of two SMBus
addresses to which the LT1786F will respond. If the ADR
8
SCL (Pin 10): This is the SMBus clock input pin. Data is
shifted into the SDA pin at the rising edges of the SCL clock
during data transfer. SCL is a high impedance pin. SCL
requires a pull-up resistor or current source to VCC.
IOUT (Pin 11): This pin is the current output for the DAC and
provides a full-scale output current of 100µA ±4µA over
temperature. Initial accuracy is 100µA ±2µA.The pin can
be biased from – 10V to (VCC – 1.3V). This pin is typically
tied directly to the ICCFL pin and provides the programming
current which sets the operating lamp current. The IOUT
pin has very little bias voltage change when tied to the ICCFL
pin as ICCFL is regulated. The programming current is
sourced from the IOUT pin and sunk by the ICCFL pin.
VCC (Pin 12): This is the supply pin for the LT1786F. The
IC accepts an input voltage range of 3V minimum to 6.5V
maximum with little change in quiescent current (zero
switch current). An internal, low-dropout regulator provides a 2.4V supply for most of the internal circuitry.
Supply current increases as switch current increases at a
rate approximately 1/50 of switch current. This corresponds to a forced Beta of 50 for the power switch. The IC
incorporates undervoltage lockout by sensing regulator
dropout and locking out switching for input voltages
below 2.5V. Hysteresis is not used to maximize the range
of input voltage. The typical input voltage is a 3.3V or 5V
logic supply.
LT1786F
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ROYER (Pin 13): This pin connects to the center-tapped
primary of the Royer converter and is used with the BAT
pin in a floating-lamp configuration where lamp current is
controlled by sensing Royer primary-side converter current. This pin is the inverting terminal of a high-side
current sense amplifier. The typical quiescent current is
50µA into the pin. If the CCFL regulator is not used in a
floating-lamp configuration, tie the Royer and BAT pins
together.
BAT (Pin 14): This pin connects to the battery or AC wall
adapter voltage from which the CCFL Royer converter
operates. This voltage is typically higher than the VCC
supply voltage but can equal VCC if VCC is a 5V logic supply.
The BAT voltage must be at least 2.1V greater than the
internal 2.4V regulator or 4.5V. This pin provides biasing
for the lamp-current programming block, is used with the
Royer pin for floating-lamp configurations and connects
to one input for the open-lamp protection circuitry. For
floating-lamp configurations, this pin is the noninverting
terminal of a high-side current sense amplifier. The typical
quiescent current is 50µA into the pin. The BAT and Royer
pins monitor the primary-side Royer converter current
through an internal 0.1Ω topside current sense resistor. A
0A to 1A primary-side, center tap converter current is
translated to an input signal range of 0mV to 100mV for the
current sense amplifier. This input range translates to a
0µA to 500µA sink current at the CCFL VC pin that nulls
against the source current provided by the programmer
circuit. The BAT pin also connects to the topside of the
internal clamp between the BAT and BULB pins that is used
for open-lamp protection.
BULB (Pin 15): This pin connects to the low side of a 7V
threshold comparator between the BAT and BULB pins.
This circuit sets the maximum voltage level across the
primary side of the Royer converter under all operating
conditions and limits the maximum secondary output
under start-up conditions or open-lamp conditions. This
eases transformer voltage rating requirements. Set the
voltage limit to ensure lamp start-up with worst-case,
lamp start voltages and cold temperature, system operating conditions. The BULB pin connects to the junction of
an external divider network. The divider network connects
from the center tap of the Royer transformer or the actual
battery supply voltage to the topside of the current source
“tail inductor.” A capacitor across the top of the divider
network filters switching ripple and sets a time constant
that determines how quickly the clamp activates. When
the comparator activates, sink current is generated to pull
the CCFL VC pin down. This action transfers the entire
regulator loop from current mode operation into voltage
mode operation.
CCFL VSW (Pin 16): This pin is the collector of the internal
NPN power switch for the CCFL regulator. The power
switch provides a minimum of 1.25A. Maximum switch
current is a function of duty cycle as internal slope compensation ensures stability with duty cycles greater than
50%. Using a driver loop to automatically adapt base drive
current to the minimum required to keep the switch in a
quasi-saturation state yields fast switching times and high
efficiency operation. The ratio of switch current to driver
current is about 50:1.
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BLOCK DIAGRAM
LT1786F SMBus Programmable CCFL Switching Regulator
VCC
BAT
14
12
SHDN 6
SHUTDOWN
UNDERVOLTAGE
LOCKOUT
2.4V
REGULATOR
ROYER
13
THERMAL
SHUTDOWN
CCFL
VSW
16
200kHz
OSC
LOGIC
R4
0.1Ω
D2
6V
ANTISAT
+
–
gm
COMP
R3
1k
0µA TO 100µA FROM IOUT
Q5
1×
Q6
2×
+
Q4
5×
CCFL
Q11
–
–
GAIN = 4.4
Q8
1×
Q7
9×
Q10
2×
Q9
3×
V1
0.465V
R1
0.125Ω
CURRENT
AMP
Q3
2×
+
Q1
DRIVE
D1
2
5
3
15
4
1
ICCFL
AGND
DIO
BULB
CCFL
VC
CCFL
PGND
SMBSUS
7
SD
POWER-ON
RESET
SHUTDOWN
SCL 10
SDA 9
SMBus
INTERFACE
1
1
REGISTER A
3-BIT LATCH
EN1
SD
VOLTAGE
REFERENCE
REGISTER B
1-BIT LATCH
EN2
RADJ
SD
SD
6
8
ADR
10
REGISTER C
6-BIT LATCH
EN2
6
6-BIT
CURRENT
DAC
11 IOUT
1786 BD
LT1786F
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Timing for SMBus Interface
t BUF
SDA
t HD:STA
t HD:STA
tf
tr
SCL
t LOW
STOP
t HIGH
START
t SU:STO
t SU:STA
t HD:DAT
t SU:DAT
1786 TD01
START
STOP
Operating Sequence
SMBus Write Byte Protocol, with SMBus Address = 0101101B,
Command Byte = 0XXXXXXXB and Data Byte = 111111XXB
VCC
ADR
GND
SCL
S
1
2
3
1
4
1
5
0
6
1
7
8
9
X
X
X
X
X
X
X
1
1
1
1
1
1
X
X
ACK
0
DATA BYTE
ACK
1
SHDN
0
ACK
SDA
COMMAND BYTE
WR
SMBUS ADDRESS
*
P
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
IOUT
1786
S = START
P = STOP
* = OPTIONAL
TD02
FULL-SCALE
CURRENT
ZERO-SCALE
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Introduction
Current generation portable computers and instruments
use backlit Liquid Crystal Displays (LCDs). Cold Cathode
Fluorescent Lamps (CCFLs) provide the highest available
efficiency in back lighting the display. Providing the most
light out for the least amount of input power is the most
important goal. These lamps require high voltage AC to
operate, mandating an efficient high voltage DC/AC converter. The lamps operate from DC, but migration effects
damage the lamp and shorten its lifetime. Lamp drive
should contain zero DC component. In addition to good
efficiency, the converter should deliver the lamp drive in
the form of a sine wave. This minimizes EMI and RF
emissions. Such emissions can interfere with other
devices and can also degrade overall operating efficiency.
Sinusoidal CCFL drive maximizes current-to-light conversion in the lamp. The circuit should also permit lamp
intensity control from zero to full brightness with no
hysteresis or “pop-on.”
The small size and battery-powered operation associated
with LCD equipped apparatus dictate low component
count and high efficiency for these circuits. Size constraints place severe limitations on circuit architecture and
long battery life is a priority. Laptop and handheld portable
computers offer an excellent example. The CCFL and its
power supply can be responsible for almost 50% of the
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battery drain. Additionally, all components, including PC
board and hardware, usually must fit within the LCD
enclosure with a height restriction of 5mm to 10mm.
The CCFL regulator drives an inductor that acts as a
switched-mode current source for a current-driven Royerclass converter with efficiencies as high as 90%. The
control loop forces the CCFL PWM to modulate the average inductor current to maintain constant current in the
lamp. The constant current value, and thus lamp intensity,
is programmable. This drive technique provides a wide
range of intensity control. A unique lamp-current programming block permits either grounded lamp or floating
lamp configurations. Grounded lamp circuits directly sense
one-half of average lamp current. Floating lamp circuits
directly sense the Royer’s primary-side converter current.
Floating-lamp circuits provide symmetric differential drive
to the lamp and reduce the parasitic loss from stray lampto-frame capacitance, extending illumination range.
supply current to 150µA by shutting off the 2.4V regulator
and locks out switching action for standby operation. The
IC incorporates undervoltage lockout by sensing regulator
dropout and locking out switching below about 2.5V. The
regulator also provides thermal shutdown protection that
locks out switching in the presence of excessive junction
temperatures.
A 200kHz oscillator is the basic clock for all internal timing.
The oscillator turns on the output switch via its own logic
and driver circuitry. Adaptive anti-sat circuitry detects the
onset of saturation in the power switch and adjusts base
drive current instantaneously to limit switch saturation.
This minimizes driver dissipation and provides rapid turnoff of the switch. The CCFL power switch is guaranteed to
provide a minimum of 1.25A in the LT1786F. The antisat
circuitry provides a ratio of switch current to driver current
of about 50:1.
Digital Interface
Block Diagram Operation
The LT1786F is a fixed frequency, current mode switching
regulator. A fixed frequency, current mode switcher controls switch duty cycle directly by switch current rather
than by output voltage. Referring to the block diagram for
the LT1786F, the switch turns ON at the start of each
oscillator cycle. The switch turns OFF when switch current
reaches a predetermined level. The control of output lamp
current is obtained by using the output of a unique
programming block to set current trip level. The current
mode switching technique has several advantages. First,
it provides excellent rejection of input voltage variations.
Second, it reduces the 90° phase shift at mid-frequencies
in the energy storage inductor. This simplifies closed-loop
frequency compensation under widely varying input
voltage or output load conditions. Finally, it allows simple
pulse-by-pulse current limiting to provide maximum
switch protection under output overload or short-circuit
conditions.
The LT1786F incorporates a low dropout internal regulator that provides a 2.4V supply for most of the internal
circuitry. This low dropout design allows input voltage to
vary from 3V to 6.5V with little change in quiescent
current. An active low shutdown pin typically reduces total
12
The LT1786F communicates with an SMBus host using
the standard 2-wire SMBus interface. The Timing Diagram
shows the signals on the SMBus. The two bus lines SDA
and SCL must be high when the bus is not in use. External
pull-up resistors or current sources are required at these
lines.
The LT1786F is a receive-only (slave) device. The master
must apply the following Write Byte protocol to communicate with the LT1786F:
1
7
1
S Slave Address WR
1
8
1
A Command Byte A
8
1
1
Data Byte
A
P
S = Start Conditon, WR = Write Bit, A = Acknowledge Bit, P = Stop Condition
The master initiates communication with the LT1786F
with a START condition (see SMBus Operating Sequence)
and a 7-bit address followed by the write bit = 0. The
LT1786F acknowledges and the master delivers the
command byte. The LT1786F acknowledges and latches
the active bits of the command byte into register A (see
Block Diagram) at the falling edge of the acknowledge
pulse. The master sends the data byte and the LT1786F
acknowledges the data byte. The data byte is latched into
register C at the falling edge of the final acknowledge pulse
and the DAC current output assumes the new 6-bit data
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value (see Block Diagram). A STOP condition is optional.
The command code and data byte are defined with the
following format:
Command Code
7
Data Byte
6 5 4 3 2 1 0
SHDN X X X X X X X
7
6
5
4
3
2
1
0
D5 D4 D3 D2 D1 D0 X
X
SHDN: 0 for normal operation, 1 for shutdown
D5 to D0: DAC Data Byte Bits, D5 is the Most Significant Bit
START and STOP Conditions
At the beginning of any SMBus communication, the master must transmit a START condition by switching SDA
from high to low while SCL is high. When a master has
finished communicating with a slave device, a STOP
condition is issued by switching SDA from low to high
while SCL is high. The SMBus is then free for communication with another SMBus device.
Early STOP Conditions
The LT1786F recongnizes a STOP condition at any point in
the SMBus communication sequence. If the STOP occurs
prematurely before the data byte is acknowledged in the
Write Byte protocol, the DAC output current value is not
updated; otherwise internal register C is updated with the
new data and the DAC output current changes
correspondingly.
The Slave Address
The LT1786F responds to one of two 7-bit addresses. The
first five bits have been factory programmed to 01011. The
last two address bits are programmed by the user by tying
the ADR pin to VCC or GND (see functional table)
ADR
SMBus ADDRESS
DAC POWER-UP VALUE
GND
0101101
Zero Scale
VCC
0101100
Midscale
6-Bit Current Output DAC
The 6-bit current output DAC is guaranteed monotonic and
is digitally adjustable in 63 equal steps. On power-up, if
ADR connects to VCC, the 6-bit internal register C (see
Block Diagram) resets to 100000B and the DAC output is
set to midrange. On power-up, if ADR connects to ground,
register C resets to 000000B and the DAC output is set to
zero. For the LT1786F, the current source output (IOUT)
can be biased from – 10V to (VCC – 1.3V). Full-scale
current is trimmed to ±2% at room temperature and ±4%
over the commercial temperature range.
Shutdown
Three methods may be employed to shut down the LT1786F
(see Block Diagram).
The LT1786F enters SMBus suspend mode if a logic low
level is applied to the SMBSUS pin or a logic high level is
applied to Bit 7 in the Command Byte of the SMBus
communication sequence. In SMBus suspend mode, supply current typically drops to 40µA and the last output
current setting is internally retained. The DAC resumes
this level upon its return to normal operation. Enabling an
SMBus suspend condition also turns on an open-drain
N-channel MOSFET which pulls the SHDN pin low. The
N-channel device sinks up to 1.6mA at the SHDN pin and
its logic low level is guaranteed to less than 0.4V.
The LT1786F enters regulator shutdown mode if a logic
low level is applied to the SHDN pin. In this mode, supply
current typically drops to 150µA, the switching regulator
circuitry is shut down and the DAC is kept alive. The DAC
output current setting is maintained. This feature can be
used to program the DAC to a desired output current level
(other than the preset zero-scale or midscale level defined
by the selected SMBus address) before allowing the CCFL
regulator to turn on.
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LT1786F SMBus Lookup Tables
SMBus Address Byte Table
SMBus Command Byte Table
ADR
DECIMAL
BINARY
HEX
IOUT POWER-UP VALUE
DECIMAL
BINARY
HEX
MODE
GND
90
0101101
5A
Zero Scale
0-127
0XXXXXXX
00-7F
Normal
VCC
88
0101100
58
Midscale
128-255
1XXXXXXX
80-FF
Shutdown
Bit 0 (LSB) in the SMBus Address is the Write Bit = 0
X = Don’t Care
SMBus Data Byte Table
DECIMAL
BINARY
HEX
IOUT (µA)
DECIMAL
BINARY
HEX
IOUT (µA)
0
000000XX
00-03
0.000
32
100000XX
80-83
50.794
1
000001XX
04-07
1.587
33
2
000010XX
08-0B
3.175
34
100001XX
84-87
52.381
100010XX
88-8B
53.968
3
000011XX
0C-0F
4.762
35
100011XX
8C-8F
55.556
4
000100XX
10-13
6.349
5
000101XX
14-17
7.937
36
100100XX
90-93
57.143
37
100101XX
94-97
58.730
6
000110XX
18-1B
9.524
38
100110XX
98-9B
60.317
7
000111XX
8
001000XX
1C-1F
11.111
39
100111XX
9C-9F
61.905
20-23
12.698
40
101000XX
A0-A3
63.492
9
001001XX
24-27
14.286
41
101001XX
A4-A7
65.079
10
001010XX
11
001011XX
28-2B
15.873
42
101010XX
A8-AB
66.667
2C-2F
17.460
43
101011XX
AC-AF
68.254
12
001100XX
30-33
19.048
44
101100XX
B0-B3
69.841
13
001101XX
34-37
20.635
45
101101XX
B4-B7
71.429
14
001110XX
38-3B
22.222
46
101110XX
B8-BB
73.016
15
001111XX
3C-3F
23.810
47
101111XX
BC-BF
74.603
16
010000XX
40-43
25.397
48
110000XX
C0-C3
76.190
17
010001XX
44-47
26.984
49
110001XX
C4-C7
77.778
18
010010XX
48-4B
28.571
50
110010XX
C8-CB
79.365
19
010011XX
4C-4F
30.159
51
110011XX
CC-CF
80.952
20
010100XX
50-53
31.746
52
110100XX
D0-D3
82.540
21
010101XX
54-57
33.333
53
110101XX
D4-D7
84.127
22
010110XX
58-5B
34.921
54
110110XX
D8-DB
85.714
23
010111XX
5C-5F
36.508
55
110111XX
DC-DF
87.302
24
011000XX
60-63
38.095
56
111000XX
E0-E3
88.889
25
011001XX
64-67
39.683
57
111001XX
E4-E7
90.476
26
011010XX
68-6B
41.270
58
111010XX
E8-EB
92.063
27
011011XX
6C-6F
42.857
59
111011XX
EC-EF
93.651
28
011100XX
70-73
44.444
60
111100XX
F0-F3
95.238
29
011101XX
74-77
46.032
61
111101XX
F4-F7
96.825
30
011110XX
78-7B
47.619
62
111110XX
F8-FB
98.413
31
011111XX
7C-7F
49.206
63
111111XX
FC-FF
100.000
X = Don’t Care, IOUT = Ideal Value
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Simplified Lamp Current Programming
A programming block in the LT1786F controls lamp
current, permitting either grounded lamp or floating lamp
configurations. Grounded configurations control lamp
current by directly controlling one-half of actual lamp
current and converting it to a feedback signal to close a
control loop. Floating configurations control lamp current
by directly controlling the Royer’s primary-side converter
current and generating a feedback signal to close a control
loop.
Previous backlighting solutions have used a traditional
error amplifier in the control loop to regulate lamp current.
This approach converted an RMS current into a DC voltage
for the input of the error amplifier. This approach used
several time constants in order to provide stable loop
frequency compensation. This compensation scheme
meant that the loop had to be fairly slow and that output
overshoot with start-up or overload conditions had to be
carefully evaluated in terms of transformer stress and
breakdown voltage requirements.
The LT1786F eliminates the error amplifier concept
entirely and replaces it with a lamp current programming
block. This block provides an easy-to-use interface to
program lamp current. The programmer circuit also
reduces the number of time constants in the control loop
by combining the error signal conversion scheme and
frequency compensation into a single capacitor. The control loop thus exhibits the response of a single pole
system, allows for faster loop transient response and
virtually eliminates overshoot under start-up or overload
conditions.
Lamp current is programmed at the input of the programmer block, the ICCFL pin. This pin is the input of a shunt
regulator and accepts a DC input current signal of 0µA to
100µA from the DAC. This input signal is converted to a
0µA to 500µA source current at the CCFL VC pin. The
programmer circuit is simply a current-to-current converter with a gain of five.
The ICCFL pin is sensitive to capacitive loading and will
oscillate with capacitance greater than 10pF. For example,
loading the ICCFL pin with a 1× or 10× scope probe causes
oscillation and erratic CCFL regulator operation because
of the probe’s respective input capacitance. A current
meter in series with the ICCFL pin will also produce oscillation due to its shunt capacitance. Use a decoupling
resistor of several kilohms between the ICCFL pin and the
IOUT pin if excessive trace stray capacitance exists. Normally, this resistor is not required.
In some applications, the maximum programming current
required at the ICCFL pin for a maximum lamp current will be
less than the full-scale output current of the DAC, which is
100µA. The system designer can either limit the maximum
programming current through software built into the system,
or use a current splitter which shunts a percentage of the fullscale current from the ICCFL pin. A splitter circuit is illustrated
in Figure 1. A divider string is used from a reference voltage
to set up a voltage level equal to the ICCFL summing voltage,
or 465mV. The main current flowing in the divider string
should be chosen to swamp out the effects of the shunted
current into the divider string.
IOUT FULL-SCALE
100µA
R1
V(ICCFL)
465mV
V1
XI
VREF
I
R2
I1
R3
V(ICCFL)
(1 – X)I
R4
I = 100µA
0 (1 – X)I
R1 = (V1 – 0.465)/[(X)(100µA)]
R2 = (V1 – 0.465)/[(1 – X)(100µA)]
R3 = (VREF – 0.465)/I1
R4 = 0.465R3/[(1 – X)(100µA)(R3)
+ (VREF – 0.465)]
1786 F01
Figure 1
Grounded Lamp Configuration
In a grounded lamp configuration, the low voltage side of
the lamp connects directly to the LT1786F DIO pin. This
pin is the common connection between the cathode and
anode of two internal diodes. In previous grounded lamp
solutions, these diodes were discrete units and are now
integrated onto the IC, saving cost and board space.
Bidirectional lamp current flows in the DIO pin and thus,
the diodes conduct alternately on half cycles. Lamp current is controlled by monitoring one-half of the average
lamp current. The diode conducting on negative half
cycles has one-tenth of its current diverted to the CCFL VC
pin and nulls against the source current provided by the
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lamp current programmer circuit. The compensation capacitor on the CCFL VC pin provides stable loop compensation and an averaging function to the rectified sinusoidal
lamp current. Therefore, input programming current relates to one-half of average lamp current.
The transfer function between lamp current and input
programming current must be empirically determined and
is dependent on the particular lamp/display housing combination used. The lamp and display housing are a distributed loss structure due to parasitic lamp-to-frame capacitance. This means that the current flowing at the highvoltage side of the lamp is higher than what is flowing at
the DIO pin side of the lamp. The input programming
current is set to control lamp current at the high-voltage
side of the lamp, even though the feedback signal is the
lamp current at the bottom of the lamp. This ensures that
the lamp is not overdriven which can degrade the lamp’s
operating lifetime. Therefore, the full scale current of the
DAC does not necessarily correspond to the current
required to set maximum lamp current.
Floating Lamp Configuration
In a floating lamp configuration, the lamp is fully floating
with no galvanic connection to ground. This allows the
transformer to provide symmetric differential drive to the
lamp. Balanced drive eliminates the field imbalance associated with parasitic lamp-to-frame capacitance and reduces “thermometering” (uneven lamp intensity along the
lamp length) at low lamp currents.
Carefully evaluate display designs in relation to the physical layout of the lamp, its leads and the construction of the
display housing. Parasitic capacitance from any high
voltage point to DC or AC ground creates paths for
unwanted current flow. This parasitic current flow
degrades electrical efficiency and losses up to 25% have
been observed in practice. As an example, at a Royer
operating frequency of 60kHz, 1pF of stray capacitance
represents an impedance of 2.65MΩ. With an operating
lamp voltage of 400V and an operating lamp current of
6mA, the parasitic current is 150µA. This additional current must be supplied by the transformer secondary.
Layout techniques that increase parasitic capacitance
include long high voltage lamp leads, reflective metal foil
16
around the lamp and displays supplied in metal enclosures. Losses for a good display are under 5%, whereas,
losses for a bad display range from 5% to 25%. Lossy
displays are the primary reason to use a floating lamp
configuration. Providing symmetric, differential drive to
the lamp reduces the total parasitic loss by one-half.
Maintaining closed-loop control of lamp current in a
floating lamp configuration necessitates deriving a feedback signal from the primary side of the Royer transformer. Previous solutions have used an external precision shunt and high-side sense amplifier configuration.
This approach has been integrated onto the LT1786F for
simplicity of design and ease of use. An internal 0.1Ω
resistor monitors the Royer converter current and connects between the input terminals of a high-side sense
amplifier. A 0 – 1 Amp Royer primary-side, center-tap
current is translated to a 0µA to 500µA sink current at the
CCFL VC pin to null against the source current provided by
the lamp current programmer circuit. The compensation
capacitor on the CCFL VC pin provides stable loop compensation and an averaging function to the error sink
current. Therefore, input programming current is related
to average Royer converter current. Floating lamp circuits
operate similarly to grounded lamp circuits except for the
derivation of the feedback signal.
The transfer function between lamp current and input
programming current must be empirically determined and
is dependent upon a myriad of factors including lamp
characteristics, display construction, transformer turns
ratio and the tuning of the Royer oscillator. Once again,
lamp current will be slightly higher at one end of the lamp
and input programming current should be set for this
higher level to ensure that the lamp is not overdriven.
The internal 0.1Ω high-side sense resistor on the LT1786F
is rated for a maximum DC current of 1A. This resistor can
be damaged by extremely high surge currents at start-up.
The Royer converter typically uses a few microfarads of
bypass capacitance at the center tap of the transformer.
This capacitor charges up when the system is first powered by the battery pack or an AC wall adapter. The amount
of current delivered at start-up can be very large if the total
impedance in this path is small and the voltage source has
high current capability. Linear Technology recommends
LT1786F
U
W
U
U
APPLICATIONS INFORMATION
the use of an aluminum electrolytic for the transformer
center-tap bypass capacitor with an ESR greater than or
equal to 0.5Ω. This lowers the peak surge currents to an
acceptable level. In general, the wire and trace inductance
in this path also help reduce the di/dt of the surge current.
This issue only exists with floating lamp circuits as
grounded lamp circuits do not make use of the high-side
sense resistor.
Input Capacitor Type
Caution must be used in selecting the input capacitor type
for switching regulators. Aluminum electrolytics are electrically rugged and the lowest cost, but are physically large
to meet required ripple current ratings, and size constraints (especially height) may preclude their use. Ceramic capacitors are now available in larger values and
their high ripple current and voltage rating make them
ideal for input bypassing.
Solid tantalum capacitors would be a good choice except
for a history of occasional failure when subjected to large
current surges during start-up. The input bypass capacitor of regulators can see these high surges when a battery
or high capacitance source is connected. Some manufacturers have developed tantalum capacitor lines specially
tested for surge capability (AVX TPS series for instance),
but even these units may fail if the input voltage surge
approaches the capacitor’s maximum voltage rating. AVX
recommends derating the capacitor voltage by 2:1 for high
surge applications.
Applications Support
Linear Technology invests an enormous amount of time,
resources and technical expertise in understanding,
designing and evaluating backlight/LCD contrast solutions for system designers. The design of an efficient and
compact LCD backlight system is a study of compromise
in a transduced electronic system. Every aspect of the
design is interrelated and any design change requires
complete re-evaluation for all other critical design parameters. Linear Technology has engineered one of the most
complete test and evaluation setups for backlight designs
and understands the issues and tradeoffs in achieving a
compact, efficient and economical customer solution.
Linear Technology welcomes the opportunity to discuss,
design, evaluate and optimize any backlight/LCD contrast
system with a customer. For further information on backlight/LCD contrast designs, consult the References.
References
1. Williams, Jim. August 1992. Illumination Circuitry for
Liquid Crystal Displays. Linear Technology Corporation,
Application Note 49.
2. Williams, Jim. August 1993. Techniques for 92% Efficient LCD Illumination. Linear Technology Corporation,
Application Note 55.
3. Bonte, Anthony. March 1995. LT1182 Floating CCFL
with Dual Polarity Contrast. Linear Technology Corporation, Design Note 99.
4. Williams, Jim. April 1995. A Precision Wideband Current Probe for LCD Backlight Measurement. Linear Technology Corporation, Design Note 101.
5. LT1182/LT1183/LT1184/LT1184F Data Sheet. CCFL/
LCD Contrast Switching Regulators. April 1995. Linear
Technology Corporation.
6. Williams, Jim. November 1995. A Fourth Generation of
LCD Backlight Technology. Linear Technology Corporation, Application Note 65.
17
LT1786F
U
TYPICAL APPLICATION
nating high voltage lead length. Additionally, although the
lamp receives differential drive, with its attendant low
parasitic losses, the feedback signal is ground referred.
Thus, the stacked secondaries afford floating lamp operating efficiency with grounded mode current certainty and
line regulation.
Dual Transformer CCFL Power Supply
Space constraints may dictate utilization of two small
transformers instead of a single, larger unit. Although this
approach is somewhat more expensive, it can solve space
problems and offers other attractive advantages. Figure
2’s approach is essentially a “grounded lamp” LT1786Fbased circuit. The transistors drive two transformer primaries in parallel. The transformer secondaries, stacked
in series, provide the output. The relatively small transformers, each supplying half the load power, may be
located directly at the lamp terminals. Aside from the
obvious space advantage (particularly height), this
arrangement minimizes parasitic wiring losses by elimi-
L1 is directly driven, with winding 4-5 furnishing feedback
in the normal fashion. L3, “slaved” to L1’s and L3’s
interconnects must be laid out for low inductance to
maintain waveform purity. The traces should be as wide as
possible (e.g., 1/8") and overlaid to cancel inductive
effects.
27pF
LAMP
10
6
6
10
L3
L1
3
2
1
+
4
5
3
WIDE TRACE
1
2.2µF
2.2µF
SEE TEXT AND NOTES
220k
1000pF
2
+
4
NC
0.1µF*
WIDE TRACE
0.1µF*
100k
750Ω
Q1-Q2
ZDT1048
BAT54
+VBAT
L2
MBRS130L
1
2
3
4
1µF
SHUTDOWN
5
6
7
8
PGND
VSW
ICCFL
BULB
DIO
VC
16
15
14
BAT
LT1786F
13
ROYER
AGND
VIN
SHDN
IOUT
SMBSUS
SCL
ADR
SDA
12
11
+
2.2µF
16V
VIN
5V
10
9
TO
SMBus
HOST
L1, L3 = COILTRONICS CTX110605
OVERLAY INDICATED TRACES BETWEEN L1 AND L3
L2 = COILTRONICS CTX100-4
* = WIMA MKI OR MKP-20
= PANASONIC ECH-U
COILTRONICS (561) 241-7876
Figure 2. Dual Transformers Save Space and Minimize parasitic Losses While Maintaining
Current Accuracy and Line Regulation. Trade-Off Is Increased Cost
18
1786 F02
5
NC
LT1786F
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PACKAGE DESCRIPTION
S Package
16-Lead Plastic Small Outline (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1610)
0.386 – 0.394*
(9.804 – 10.008)
16
15
14
13
12
11
10
9
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
2
3
4
5
6
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
TYP
8
0.004 – 0.010
(0.101 – 0.254)
0° – 8° TYP
0.016 – 0.050
(0.406 – 1.270)
7
0.050
(1.270)
BSC
S16 1098
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT1786F
U
TYPICAL APPLICATION
90% Efficient Grounded CCFL with 2-Wire SMBus Control of Lamp Current
LAMP
10
C2
27pF
3kV
6
L1
3
2
C7
1µF
CCFL VSW
ICCFL
BULB
BAT
8V TO 28V
R1
750Ω
1%
Q1
16
L2
100µH
15
D1
1N5818
14
BAT
LT1786F
13
4
CCFL VC
ROYER
3
5
SHUTDOWN
CCFL
PGND
5
C3
2.2µF
35V
Q2
1
4
C1
0.068µF
R3
100k
1%
D1
BAT85
1
+
R2
221k
1%
C5
1000pF
2
6
7
8
DIO
AGND
VCC
SHDN
IOUT
SMBSUS
SCL
ADR
SDA
1786F TA03
12
11
+
3V < VCC
< 6.5V
C4
2.2µF
10
9
TO
SMBus
HOST
C1 MUST BE A LOW LOSS CAPACITOR
C1 = WIMA MKI OR MKP-20
PANASONIC ECH-U
L1 = COILTRONICS CTX210605
L2 = COILTRONICS CTX100-4
(DO NOT SUBSTITUTE COMPONENTS
COILTRONICS (561) 241-7876)
Q1, Q2 = ZETEX ZTX849 OR ROHM 25C5001
RELATED PARTS
PART NUMBER
DESCRIPTION
LT1172
Current Mode Switching Regulator for CCFL or LCD Contrast Control
1.25A, 100kHz
LT1173
Micropower DC/DC Converter for LCD Contrast Control
1A, 24kHz, Hysteretic
LT1182
Dual Current Mode Switching Regulator for CCFL and LCD Contrast Control
1.25A, 0.625A, 200kHz
LT1183
Dual Current Mode Switching Regulator for CCFL and LCD Contrast Control
1.25A, 0.625A, 200kHz
LT1184
Current Mode Switching Regulator for CCFL Control
1.25A, 200kHz
LT1184F
Current Mode Switching Regulator for CCFL Control
1.25A, 200kHz
LT1186F
DAC Programmable Current Mode Switching Regulator for CCFL Control
1.25A, 200kHz, SPI or Pulse Mode
LT1316
Micropower DC/DC Converter for LCD Contrast Control
Programmable Peak Current Limit
LT1372
Current Mode Switching Regulator for CCFL or LCD Contrast Contol
1.5A, 500kHz
LT1786
CCFL Controller for Wide Dimming Range and Maximum Lamp Lifetime
Multiple Lamp Capability, No Lamp Flicker, Precision
Maximum and Minimum Lamp Currents Maximize
Lamp Life
20
COMMENTS
Linear Technology Corporation
1786fa LT/CP 0801 1.5K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1998
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