Datasheet

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LT1768

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.

■

Ultrawide Multimode Dimming

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

High Power CCFL Controller

for Wide Dimming Range and

Maximum Lamp Lifetime

■

Desktop Flat Panel Displays

■

Multiple Lamp Displays

■

Notebook LCD Displays

■

Point of Sale Terminal Displays

Figure 1. 14W CCFL Supply Produces a 100:1 Dimming Ratio While

Maintaining Minimum and Maximum Lamp Current Specifications

1768 TA01

0.33µF

250Ω

1/4W

R5*

0.025

68µH

33pF

0.1µF

33µF

Si3456DY

MBRS130T3

LAMP

60.4k

49.9k

40.2k

16.2k

0.1µF

10µF

0.033µF

8V – 24V

PROG

0V TO 5V OR

1kHz PWM

LT1768

DI02

PGND GATE

AGND

PROG

DI01

SENSE

SHDN

MIN

MAX

PWM

FAULT

REF

2200pF

100

610

53 2

C4-WIMA MKP2

L1-COILTRONICS UP4-680

T1-2 CTX110607 IN PARALLEL

Q1-ZDT1048

*R5 CAN BE METAL PCB TRACE

LAMP CURRENT (mA)

10000

1000

100

0.1

1768 TA01b

DIMMING RATIO (NITS/NITS)

LAMP OUTPUT (NITS)

LAMP MANUFACTURERS

SPECIFIED CURRENT RANGE

Lamp Output and Dimming

Ratio vs Lamp Current

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

, LTC and LT are registered trademarks of Linear Technology Corporation.

Multimode Dimming is a trademark of Linear Technology Corporation.

*Patent Pending

Summary of content (20 pages)

PAGE 1
LT1768 High Power CCFL Controller for Wide Dimming Range and Maximum Lamp Lifetime DESCRIPTIO U 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.
PAGE 2
LT1768 U W U PACKAGE/ORDER I FOR ATIO U W W W 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 ...........................................
PAGE 3
LT1768 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VVIN = 12V, IDIO1/2 = 250µA, VPROG = 0V, VPWM = 2.5V, IRMAX = –100µA, IRMIN = –100µA, unless otherwise specified. SYMBOL PARAMETER IPWM PWM Input Bias Current PWM Duty Cycle CONDITIONS MIN ● 4 UNITS µA 45 50 55 % 90 110 130 Hz 1.7 –1.1 1.9 –1.3 V V 3.7 0.7 3.9 0.
PAGE 4
LT1768 U W TYPICAL PERFOR A CE CHARACTERISTICS VREF vs Temperature 5.06 1.28 5.04 1.27 5.02 5.00 4.98 VRMIN(V) 1.26 VRMAX(V) 1.25 1.24 4.96 1.23 4.94 1.22 4.92 1.21 4.90 –50 1.20 –50 50 0 25 75 TEMPERATURE (°C) 100 125 VSHDN = 0V 76 SHUTDOWN CURRENT (µA) 5.08 IRMIN = –100µA 1.29 IRMAX = –100µA –25 80 1.30 IREF = –1mA VOLTAGE (V) VREF VOLTAGE (V) 5.
PAGE 5
LT1768 U W TYPICAL PERFOR A CE CHARACTERISTICS FAULT Pin Saturation Voltage vs Temperature PWM Frequency vs Temperature 400 124 390 120 380 116 0.200 112 0.175 370 360 350 340 330 0.250 CT = 0.22µF VPWM = 2.5V 108 104 100 96 0.125 0.100 0.75 92 0.50 310 88 0.25 300 –50 84 –50 50 0 25 75 TEMPERATURE (°C) –25 100 125 –25 50 0 25 75 TEMPERATURE (°C) 100 0 –50 125 15.00 VSENSE = 0V 45 200 GATE CLAMP VOLTAGE (V) SENSE CURRENT (µA) 250 35 30 25 20 15 10 150 5 100 0 0.
PAGE 6
LT1768 U W TYPICAL PERFOR A CE CHARACTERISTICS VC Clamp Voltage vs Temperature 3.90 25 1.00 3.80 3.75 3.70 3.65 3.60 3.55 3.50 3.45 3.40 –50 –25 50 0 25 75 TEMPERATURE (°C) 100 0.95 0.90 PWM INPUT CURRENT (µA) VC SWITCH THRESHOLD VOLTAGE (V) IVC = 500µA 3.85 VC CLAMP(V) PWM Pin Input Current vs Voltage VC Switching Threshold vs Temperature 0.85 0.80 0.75 0.70 0.65 0.60 50 0 25 75 TEMPERATURE (°C) 100 1.10 1.00 0.90 0.80 0.70 0.60 0.
PAGE 7
LT1768 U U U PIN FUNCTIONS PGND (Pin 1): The PGND pin is the high current ground path. High switching current transients and lamp current flow through the PGND pin. DIO1/DIO2 (Pins 3/2): Each DIO pin is the common connection between the cathode and anode of two internal diodes. The remaining terminals of the diodes are connected to PGND. In a typical application, the DIO1/2 pins are connected to the low voltage side of the lamps.
PAGE 8
LT1768 U U U PIN FUNCTIONS RMAX (Pin 10): The RMAX pin outputs a regulated voltage of 1.25V that is to be loaded with an external resistor. The current through the external resistor sets the maximum lamp current. Maximum lamp current in a dual lamp application will be approximately equal to 100 times IRMAX when the voltage on the PROG pin is greater than 4V. The value of RRMAX must be greater than 5K and less than [RRMIN • 2.5 • (VPWM–1/3)] for proper PWM operation.
PAGE 9
LT1768 W BLOCK DIAGRA VIN 15 VREF SHDN 12 UNDERVOLTAGE LOCKOUT THERMAL SHUTDOWN OSC VIN GATE SW BLANK 16 GATE VREF 14 RMIN 11 1.26V IRMIN S RMAX 10 1.25V Q IRMAX 0 1V 4V R MODE CONTROL PROG 8 IVC SLOPE VCCLAMP FAULT 4 SENSE VPWM PWM 9 1V CT 7 PWM PERIOD (IDIO1 + IDIO2) GAIN MULTI-MODE DIMMING BLOCK 1 PGND IDIO1 < 125µA IVC 13 FAULT IDIO2 < 125µA 1768 BD 5 6 3 2 VC AGND DI01 DIO2 Figure 2.
PAGE 10
LT1768 U W U U APPLICATIONS INFORMATION The current mode pulse width modulation produces an average current in inductor L1 proportional to the VC voltage. Inductor L1 then acts as a switched mode current source for a current driven Royer class converter with efficiencies as high as 90%. T1, C4 and Q1 comprise the Royer class converter which provides the CCFLs with a zero DC, 60kHz sinusoidal waveform whose amplitude is based on the average current in L1.
PAGE 11
LT1768 U W U U APPLICATIONS INFORMATION per lamp to 1mA for all PROG voltages but limits the dimming ratio to 6:1. Trace B in Figures 3a and 3b shows Figure 1’s CCFL current waveform operating at 1mA in PWM mode. Maximum current mode (VPROG > 4V) sets the VC source current to five times the current out of the 1.25V referenced RMAX pin. Setting RRMAX to produce CCFL current equal to the manufacturer’s maximum rating in this mode insures no degradation in the specified lamp lifetime.
PAGE 12
LT1768 U U W U APPLICATIONS INFORMATION and is dependant on the operating mode. For dual lamp displays, the transfer function for minimum current mode (IDIO/IRMIN) is equal to 10A/A, and for maximum current mode (IDIO/IRMAX) is equal to 100A/A. The transfer functions discussed above are between RMAX and RMIN current and average lamp current not RMS lamp current.
PAGE 13
LT1768 U W U U APPLICATIONS INFORMATION voltage and DIO lines can cause enough current flow to fool the open lamp detection. In situations where coupling can’t be avoided, resistors can be added from the DIO pins to ground to increase the open lamp threshold. When resistors from the DIO pins to ground are added, the values for RRMAX and RRMIN may need to be increased from their nominal values to compensate for the additional current.
PAGE 14
LT1768 U W U U APPLICATIONS INFORMATION GATE The LT1768 has a single high current totem pole output stage. This output stage is capable of driving up to ±1.5A of output current. Cross-conduction current spikes in the totem pole output have been eliminated. The GATE pin is intended to drive an N-channel MOSFET switch. Rise and fall times are typically 50ns with a 3000pF load. A clamp is built into the device to prevent the GATE pin from rising above 13V in order to protect the gate of the MOSFET switch.
PAGE 15
LT1768 U W U U APPLICATIONS INFORMATION Supply Bypass and Layout Considerations Proper supply bypassing and layout techniques must be used to insure proper regulation, avoid display flicker, and insure long term reliability. Figure 8 shows the application’s critical high current paths in thick lines. Ideally, all components in the high current path should be placed as close as possible and connected with short thick traces.
PAGE 16
LT1768 U TYPICAL APPLICATIONS DC Intensity Control VREF R1 100k POT PROG LT1768 AGND 1768 TA04 PWM Intensity Control 0 – >5V 1kHz PWM VREF R1 49.9k PROG C1 10µF LT1768 AGND 1768 TA05 PWM Intensity Control From 3.3V or 5V Logic VREF R1 10k 0 – >3.3V OR 0 – >5V 1kHz PWM PROG R1 49.
PAGE 17
LT1768 U TYPICAL APPLICATIONS 2-Wire Serial interface Intensity Control VCC SCL VREF PROG LT1768 AGND VOUT SDA LTC1663 GND 1768 TA08 Pushbutton Intensity Control R1 50k S1 CLK1 S2 SHDN VCC CLK2 LTC1426 AGND VREF PWM2 PWM1 VREF PROG LT1768 AGND R1 49.
PAGE 18
PROG 0V TO 5V OR 1kHz PWM C3 0.1µF C2 0.047µF R1 49.9k DI01 DI02 PGND VREF VIN GATE 14 15 16 8 7 6 4 C4 10µF PROG CT AGND PWM RMAX RMIN SHUTDOWN FAULT 9 C1 33µF D4 BAT54 R11 1k C6 1µF R4 10 11.3k 11 13 SENSE FAULT LT1768 12 5 VC SHDN 3 2 1 VIN = 12V D3 BAT54 R10 1k R3 69.8k R2 30.1k 5V R5 125k C5 0.1µF X4 X3 X2 X1 R6 499Ω R7 499Ω LAMP LAMP LAMP LAMP C7 2200pF D2 MBRS130LT3 T1 C15, 22pF C14, 22pF C13, 22pF C12, 22pF R8 100Ω Q1A ZDT1048 R9 0.
PAGE 19
LT1768 U PACKAGE DESCRIPTIO GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) 0.189 – 0.196* (4.801 – 4.978) 16 15 14 13 12 11 10 9 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.007 – 0.0098 (0.178 – 0.249) 0.009 (0.229) REF 2 3 4 5 6 7 0.053 – 0.068 (1.351 – 1.727) 8 0.004 – 0.0098 (0.102 – 0.249) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.008 – 0.012 (0.203 – 0.305) 0.0250 (0.
PAGE 20
LT1768 U TYPICAL APPLICATION 4 Watt Single Lamp CCFL Supply C9 33pF X1 LAMP T1 CTX110607 VIN = 9V TO 24V C1 33µF C7 0.33µF 1 2 3 4 5 C2 0.047µF 6 7 C3 0.22µF PROG 0V TO 5V OR 1kHz PWM R1 49.9k 8 PGND GATE 16 15 DI02 VIN DI01 VREF 14 AGND RMIN CT RMAX PROG PWM FAULT R5 124k SHUTDOWN 11 Q2 Si3456DV R2 39.2k C6 1µF C4 10µF Q1B ZDT1048 L1 33µH D2 MBRS130LT3 R4 10 31.6k 9 Q1A ZDT1048 C5 0.1µF 13 SENSE FAULT LT1768 12 VC SHDN R7 499Ω 5V R3 61.