LT1786F
SMBus Programmable
CCFL Switching Regulator
FEATURES
s
s
s
s
s
s
s
Wide Battery Input Range: 4.5V to 30V
Grounded Lamp or Floating Lamp Configurations
Open Lamp Protection
Precision 100µA Full-Scale DAC
Programming Current
2-Wire SMBus Interface
Two Selectable SMBus Addresses
DAC Setting Is Retained in Shutdown
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 switch-
ing 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.
APPLICATIONS
s
s
s
Notebook and Palmtop Computers
Portable Instruments
Personal Digital Assistants
DESCRIPTION
The LT
®
1786F is a fixed frequency, current mode, switch-
ing 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 full-
scale current output DAC. The DAC provides simple “bits-
TYPICAL APPLICATION
90% Efficient Floating CCFL with 2-Wire SMBus Control of Lamp Current
D1
BAT85
CCFL BACKLIGHT APPLICATION CIRCUITS
CONTAINED IN THIS DATA SHEET ARE COVERED
BY U.S. PATENT NUMBER 5408162
AND OTHER PATENTS PENDING
1
2
3
C7, 1µF
4
5
SHUTDOWN
6
7
8
CCFL
PGND
I
CCFL
DIO
CCFL V
SW
BULB
BAT
16
15
14
R2
220k
C5
1000pF
3
2
1
C3B
2.2µF
35V
R1
750Ω
4
5
LAMP
10
6
L1 = COILTRONICS CTX210605
C2
27pF
3kV
L1
L2 = COILTRONICS CTX100-4
*DO
NOT SUBSTITUTE COMPONENTS
COILTRONICS (561) 241-7876
BAT
8V TO 28V
LT1786F
13
CCFL V
C
ROYER
AGND
SHDN
SMBSUS
ADR
V
CC
I
OUT
SCL
SDA
12
11
10
9
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
Q1, Q2 = ZETEX ZTX849 OR ROHM 2SC5001
U
U
U
+
+
C3A
2.2µF
35V
+
C4
2.2µF
3V
≤
V
CC
≤
6.5V
C1*
0.068µF
R3
100k
Q2*
Q1*
L2
100µH
D1
1N5818
1786F TA01
0µA TO 50µA I
CCFL
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
1
LT1786F
ABSOLUTE
MAXIMUM
RATINGS
V
CC
........................................................................... 7V
BAT, Royer, BULB .................................................. 30V
CCFL V
SW
............................................................... 60V
Shutdown ................................................................. 6V
I
CCFL
Input Current .............................................. 10mA
DIO Input Current (Peak, < 100ms).................... 100mA
Digital Inputs .............................. – 0.3V to (V
CC
+ 0.3V)
Digital Outputs ............................ – 0.3V to (V
CC
+ 0.3V)
DAC Output Voltage ..................... – 15V to (V
CC
+ 0.3V)
Junction Temperature (Note 1) ............................ 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
PACKAGE/ORDER INFORMATION
TOP VIEW
CCFL PGND 1
I
CCFL
2
DIO 3
CCFL V
C
4
AGND 5
SHDN 6
SMBSUS 7
ADR 8
16 CCFL V
SW
15 BULB
14 BAT
13 ROYER
12 V
CC
11 I
OUT
10 SCL
9
SDA
ORDER PART
NUMBER
LT1786FCS
S PACKAGE
16-LEAD PLASTIC SO
T
JMAX
= 100°C,
θ
JA
= 100°C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
CC
= SHUTDOWN = SMBSUS = SCL = SDA = 3.3V, BAT = Royer = BULB = 12V, I
CCFL
= CCFL V
SW
= Open,
DIO = I
OUT
= GND, CCFL V
C
= 0.5V, unless otherwise specified.
SYMBOL
I
Q
ISUS
I
SHDN
PARAMETER
Supply Current
SMBSUS Supply Current
SHUTDOWN Supply Current
SHUTDOWN Input Bias Current
SHUTDOWN Threshold Voltage
f
DC(MAX)
BV
Switching Frequency
Maximum Switch Duty Cycle
Switch Breakdown Voltage
Switch Leakage Current
I
CCFL
Summing Voltage
∆I
CCFL
Summing Voltage for
∆Input
Programming Current
CCFL V
C
Offset Sink Current
∆CCFL
V
C
Source Current for
∆I
CCFL
Programming Current
CCFL V
C
to DIO Current Servo Ratio
CCFL V
C
Low Clamp Voltage
CCFL V
C
High Clamp Voltage
CCFL V
C
Switching Threshold
CCFL High-Side Sense Servo Current
Measured at CCFL V
SW
, I
SW
= 50mA,
I
CCFL
= 100µA, CCFL V
C
= Open
Measured at CCFL V
SW
q
CONDITIONS
3V
≤
V
CC
≤
6.5V, I
OUT
= 0µA
SMBSUS = 0V or Command Code Bit 7 = 1,
CCFL V
C
= Open (Note 2)
SHUTDOWN = 0V, CCFL V
C
= Open (Note 2)
SHUTDOWN = 0V, CCFL V
C
= Open
q
q
q
q
q
MIN
TYP
6
40
150
5
MAX
9.5
100
300
10
1.2
225
240
UNITS
mA
µA
µA
µA
V
kHz
kHz
%
%
V
0.45
175
160
80
75
60
0.85
200
200
85
85
70
Measured at CCFL V
SW
V
SW
= 12V, Measured at CCFL V
SW
V
SW
= 30V, Measured at CCFL V
SW
3V
≤
V
CC
≤
6.5V
q
20
40
0.425
0.385
0.465
0.465
5
–5
q
q
q
q
q
q
0.505
0.555
15
15
5.20
104
0.3
2.4
1.3
1.07
I
CCFL
= 0µA to 100µA
CCFL V
C
= 1.5V, Positive Current Measured into Pin
I
CCFL
= 25µA, 50µA, 75µA, 100µA,
CCFL V
C
= 1.5V
DIO = 5mA out of Pin, Measure I(V
C
) at CCFL V
C
= 1.5V
V
BAT
– V
BULB
= BULB Protect Servo Voltage
I
CCFL
= 100µA
CCFL V
SW
DC = 0%
I
CCFL
= 100µA, I(V
C
) = 0µA at CCFL V
C
= 1.5V
5
4.95
99
0.1
2.1
0.95
1.00
4.70
94
1.7
0.6
0.93
µA/µA
µA/mA
V
V
V
A
2
U
W
U
U
W W
W
µA
µA
V
V
mV
µA
LT1786F
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
CC
= SHUTDOWN = SMBSUS = SCL = SDA = 3.3V, BAT = Royer = BULB = 12V, I
CCFL
= CCFL V
SW
= Open,
DIO = I
OUT
= GND, CCFL V
C
= 0.5V, unless otherwise specified.
SYMBOL
PARAMETER
CCFL High-Side Sense Servo Current
Line Regulation
BULB Protect Servo Voltage
BULB Input Bias Current
I
LIM
V
SAT
∆I
Q
∆I
SW
CCFL Switch Current Limit
CCFL Switch On Resistance
Supply Current Increase During
CCFL Switch On Time
DAC Resolution
DAC Full-Scale Current
DAC Zero Scale Current
DAC Differential Nonlinearity
DAC Supply Voltage Rejection
I
IN
V
IH
Logic Input Current
High Level Input Voltage
3V
≤
V
CC
≤
6.5V, I
OUT
= Full Scale, V(I
OUT
) = 0.465V
0V
≤
V
IN
≤
V
CC
ADR
SMBSUS
SCL, SDA
SMBSUS, ADR
SCL, SDA
I
OUT
= 3mA, SDA Only
I
OUT
= 1.6mA, SMBSUS = 0V, Measured at SHDN Pin
V(I
OUT
) = 0.465V
q
CONDITIONS
BAT = 5V to 30V, I
CCFL
= 100µA,
I(V
C
) = 0µA at CCFL V
C
= 1.5V
q
q
MIN
TYP
0.1
MAX
0.16
150
7.5
9
3.0
2.6
1.0
30
UNIT
%/V
µA
V
µA
A
A
Ω
mA/A
Bits
CCFL High-Side Sense Supply Current Current Measured into BAT and Royer Pins
I
CCFL
= 100µA, I(V
C
) = 0µA at CCFL V
C
= 1.5V,
Servo Voltage Measured between BAT and BULB Pins
I
CCFL
= 100µA, I(V
C
) = 0µA at CCFL V
C
= 1.5V
Duty Cycle = 50%
Duty Cycle = 75% (Note 3)
CCFL I
SW
= 1A
CCFL I
SW
= 1A
50
6.5
100
7.0
5
q
q
q
1.25
0.9
1.9
1.6
0.6
20
6
98
96
100
100
±0.1
0.2
102
104
±200
±1
2
±1
µA
µA
nA
LSB
LSB
µA
V
V
V
V(I
OUT
) = 0.465V
q
q
q
q
q
q
q
q
q
q
q
V
CC
– 0.3
2.4
1.4
0.8
0.6
0.4
0.4
10
4.7
4.0
4.7
4.0
300
250
4.7
4.0
50
300
1000
100
V
IL
V
OL
Low Level Input Voltage
Low Level Output Voltage
V
V
V
V
kHz
µs
µs
µs
µs
ns
ns
µs
µs
ns
ns
SMBus Timing (Notes 4, 5)
f
SMB
t
BUF
t
HD:STA
t
SU:STA
t
SU:STO
t
HD:DAT
t
SU:DAT
t
LOW
t
HIGH
t
f
t
r
SMB Operating Frequency
Bus Free Time Between Stop and Start Condition
Hold Time After (Repeated) Start Condition
Repeated Start Condition Setup Time
Stop Condition Setup Time
Data Hold Time
Data Setup Time
Clock Low Period
Clock High Period
Clock/Data Fall Time
Clock/Data Rise Time
q
q
q
q
q
q
q
q
q
q
q
The
q
denotes specifications which apply over the specified operating
temperature range.
Note 1:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
LT1786FCS: T
J
= T
A
+ (P
D
)(100°C/W)
Note 2:
Does not include switch leakage.
Note 3:
For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by I
LIM
= 1.4(1.393 – DC) for the
LT1786F due to internal slope compensation circuitry.
Note 4:
Timings for all signals are referenced to V
IH
and V
IL
signals.
Note 5:
These parameters are guaranteed by design and are not tested in
production. Refer to the Timing Diagrams for additional information.
3
LT1786F
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current
vs Temperature
10
9
8
100
90
8O
70
ISUS (µA)
SHDN SUPPLY CURRENT (µA)
SUPPLY CURRENT (mA)
7
6
5
4
3
2
1
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G01
SHDN Input Bias Current
vs Temperature
6
1.2
SHDN INPUT BIAS CURRENT (µA)
SHDN THRESHOLD VOLTAGE (V)
5
4
3
2
1
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G04
1.0
0.9
0.8
0.7
0.6
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G05
CCFL FREQUENCY (kHz)
V
CC
= 5V
V
CC
= 3V
Maximum Duty Cycle
vs Temperature
95
93
CCFL MAXIMUM DUTY CYCLE (%)
I
CCFL
SUMMING VOLTAGE (V)
91
89
87
85
83
81
79
77
75
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G07
∆I
CCFL
SUMMING VOLTAGE (mV)
4
U W
ISUS Current vs Temperature
300
SMBSUS = 0V
270
240
210
180
150
120
90
60
30
–5
25 50 75 100 125 150
TEMPERATURE (°C)
1786 G02
SHDN Supply Current
vs Temperature
SHDN = 0V
60
50
40
30
20
10
0
–55 –25
V
CC
= 5V
V
CC
= 3V
V
CC
= 5V
V
CC
= 3V
0
–55 –25
–5
25 50 75 100 125 150
TEMPERATURE (°C)
1786 G03
SHDN Threshold Voltage
vs Temperature
240
230
220
210
200
190
180
170
Frequency vs Temperature
1.1
160
–75 –50 –25
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G06
I
CCFL
Summing Voltage
vs Temperature
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
I
CCFL
Summing Voltage
Load Regulation
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
0
T = –55°C
T = 25°C
T = 125°C
20 40 60 80 100 120 140 160 180 200
I
CCFL
PROGRAMMING CURRENT (µA)
1786 G09
LT1786F
TYPICAL PERFORMANCE CHARACTERISTICS
V
C
Sink Offset Current
vs Temperature
10
CCFL V
C
SINK OFFSET CURRENT (µA)
POSITIVE DIO VOLTAGE (V)
9
8
7
6
5
4
CCFL V
C
= 1.5V
∆CCFL
V
C
SOURCE CURRENT FOR
∆I
CCFL
PROGRAMMING CURRENT (µA/µA)
CCFL V
C
= 1.0V
3
2
1
CCFL V
C
= 0.5V
0
–1
–2
–3
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G10
Negative DIO Voltage
vs Temperature
CCFL V
C
DIO CURRENT SERVO RATIO (µA/mA)
1.6
1.4
103
102
101
100
99
98
97
96
CCFL V
C
LOW CLAMP VOLTAGE (V)
NEGATIVE DIO VOLTAGE (V)
I(DIO) = 10mA
1.2
1.0
0.8
0.6
0.4
0.2
0
–75 –50 –25
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G13
I(DIO) = 5mA
I(DIO) = 1mA
V
C
High Clamp Voltage
vs Temperature
CCFL V
C
SWITCHING THRESHOLD VOLTAGE (V)
2.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
CCFL V
C
HIGH CLAMP VOLTAGE (V)
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
BULB PROTECT SERVO VOLTAGE (V)
U W
∆CCFL
V
C
Source Current for
∆I
CCFL
Programming Current
vs Temperature
5.10
5.05
5.00
4.95
4.90
4.85
4.80
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G11
Positive DIO Voltage
vs Temperature
1.2
1.0
I(DIO) = 10mA
0.8
0.6
0.4
0.2
0
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G12
I
CCFL
= 100µA
I
CCFL
= 50µA
I
CCFL
= 10µA
I(DIO) = 5mA
I(DIO) = 1mA
V
C
to DIO Current Servo Ratio
vs Temperature
0.30
0.25
0.20
0.15
0.10
0.05
V
C
Low Clamp Voltage
vs Temperature
I(DIO) = 10mA
I(DIO) = 1mA
I(DIO) = 5mA
95
–75 – 50 –25
0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G14
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G15
V
C
Switching Threshold
vs Temperature
7.5
7.4
7.3
7.2
7.1
7.0
6.9
6.8
6.7
6.6
BULB Protect Servo Voltage
vs Temperature
I
CCFL
= 100µA
I
CCFL
= 50µA
I
CCFL
= 10µA
0.6
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G17
6.5
–75 – 50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
1786 G18
5
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