LT1768
High Power CCFL Controller
for Wide Dimming Range and
Maximum Lamp Lifetime
DESCRIPTIO
The LT
®
1768 is designed to control single or multiple cold
cathode fluorescent lamp (CCFL) displays. A unique Mul-
timode Dimming scheme* combines both linear and PWM
control functions to maximize lamp life, efficiency, and
dimming range. Accurate maximum and minimum lamp
currents can be easily set. The LT1768 can detect and
protect against lamp failures and overvoltage start-up
conditions. It is designed to provide maximum flexibility
with a minimum number of external components.
The LT1768 is a current mode PWM controller with a 1.5A
MOSFET driver for high power applications. It contains a
350kHz oscillator, 5V reference, and a current sense
comparator with a 100mV threshold. It operates from an
8V to 24V input voltage. The LT1768 also has undervoltage
lockout, thermal limit, and a shutdown pin that reduces
supply current to 65µA. It is available in a small 16-lead
SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Multimode Dimming is a trademark of Linear Technology Corporation.
*Patent Pending
FEATURES
s
s
s
s
s
s
s
s
s
s
s
Ultrawide Multimode Dimming
TM
Range
Multiple Lamp Capability
Programmable PWM Dimming Range and
Frequency
Precision Maximum and Minimum Lamp
Currents Maximize Lamp Lifetime
No Lamp Flicker Under All Supply and Load
Conditions
Open Lamp Detection and Protection
350kHz Switching Frequency
1.5A MOSFET Gate Driver
100mV Current Sense Threshold
5V Reference Voltage Output
The 16-Lead SSOP Package
APPLICATIO S
s
s
s
s
Desktop Flat Panel Displays
Multiple Lamp Displays
Notebook LCD Displays
Point of Sale Terminal Displays
TYPICAL APPLICATIO
C4-WIMA MKP2
L1-COILTRONICS UP4-680
T1-2 CTX110607 IN PARALLEL
Q1-ZDT1048
*R5 CAN BE METAL PCB TRACE
33pF
LAMP
6
LAMP
V
IN
8V – 24V
C1
33µF
PGND
DI02
DI01
GATE
V
IN
V
REF
5V
0.1µF
33pF
4
T1
10
5
3
2
C4
0.33µF
1
250Ω
1/4W
Q1
Q1
C2
0.033µF
SENSE FAULT
LT1768
V
C
SHDN
AGND
C
T
PROG
C4
10µF
R
MIN
R
MAX
PWM
R4
16.2k
MBRS130T3
R2
40.2k
L1
68µH
C3
0.1µF
PROG
0V TO 5V OR
1kHz PWM
Si3456DY
100
R3
60.4k
R1
49.9k
2200pF
R5*
0.025
1768 TA01
Figure 1. 14W CCFL Supply Produces a 100:1 Dimming Ratio While
Maintaining Minimum and Maximum Lamp Current Specifications
U
U
U
Lamp Output and Dimming
Ratio vs Lamp Current
10000
1000
DIMMING RATIO (NITS/NITS)
100
LAMP OUTPUT (NITS)
10
1
LAMP MANUFACTURERS
SPECIFIED CURRENT RANGE
0.1
0
2
6
8
4
LAMP CURRENT (mA)
10
1768 TA01b
1
LT1768
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PGND
DI01
DI02
SENSE
VC
AGND
C
T
PROG
1
2
3
4
5
6
7
8
16 GATE
15 V
IN
14 V
REF
13 FAULT
12 SHDN
11 R
MIN
10 R
MAX
9
PWM
Input Voltage (V
IN
Pin) ............................................ 28V
SHDN Pin Voltage .................................................... 28V
FAULT Pin Voltage ................................................... 28V
PROG Pin Voltage ................................................... 5.5V
PWM Pin Voltage .................................................... 4.5V
C
T
Pin Voltage ........................................................ 4.5V
SENSE Pin Voltage .................................................... 1V
DIO1, DIO2 Input Current ...................................
±50mA
R
MAX
Pin Source Current ..................................... 750µA
R
MIN
Pin Source Current ..................................... 750µA
V
REF
Pin Source Current ....................................... 10mA
Operating Junction Temperature Range
LT1768C ................................................ 0°C to 125°C
LT1768I ............................................ – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering 10 sec)................... 300°C
ORDER PART
NUMBER
LT1768CGN
LT1768IGN
GN PART
MARKING
1768
1768I
GN PACKAGE
16-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 100°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
VIN
= 12V, I
DIO1/2
= 250µA, V
PROG
= 0V, V
PWM
= 2.5V, I
RMAX
= –100µA,
I
RMIN
= –100µA, unless otherwise specified.
SYMBOL
I
Q
I
SHDN
PARAMETER
Supply Current
Supply Current in Shutdown
SHDN Pin Pull-Up Current
SHDN Threshold Voltage
SHDN Threshold Hysteresis
V
IN
Undervoltage Lockout
V
IN
Undervoltage Lockout
V
REF
REF Voltage
REF Line Regulation
REF Load Regulation
V
RMAX
V
RMIN
FSW
R
MAX
Pin Voltage
R
MIN
Pin Voltage
Switching Frequency
Maximum Duty Cycle
Minimum ON Time
I
PROG
V
PROG
PROG Pin Input Bias Current
PROG Pin Voltage for Zero Lamp Current
PROG Pin Voltage for Minimum Lamp Current
PROG Pin Voltage for Maximum Lamp Current
V
PROG
= 0.75V, V
SENSE
= 0V
V
PROG
= 0.75V, V
SENSE
= 0V
V
PROG
= 0.75V, V
SENSE
= 150mV
V
PROG
= 5V
(Note 2)
(Note 3)
(Note 4)
q
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
9V< V
VIN
< 24V
V
SHDN
= 0V
V
SHDN
= 0V
V
SHDN
Off to On
V
IN
Off to On
V
IN
On to Off
I
REF
= –1mA
∆V
VIN
8V to 24V I
REF
= –1mA
∆I
REF
–1mA to –10mA
q
q
q
q
q
q
q
q
q
q
q
q
q
MIN
TYP
7
65
MAX
8
100
12
1.8
300
8.2
7.6
5.1
20
20
1.275
1.30
410
UNITS
mA
µA
µA
V
mV
V
V
V
mV
mV
V
V
kHz
%
ns
4
0.6
100
7.2
7.1
4.9
7
1.26
200
7.9
7.4
5
7
10
1.225
1.22
300
1.25
1.26
350
93
125
100
500
0.55
1.1
4.2
0.45
0.9
3.8
0.5
1
4
2
U
nA
V
V
V
W
U
U
W W
W
LT1768
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
VIN
= 12V, I
DIO1/2
= 250µA, V
PROG
= 0V, V
PWM
= 2.5V, I
RMAX
= –100µA,
I
RMIN
= –100µA, unless otherwise specified.
SYMBOL
I
PWM
PARAMETER
PWM Input Bias Current
PWM Duty Cycle
PWM Frequency
V
DIO1/2
V
VCCLAMP
I
SENSE
V
SENSE
DIO1/2 Positive Voltage
DIO1/2 Negative Voltage
VC High Clamp Voltage
VC Switching Threshold
SENSE Input Bias Current
SENSE Threshold for Current Limit
I
DIO1/2
to I
RMAX
Ratio
V
PROG
= 1.75
C
T
= 0.22µF (Note 7)
I
DIO
= 14mA
I
DIO
= –14mA
V
PROG
= 4.5V (Note 8)
V
PROG
= 4.5V (Note 8)
V
SENSE
= 0V
V
VC
= V
VCCLAMP
, Duty Cycle < 50%, V
PROG
= 1V
V
VC
= V
VCCLAMP
, Duty Cycle 80%, V
PROG
= 1V
V
PROG
= 4.5V (Note 5)
V
PROG
= 4.5V, I
DIO1
or I
DIO2
= 0, V
VC
= 2.5V,
(Note 5)
I
DIO1/2
to I
RMIN
Ratio
V
PROG
< 0.75V (Note 6)
V
PROG
< 0.75V, I
DIO1
or I
DIO2
= 0, V
VC
= 2.5V,
(Note 6)
I
GATE
GATE Drive Peak Source Current
GATE Drive Peak Sink Current
GATE Drive Saturation Voltage
GATE Drive Clamp Voltage
GATE Drive Low Saturation Voltage
Open LAMP Threshold
FAULT Pin Saturation Voltage
FAULT Pin Leakage Current
Thermal ShutdownTemperature
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
This is the threshold voltage where the lamp current switches
from zero current to minimum lamp current. For V
PROG
less than the
threshold voltage, lamp current will be at zero. For V
PROG
greater than the
threshold voltage, lamp current will be equal to the minimum lamp
current. Minimum lamp current is set by the value of the resistor from the
R
MIN
pin to ground. See Applications Information for more details.
Note 3:
This is the threshold voltage where the device starts to pulse width
modulate the lamp current. For V
PROG
less than the threshold voltage,
lamp current will be equal to the minimum lamp current. For V
PROG
greater than the threshold voltage, lamp current will be pulse width
modulated between the minimum lamp current and some higher value.
Minimum lamp current is set by the value of the resistor from the R
MIN
pin
to ground. The higher value lamp current is a function of the R
MAX
resistor
to ground value, and the voltages on the PWM and PROG pins. See
Applications Information for more details.
Note 4:
This is the threshold voltage where the lamp current reaches its
maximum value. For V
PROG
greater than the threshold voltage, there will
be no increase in lamp current. For V
PROG
less than the threshold voltage,
lamp current will be at some lower value. Maximum lamp current is set by
V
VIN
= 12V, I
GATE
= –100mA, V
PROG
= 4.5V
V
VIN
= 24V, I
GATE
= –10mA, V
PROG
= 4.5V
I
GATE
= 100mA
(Note 9)
I
FAULT
= 1mA, I
DI01
, I
DI02
= 0µA, V
PROG
= 4.5V
V
FAULT
= 5V
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
q
MIN
45
90
TYP
0.6
50
110
1.7
–1.1
MAX
4
55
130
1.9
–1.3
3.9
0.95
–30
115
104
55
11
11
UNITS
µA
%
Hz
V
V
V
V
µA
mV
mV
A/A
A/A
A/A
A/A
A
A
V
3.6
0.5
85
94
45
9
9
3.7
0.7
–25
100
90
98
49
10
10
1.5
1.5
9.8
10.2
12.5
0.4
14
0.6
150
0.3
100
V
V
µA
V
nA
°C
100
125
0.2
20
160
the value of the resistor from the R
MAX
pin to ground. The lower value
lamp current is a function of the R
MIN
and R
MAX
resistors, and the
voltages on the PWM and PROG pins. See Applications Information for
more details.
Note 5:
I
DIO1/2
to I
RMAX
ratio is determined by setting I
RMAX
to –100µA,
V
PROG
to 4.5V, V
VC
to 2.5V, and then ramping a DC current out of the
DIO1/2 pins from zero until the DC current in the VC voltage source
current equals zero. The I
DIO1/2
to I
RMAX
ratio is then defined as (I
DIO1
+
I
DIO2
)/I
RMAX
. See Applications Information for more details.
Note 6:
I
DIO1/2
to I
RMIN
ratio is determined by setting I
RMIN
to –100µA,
V
PROG
to 0.75V, V
VC
to 2.5V, and then ramping a DC current out of the
DIO1/2 pins from zero until the DC current in the VC voltage source
current equals zero. The I
DIO1/2
to I
RMIN
ratio is then defined as (I
DIO1
+
I
DIO2
)/I
RMIN
. See Applications Information for more details.
Note 7:
The PWM frequency is set by the equation PWMFREQ = 22Hz/
C
T
(µF).
Note 8:
For VC voltages less than the switching threshold, GATE switching
is disabled.
Note 9:
An open lamp will be detected if either I
DIO1
or I
DIO2
is less than
the threshold current for at least 1 full PWM cycle.
3
LT1768
TYPICAL PERFOR A CE CHARACTERISTICS
V
REF
vs Temperature
5.10
5.08
5.06
I
REF
= –1mA
1.30
SHUTDOWN CURRENT (µA)
V
REF
VOLTAGE (V)
5.04
VOLTAGE (V)
5.02
5.00
4.98
4.96
4.94
4.92
4.90
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
Supply Current vs Input Voltage
10
7.40
7.30
SUPPLY CURRENT (mA)
7.20
7.10
7.00
6.90
6.80
6.70
6.60
6.50
6
SHUTDOWN CURRENT (µA)
8
SUPPLY CURRENT (mA)
4
2
0
0
5
15
INPUT VOLTAGE (V)
10
SHDN Pull-Up Current
vs Input Voltage
10
SHDN PULL-UP CURRENT (µA)
V
SHDN
= 0V
SHUTDOWN VOLTAGE (V)
8
UNDERVOLTAGE LOCKOUT (V)
6
4
2
0
0
5
15
10
INPUT VOLTAGE (V)
20
25
1768 G07
4
U W
1768 G01
V
RMIN
, V
RMAX
vs Temperature
I
RMIN
= –100µA
1.29
I
RMAX
= –100µA
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
V
RMIN(V)
V
RMAX(V)
Supply Current in Shutdown vs
Temperature
80
76
72
68
64
60
56
52
48
44
40
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
V
SHDN
= 0V
1768 G02
1768 G03
Supply Current vs Temperature
100
Supply Current in Shutdown vs
Input Voltage
V
SHDN
= 0V
80
60
40
20
20
25
1768 G04
6.40
–50
0
–25
0
75
TEMPERATURE (°C)
25
50
100
125
0
5
15
INPUT VOLTAGE (V)
10
20
25
1768 G06
1768 G05
Shutdown Threshold Voltage vs
Temperature
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0
–50
Undervoltage Lockout Threshold
vs Temperature
8.20
8.10
V
SHDN
OFF TO ON
8.00
7.90
7.80
7.70
7.60
7.50
7.40
7.30
V
UVL
OFF TO ON
V
SHDN
ON TO OFF
V
UVL
ON TO OFF
–25
50
0
25
75
TEMPERATURE (°C)
100
125
7.20
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
1768 G08
1768 G09
LT1768
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency vs Temperature
400
390
SWITCHING FREQUENCY (kHz)
380
PWM FREQUENCY (Hz)
360
350
340
330
320
310
300
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
108
104
100
96
92
88
84
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
FAULT VOLTAGE (V)
370
FAULT Pin Saturation Voltage vs
Current
450
400
I
DIO1
= 0µA
I
DIO2
= 0µA
350
300
250
200
150
100
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
I
FAULT
(mA)
1768
G13
35
30
25
20
15
10
5
0
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
GATE CLAMP VOLTAGE (V)
FAULT VOLTAGE (mV)
SENSE CURRENT (µA)
DIO Pin Voltage vs Current
2.0
1.8
1.6
DIO VOLTAGE (V)
DIO VOLTAGE (V)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
2
4
6 8 10 12 14 16 18 20
DIO CURRENT (mA)
1768 G24
–1.4
–1.2
–1.0
– 0.8
– 0.6
– 0.4
– 0.2
0
0
– 2 – 4 – 6 – 8 –10 –12 –14 –16 –18 –20
DIO CURRENT (mA)
1768 G20
V
C
CLAMP VOLTAGE (V)
U W
1768 G10
PWM Frequency vs Temperature
124
120
116
112
C
T
= 0.22µF
V
PWM
= 2.5V
0.250
0.225
0.200
0.175
0.150
0.125
0.100
0.75
0.50
0.25
FAULT Pin Saturation Voltage vs
Temperature
I
DIO1
= 0µA
I
DIO2
= 0µA
I
FAULT
= 1mA
–25
50
0
25
75
TEMPERATURE (°C)
100
125
0
–50
1768 G11
1768 G12
Sense Pin Bias Current vs
Temperature
50
45
40
V
SENSE
= 0V
Maximum Gate Voltage vs
Temperature
15.00
14.50
14.00
13.50
13.00
12.50
12.00
11.50
11.00
10.50
10.00
–50
–25
50
0
25
75
TEMPERATURE (°C)
100
125
V
IN
= 12V
V
IN
= 24V
I
GATE
= –10mA
1768 G14
1768 G15
DIO Pin Voltage vs Current
– 2.0
–1.8
–1.6
3.75
3.74
3.73
3.72
3.71
3.70
3.69
3.68
3.67
3.66
3.65
VC Clamp Voltage vs Current
0
50 100 150 200 250 300 350 400 450 500
V
C
CURRENT (µA)
1768 G25
5
京公网安备 11010802033920号
EEWORLD
