LINEAR TECHNOLOGY LT1768 Technical data

LT1768

High Power CCFL Controller

for Wide Dimming Range and

FEATURES

■

Ultrawide Multimode DimmingTM 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

U

APPLICATIO S

■

Desktop Flat Panel Displays

■

Multiple Lamp Displays

■

Notebook LCD Displays

■

Point of Sale Terminal Displays

Maximum Lamp Lifetime

U

DESCRIPTIO

The LT®1768 is designed to control single or multiple cold
cathode fluorescent lamp (CCFL) displays. A unique Mul­timode 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

TYPICAL APPLICATIO

C4-WIMA MKP2
L1-COILTRONICS UP4-680
T1-2 CTX110607 IN PARALLEL
Q1-ZDT1048
*R5 CAN BE METAL PCB TRACE

PGND GATE

DI02

DI01

SENSE

C4
10µF

V

AGND

C

PROG

PROG

0V TO 5V OR

1kHz PWM

C2

0.033µF

C3

0.1µF

R1

49.9k

Figure 1. 14W CCFL Supply Produces a 100:1 Dimming Ratio While
Maintaining Minimum and Maximum Lamp Current Specifications

C

T

LT1768

V

FAULT

SHDN

R

MIN

R

MAX

PWM

V

IN

REF

LAMP

16.2k

U

33pF

LAMP

V

IN

8V – 24V

C1
33µF

5V

0.1µF

R4

R2

40.2k

R3

60.4k

33pF

250Ω
1/4W

T1

4

MBRS130T3

2200pF

610

53 2

100

C4

0.33µF

Q1

L1
68µH

Si3456DY

R5*

0.025

1

Lamp Output and Dimming

Ratio vs Lamp Current

10000

1000

Q1

1768 TA01

DIMMING RATIO (NITS/NITS)

100

10

1

0.1

LAMP MANUFACTURERS

SPECIFIED CURRENT RANGE

2

0

LAMP CURRENT (mA)

LAMP OUTPUT (NITS)

6

4

8

10

1768 TA01b

1

LT1768

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

Input Voltage (VIN Pin) ............................................ 28V

SHDN Pin Voltage.................................................... 28V

FAULT Pin Voltage ................................................... 28V

PROG Pin Voltage................................................... 5.5V

PWM Pin Voltage.................................................... 4.5V

CT Pin Voltage ........................................................ 4.5V

SENSE Pin Voltage .................................................... 1V

DIO1, DIO2 Input Current ................................... ±50mA

R

Pin Source Current..................................... 750µA

MAX

R

Pin Source Current ..................................... 750µA

MIN

V

Pin Source Current ....................................... 10mA

REF

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

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

PGND

1

DI01

2

DI02

3

SENSE

4

VC

5

AGND

6

C

7

T

PROG

8

GN PACKAGE

16-LEAD PLASTIC SSOP

T

= 125°C, θJA = 100°C/W

JMAX

Consult LTC Marketing for parts specified with wider operating temperature ranges.

GATE

16

V

15

IN

V

14

REF

FAULT

13

SHDN

12

R

11

MIN

R

10

MAX

PWM

9

ORDER PART

NUMBER

LT1768CGN
LT1768IGN

GN PART

MARKING

1768
1768I

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T
I

= –100µA, unless otherwise specified.

RMIN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

I

SHDN

V

REF

V

RMAX

V

RMIN

FSW Switching Frequency V

I

PROG

V

PROG

Supply Current 9V< V
Supply Current in Shutdown V
SHDN Pin Pull-Up Current V
SHDN Threshold Voltage V
SHDN Threshold Hysteresis ● 100 200 300 mV
VIN Undervoltage Lockout V
VIN Undervoltage Lockout V
REF Voltage I
REF Line Regulation ∆V
REF Load Regulation ∆I
R

Pin Voltage ● 1.225 1.25 1.275 V

MAX

R

Pin Voltage ● 1.22 1.26 1.30 V

MIN

Maximum Duty Cycle V
Minimum ON Time V
PROG Pin Input Bias Current V
PROG Pin Voltage for Zero Lamp Current (Note 2) ● 0.45 0.5 0.55 V

PROG Pin Voltage for Minimum Lamp Current (Note 3)
PROG Pin Voltage for Maximum Lamp Current (Note 4) ● 3.8 4 4.2 V

The ● denotes the specifications which apply over the full operating

= 25°C, V

A

= 12V, I

VIN

< 24V ● 78 mA

VIN

= 0V ● 65 100 µA

SHDN

= 0V ● 4712 µA

SHDN

Off to On ● 0.6 1.26 1.8 V

SHDN

Off to On ● 7.2 7.9 8.2 V

IN

On to Off ● 7.1 7.4 7.6 V

IN

= –1mA ● 4.9 5 5.1 V

REF

8V to 24V I

VIN

–1mA to –10mA ● 10 20 mV

REF

= 0.75V, V

PROG

= 0.75V, V

PROG

= 0.75V, V

PROG

= 5V ● 100 500 nA

PROG

DIO1/2

= 250µA, V

= –1mA ● 720 mV

REF

= 0V ● 300 350 410 kHz

SENSE

= 0V 93 %

SENSE

= 150mV 125 ns

SENSE

= 0V, V

PROG

● 0.9 1 1.1 V

PWM

= 2.5V, I

RMAX

= –100µA,

2

LT1768

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T
I

= –100µA, unless otherwise specified.

RMIN

The ● denotes the specifications which apply over the full operating

= 25°C, V

A

= 12V, I

VIN

DIO1/2

= 250µA, V

PROG

= 0V, V

PWM

= 2.5V, I

RMAX

= –100µA,

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

PWM

PWM Input Bias Current ● 0.6 4 µA
PWM Duty Cycle V

= 1.75 45 50 55 %

PROG

PWM Frequency CT = 0.22µF (Note 7) 90 110 130 Hz

V

DIO1/2

V

VCCLAMP

I

SENSE

V

SENSE

DIO1/2 Positive Voltage I
DIO1/2 Negative Voltage I

VC High Clamp Voltage V
VC Switching Threshold V

SENSE Input Bias Current V

= 14mA 1.7 1.9 V

DIO

= –14mA –1.1 –1.3 V

DIO

PROG
PROG

SENSE

SENSE Threshold for Current Limit VVC = V

VVC = V

I

DIO1/2

to I

Ratio V

RMAX

V

PROG

PROG

= 4.5V (Note 8) 3.6 3.7 3.9 V
= 4.5V (Note 8) 0.5 0.7 0.95 V

= 0V –25 –30 µA

, Duty Cycle <50%, V

VCCLAMP

, Duty Cycle 80%, V

VCCLAMP

= 1V 85 100 115 mV

PROG

= 1V 90 mV

PROG

= 4.5V (Note 5) ● 94 98 104 A/A
= 4.5V, I

DIO1

or I

DIO2

= 0, VVC = 2.5V,

(Note 5) 45 49 55 A/A

I

to I

DIO1/2

Ratio V

RMIN

< 0.75V (Note 6) ● 9 10 11 A/A

PROG

V

PROG

< 0.75V, I

DIO1

or I

DIO2

= 0, VVC = 2.5V,

(Note 6) 9 10 11 A/A

I

GATE

GATE Drive Peak Source Current 1.5 A
GATE Drive Peak Sink Current 1.5 A

GATE Drive Saturation Voltage V
GATE Drive Clamp Voltage V
GATE Drive Low Saturation Voltage I

= 12V, I

VIN

= 24V, I

VIN

= 100mA ● 0.4 0.6 V

GATE

= –100mA, V

GATE

= –10mA, V

GATE

= 4.5V ● 9.8 10.2 V

PROG

= 4.5V ● 12.5 14 V

PROG

Open LAMP Threshold (Note 9) 100 125 150 µA
FAULT Pin Saturation Voltage I
FAULT Pin Leakage Current V

= 1mA, I

FAULT

= 5V 20 100 nA

FAULT

DI01

, I

DI02

= 0µA, V

= 4.5V 0.2 0.3 V

PROG

Thermal ShutdownTemperature 160 °C

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
threshold voltage, lamp current will be at zero. For V

less than the

PROG

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

pin to ground. See Applications Information for more details.

R

MIN

Note 3: This is the threshold voltage where the device starts to pulse width
modulate the lamp current. For V
lamp current will be equal to the minimum lamp current. For V

less than the threshold voltage,

PROG

PROG

greater than the threshold voltage, lamp current will be pulse width
modulated between the minimum lamp current and some higher value.

MAX

pin

MIN

resistor

Minimum lamp current is set by the value of the resistor from the R
to ground. The higher value lamp current is a function of the R
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
be no increase in lamp current. For V

greater than the threshold voltage, there will

PROG

less than the threshold voltage,

PROG

lamp current will be at some lower value. Maximum lamp current is set by

the value of the resistor from the R
lamp current is a function of the R
voltages on the PWM and PROG pins. See Applications Information for
more details.

Note 5: I
V

PROG

to I

DIO1/2

to 4.5V, VVC to 2.5V, and then ramping a DC current out of the

ratio is determined by setting I

RMAX

DIO1/2 pins from zero until the DC current in the VC voltage source
current equals zero. The I

)/I

I

DIO2

Note 6: I
V

PROG

. See Applications Information for more details.

RMAX

to I

DIO1/2

to 0.75V, VVC to 2.5V, and then ramping a DC current out of the

DIO1/2

ratio is determined by setting I

RMIN

DIO1/2 pins from zero until the DC current in the VC voltage source
current equals zero. The I

)/I

I

DIO2

. See Applications Information for more details.

RMIN

DIO1/2

Note 7: The PWM frequency is set by the equation PWMFREQ = 22Hz/

(µF).

C

T

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
the threshold current for at least 1 full PWM cycle.

pin to ground. The lower value

MAX

to I

to I

MIN

RMAX

RMIN

and R

resistors, and the

MAX

ratio is then defined as (I

ratio is then defined as (I

or I

DIO1

RMAX

to –100µA,

RMIN

DIO2

to –100µA,

DIO1

DIO1

is less than

+

+

3

LT1768

UW

TYPICAL PERFOR A CE CHARACTERISTICS

V

, V

V

vs Temperature

REF

5.10
I

= –1mA

REF

5.08

5.06

5.04

5.02

5.00

VOLTAGE (V)

4.98

REF

V

4.96

4.94

4.92

4.90

–50 –25 0 25

TEMPERATURE (°C)

50

Supply Current vs Input Voltage

10

8

6

4

SUPPLY CURRENT (mA)

2

0

05

10

15 20 25

INPUT VOLTAGE (V)

75

100 125

1768 G01

1768 G04

RMIN

1.30

I

= –100µA

RMIN

= –100µA

I

1.29

RMAX

1.28

1.27

1.26

1.25

1.24

VOLTAGE (V)

1.23

1.22

1.21

1.20

–50 –25 0 25

Supply Current vs Temperature

7.40

7.30

7.20

7.10

7.00

6.90

6.80

6.70

SUPPLY CURRENT (mA)

6.60

6.50

6.40

–50 –25 0 25

vs Temperature

RMAX

V

RMIN(V)

V

RMAX(V)

TEMPERATURE (°C)

TEMPERATURE (°C)

50

75

50

75

100 125

1768 G02

100 125

1768 G05

Supply Current in Shutdown vs
Temperature

80

V

= 0V

SHDN

76
72
68
64
60
56
52

SHUTDOWN CURRENT (µA)

48
44
40

–50 –25 0 25

TEMPERATURE (°C)

50

Supply Current in Shutdown vs
Input Voltage

100

V

= 0V

SHDN

80

60

40

SHUTDOWN CURRENT (µA)

20

0

05

10

INPUT VOLTAGE (V)

100 125

75

1768 G03

15 20 25

1768 G06

SHDN Pull-Up Current
vs Input Voltage

10

V

= 0V

SHDN

8

6

4

2

SHDN PULL-UP CURRENT (µA)

0

5

0

4

15

10

INPUT VOLTAGE (V)

Shutdown Threshold Voltage vs
Temperature

2.00

1.80

1.60

1.40

1.20

1.00

0.80

0.60

SHUTDOWN VOLTAGE (V)

0.40

0.20

20

25

1768 G07

0

–50 –25 0 25

V

OFF TO ON

SHDN

V

SHDN

TEMPERATURE (

ON TO OFF

50

°C)

75

100 125

1768 G08

Undervoltage Lockout Threshold
vs Temperature

8.20

8.10

8.00

7.90

7.80

7.70

7.60

7.50

7.40

UNDERVOLTAGE LOCKOUT (V)

7.30

7.20
–50 –25 0 25

V

OFF TO ON

UVL

V

ON TO OFF

UVL

50

TEMPERATURE (°C)

75

100 125

1768 G09

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switching Frequency vs Temperature

400
390
380
370
360
350
340
330
320

SWITCHING FREQUENCY (kHz)

310
300

–50 –25 0 25

TEMPERATURE (°C)

50

75

100 125

1768 G10

PWM Frequency vs Temperature

124
120
116
112
108
104
100

96

PWM FREQUENCY (Hz)

92
88
84

–50 –25 0 25

TEMPERATURE (°C)

50

CT = 0.22µF
V

PWM

75

= 2.5V

100 125

1768 G11

FAULT Pin Saturation Voltage vs
Temperature

0.250

0.225

0.200

0.175

0.150

0.125

0.100

FAULT VOLTAGE (V)

0.75

0.50

0.25
0

–50 –25 0 25

TEMPERATURE (°C)

50

LT1768

I

= 0µA

DIO1

I

= 0µA

DIO2

= 1mA

I

FAULT

100 125

75

1768 G12

FAULT Pin Saturation Voltage vs
Current

450

I

= 0µA

DIO1

= 0µA

I

DIO2

400

350

300

250

200

FAULT VOLTAGE (mV)

150

100

0

0.5 1.0

2.0

1.8

1.6

1.4

1.2

1.0

0.8

DIO VOLTAGE (V)

0.6

0.4

0.2
0

2468

DIO CURRENT (mA)

2.0 3.0 3.5

1.5 2.5

I

(mA)

FAULT

10

12

Sense Pin Bias Current vs
Temperature

50

V

= 0V

SENSE

45
40
35
30
25
20
15

SENSE CURRENT (µA)

10

5

1768 G13

0

–50 –25 0 25

TEMPERATURE (°C)

50

75

100 125

1768 G14

DIO Pin Voltage vs Current

–2.0

–1.8
–1.6
–1.4
–1.2
–1.0

–0.8

DIO VOLTAGE (V)

–0.6
–0.4
–0.2

0

14 16 18 200

1768 G24

–2 –4 –6 –8

–10

–12

DIO CURRENT (mA)

–14 –16 –18 –200

1768 G20

Maximum Gate Voltage vs
Temperature

15.00

I

= –10mA

GATE

14.50

14.00

13.50

13.00

12.50

12.00

11.50

GATE CLAMP VOLTAGE (V)

11.00

10.50

10.00

–50 –25 0 25

TEMPERATURE (°C)

VC Clamp Voltage vs CurrentDIO Pin Voltage vs Current

3.75

3.74

3.73

3.72

3.71

3.70

3.69

CLAMP VOLTAGE (V)

3.68

C

V

3.67

3.66

3.65
0 50 100 150

200

VC CURRENT (µA)

V

= 24V

IN

V

= 12V

IN

50

75

300 350 400 450 500

250

100 125

1768 G15

1768 G25

5

LT1768

UW

TYPICAL PERFOR A CE CHARACTERISTICS

VC Clamp Voltage vs Temperature

3.90
IVC = 500µA

3.85

3.80

3.75

3.70

(V)

3.65

C CLAMP

3.60

V

3.55

3.50

3.45

3.40

–50 –25 0 25

TEMPERATURE (°C)

50

75

PWM Pin Input Current vs
Temperature

1.40

1.30

1.20

1.10

1.00

0.90

0.80

0.70

PWM INPUT CURRENT (µA)

0.60

0.50

0.40
–50 –25 0 25

TEMPERATURE (°C)

50

V

75

PWM

100 125

1768 G26

= 2.5V

100 125

1768 G29

VC Switching Threshold
vs Temperature

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

SWITCH THRESHOLD VOLTAGE (V)

C

V

0.55

0.50
–50 –25 0 25

TEMPERATURE (°C)

50

Lamp Fault Current Threshold
vs Temperature

200
180
160
140
120
100

80
60
40
20

BULB FAULT CURRENT THRESHOLD (µA)

0

–50 –25 0 25

TEMPERATURE (°C)

50

75

75

100 125

1768 G27

100 125

1768 G31

PWM Pin Input Current
vs Voltage

25

20

15

10

PWM INPUT CURRENT (µA)

5

0

1

0

2

PWM VOLTAGE (V)

Maximum Sense Threshold
vs Gate Drive Duty Cycle

120
110
100

90
80
70
60
50

SENSE THRESHOLD (mV)

40
30
20

0 102030

40

50

GATE DUTY CYCLE (%)

3

4

1768 G28

60 70 80 90 100

1768 G32

5

110
108
106
104
102

RATIO (A/A)

100

RMAX

98

TO I

96

DI01/2

I

94
92
90

6

I

to I

DIO1/2

Current

V

= 4.5V

PROG

= 2.5V

V

VC

0 –60

RMAX

–120

I

RMAX

Ratio vs R

(µA)

MAX

–180 –240 –300

1768 G33

I

DIO1/2

to I

RMAX

Ratio vs R

Current with a Lamp Fault

60

V

= 4.5V

PROG

58

= 2.5V

V

VC

OR I

DI02

= 0µA

–120

I

RMAX

I

DI01

56
54
52

RATIO (A/A)

50

RMAX

48

TO I

46

DI01/2

I

44
42
40

0 –60

MAX

–180 –240 –300

(µA)

1768 G34

I

DIO1/2

Current

11.0

V

10.8

V

10.6

10.4

10.2

RATIO (A/A)

10.0

RMIN

9.8

TO I

9.6

DI01/2

I

9.4

9.2

9.0
0 –60

PROG
VC

to I

= 0.75V

= 2.5V

Ratio vs R

RMIN

–120

I

RMIN

MIN

–180 –240 –300

(µA)

1768 G35