Texas Instruments UCC3973PW, UCC3973PWTR, UCC3973N, UCC3972PWTR, UCC3972PW Datasheet

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UCC1972/3 UCC2972/3 UCC3972/3

DESCRIPTION

Design goals for a Cold Cathode Fluorescent Lamp (CCFL) converter used in a notebook computer or portable application include small size, high effi

ciency, and low cost. The UCC3972/3 CCFL controllers provide the neces

sary circuit blocks to implement a highly efficient CCFL backlight power supply in a small footprint 8 pin TSSOP package. The BiCMOS controllers typically consume less than 1mA of operating current, improving overall system efficiency when compared to bipolar controllers requiring 5mA to 10mA of operating current.

External parts count is minimized and system cost is reduced by integrat

ing such features as a feedback controlled PWM driver stage, open lamp protection, startup delay and synchronization circuitry between the buck and push-pull stages. The UCC3972/3 include an internal shunt regulator, allowing the part to operate with input voltages from 4.5V up to 25V. The part supports both analog and externally generated low frequency dimming modes of operation.

The UCC3973 adds a programmable voltage clamp at the BUCK pin. This feature can be used to protect the transformer from overvoltage during startup or when an open lamp occurs. Transformer voltage is controlled by reducing duty cycle when an over-voltage is detected.

BiCMOS Cold Cathode Fluorescent Lamp Driver Controller

FEATURES

•

1mA Typical Supply Current

•

Accurate Lamp Current Control

•

Analog or Low Frequency Dimming Capability

•

Open Lamp Protection

•

Programmable Startup Delay

•

4.5V to 25V Operation

•

PWM Frequency Synchronized to External Resonant Tank

•

8 Pin TSSOP and SOIC Packages Available

• Internal Voltage Clamp Protects

Transformer from Over-voltage (UCC3973)

SLUS252A - JANUARY 2000

VBAT

VDD

GND

MODE

COMP

BUCK

OUT

0.1µF

R10

R11

C5 0.1 µF

68µH

R6 75Ω

R2 1k

C6 27pF

LAMP

R3 68k

R5 10k

R4 750

ANALOG DIMMING

LFD

68k

0V-5V LOW FREQUENCY CONTROL SIGNAL

C4 33nF

LOW FREQUENCY DIMMING

1µF

1kΩ

6.8µF

SYSTEM VOLTAGE (4.5V TO 25V)

1µF

UCC3972

UCC3973

NO INTERNAL VOLTAGE CLAMP

INTERNAL VOLTAGE CLAMP LIMITS TRANSFORMER VOLTAGE AT START-UP OR DURING FAULT

UCC3972/3

TYPICAL APPLICATION CIRCUIT

UDG-99154

UCC1972/3 UCC2972/3 UCC3972/3

TSSOP-8 (TOP VIEW) PW Package

ABSOLUTE MAXIMUM RATINGS

VBAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +27V VDD Maximum Forced Current . . . . . . . . . . . . . . . . . . . . 30mA Maximum Forced Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 17V BUCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5V to VBAT MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 4.0V MODE Maximum Forced Current . . . . . . . . . . . . . . . . . . 300µA Operating Junction Temperature . . . . . . . . . . –55°C to +150°C Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C

Unless otherwise indicated, currents are positive into, negative out of the specified terminal. Pulse is defined as less than 10%

duty cycle with a maximum duration of 500

s. Consult Pack

aging Section of Databook for thermal limitations and consider

ations of packages. All voltages are referenced to GND.

CONNECTION DIAGRAMS

ELECTRICAL CHARACTERISTICS:

Unless otherwise specified these specifications hold for TA=0°C to +70°C for the

UC3972/3, –40°C to +85°C for the UC2972/3, and –55°C to +125°C for the UC1972/3; T

A=TJ

; VDD=VBAT=VBUCK=12V;

MODE=OPEN. For any tests with VBAT>17V, place a 1k resistor from VBAT to VDD.

PARAMETER TEST CONDITIONS MIN TYP MAX UNITS

Input supply

VDD Supply Current VDD = 12V 1 1.5 mA

VBAT = 25V 7 10.5 mA

VBAT Supply Current VBAT = 12V 30 60

µA

VBAT = 25V 70 140

µA

VDD Regulator Turn-on Voltage I

SOURCE

= 2mA to 10mA 17 18 19 V

VDD UVLO Threshold Low to high 3.6 4 4.4 V

UVLO Threshold Hysteresis 100 200 300 mV

Output Section

Pull Down Resistance I

SINK

= 10mA to 100mA 25 50 Ω

Pull Up Resistance I

SOURCE

= 10mA to 100mA 25 50 Ω

Output Clamp Voltage VBAT = 25V, Shunt Regulator on 16 18 V

Output Low MODE = 0.5V, I

SINK

= 1mA 0.05 0.2 V

Rise Time CL = 1nF, Note 1 200 ns

Fall Time CL = 1nF, Note 1 200 ns

MODE

GND

COMP

VDD

OUT

BUCK

VBAT

DIL-8 (TOP VIEW) J, N Packages

UCC1972/3 UCC2972/3 UCC3972/3

ELECTRICAL CHARACTERISTICS:

Unless otherwise specified these specifications hold for TA=0°C to +70°C for the

UC3972/3, –40°C to +85°C for the UC2972/3, and –55°C to +125°C for the UC1972/3; T

A=TJ

; VDD=VBAT=VBUCK=12V;

MODE=OPEN. For any tests with VBAT>17V, place a 1k resistor from VBAT to VDD.

PARAMETER TEST CONDITIONS MIN TYP MAX UNITS

Oscillator Section

Minimum Frequency BUCK = VBAT– 2, V

BAT

= 12V to 25V,

= –40°C to +85°C

52 66 80 kHz

BUCK = VBAT–2, V

BAT

= 12V to 25V

= –55°C to +125°C

44 66 80 kHz

Maximum Synchronizable Frequency BUCK = VBAT, V

BAT

= 12V to 25V

= –40°C to +85°C

160 220 280 kHz

BUCK = VBAT, V

BAT

= 12V to 25V

= –55°C to +125°C

145 220 280 kHz

Maximum Duty Cycle FB = 1V, T

< 0°C 84 %

FB = 1V, T

= 0°C to 70°C 92 95 %

Minimum Duty Cycle FB = 2V 0 %

BUCK Input Bias Current BUCK = VBAT = 12V 40 90

µA

BUCK = VBAT = 25V 80 110

Zero Detect Threshold Measured at BUCK w/respect to VBAT,

VBAT=12V to 25V, T

< 0°C

–2.4 –1 –0.3 V

Measured at BUCK w/respect to VBAT, VBAT=12V to 25V, T

= 0°C to 70°C

–2.0 –1 –0.3 V

Error Amplifier

Input Voltage COMP = 2V, T

= 0°C to +70°C 1.465 1.5 1.535 V

COMP = 2V 1.455 1.545 V

Line Regulation –2 2 10 mV

Input Bias Current –500 –100 nA

Open Loop Gain COMP = 0.5V to 3.0V 60 80 dB

Output High Voltage FB = 1V 3.3 3.7 4.1 V

Output Low Voltage FB = 2V 0.15 0.35 V

Output Source Current FB = 1V, COMP = 2V –1.2 –0.4 mA

Output Sink Current FB = 2V, COMP = 2V 2 4 mA

Output Source Current FB = 1V, COMP = 2V, MODE = 0.5V –1 1

µA

Output Sink Current FB = 2V, COMP = 2V, MODE = 0.5V –1 1

µA

Unity Gain Bandwidth T

= 25C, Note 1 2 MHz

Mode Select

Output Enable Threshold 0.85 1 1.15 V

Open Lamp Detect Enable Threshold 2.75 3 3.25 V

Mode Output Current MODE = 0.5V 15 20 25

µA

MODE Clamp Voltage MODE = OPEN 3.3 3.7 4 V

Open Lamp

Open Lamp Detect Threshold Measured at BUCK with respect to VBAT,

VBAT=12V to 25V

–8 –7 –6 V

Over-voltage Clamp Threshold (UCC3973) Measured at BUCK with respect to VBAT,

VBAT=12V to 25V, I

= 100µA

–10.3 –9 –7.7 V

Note 1. Guaranteed by design. Not 100% tested in production.

UCC1972/3 UCC2972/3 UCC3972/3

BUCK: Senses the voltage on the top side of the induc

tor feeding the resonant tank. The voltage at this point is used to synchronize the internally generated ramp and to detect whether an open lamp condition exists. An open lamp condition exists when this voltage is be

low the specified threshold for seven clock cycles. If the MODE pin is held below the open lamp detect enable threshold, this protective feature is disabled.

On the UCC3973, this pin is also used to sense an over-voltage across the transformer primary. If the volt

age at this pin exceeds the clamp threshold, current will be sourced fron the FB pin.

COMP: Output of the error amplifier.Compensation components set the bandwidth of the entire system and are normally connected between COMP and FB. The error amplifier averages lamp current against a fixed in

ternal reference. The resulting voltage on the COMP pin is compared to an internally generated ramp, set

ting the PWM duty cycle. During UVLO, this pin is actively pulled low.

FB: This pin is the inverting input to the error amplifier. On the UCC3973, current is sourced form this pin if the clamp threshold is exceeded at the BUCK pin (see below). The sourced current will reduce OUT duty cycle to control transformer primary voltage. The source current is disabled on the UCC3972.

GND: Ground reference for the IC.

MODE: The voltage on this pin is used to control start-up

and various modes of operation for the part (refer to the ta

ble in the block diagram).

When the voltage is below 1V, OUT is forced low, open lamp detection is disabled and the error amplifier is tri-stated.

When the voltage is between 1V and 3V, OUT is enabled and the error amplifier output is connected to COMP. Open lamp detection is still disabled and a constant 20µA current is sourced from this pin. Placing an appropriate value external capacitor between this pin and ground al

lows the user to disable open lamp detection for a set pe

riod of time at start-up to allow the lamp to strike.

When MODE reaches 3V, open lamp detection is enabled and normal operation is activated.

OUT: Drives the buck regulator N-channel MOSFET. OUT turn-on is synchronized to twice the tank resonant frequency. OUT is actively pulled low when in UVLO, an open lamp condition has been detected or MODE is less than 1V.

VBAT: Positive input supply to power stage. This voltage is required by internal control circuitry to provide open-lamp detection and synchronization. Operating range is from 4.5V to 25V.

VDD: This pin connects to the battery voltage from which the CCFL inverter will operate. If the potential on VBAT can exceed 18V in the application, a series resistor must be placed between VBAT and this pin (see applications section). The voltage at the VDD pin will then be regulated to 18V. This pin should be bypassed with a minimum ca

pacitance of 0.1µF.

PIN DESCRIPTIONS

200

400

600

800

8.7 9.2 9.7

BAT-VBUCK

CURRENT OUT OF FB (uA)

Clamp current vs. tank voltage for UCC3973.

UCC1972/3 UCC2972/3 UCC3972/3

UDG-98154

*MODE Output Open Lamp

Detection

S2 Error Amplifier Output S1

<1V OFF DISABLED OPEN DISCONNECTED FROM COMP OPEN

1V< MODE< 3V ON DISABLED OPEN CONNECTED TO COMP CLOSED

>3V ON ENABLED CLOSED CONNECTED TO COMP CLOSED

VDD

20µA

–

1.5V

ERROR

AMPLIFIER

–

1.0V

7.0V

SYNC

OUTRAMP

0.2V

66kHz-200kHz

OSCILLATOR

*MODE

SELECT

UVLO

4.0V/3.8V

3V REF

VREF

UVLO=1

3BIT UP-DOWN COUNTER

FROM MODE SELECT

OPEN LAMP DETECT

COMPARATOR

ZERO DETECT COMPARATOR

UVLO

PWM

UVLO

OUTPUT OFF

(FROM MODE SELECT)

VDD

MODE

COMP

GND

OUT

BUCK

VDD

VBAT

18V

–

TO S3

OVER-VOLTAGE

CLAMP COMPARATOR

(ALWAYS OPEN ON UCC3972)

FROM

CLAMP

COMP

CLAMP

9.0V

BLOCK DIAGRAM

Introduction

Cold Cathode Fluorescent Lamps (CCFL) are frequently used as the backlight source for Liquid Crystal Displays (LCDs). These displays are found in numerous applica

tions such as notebook computers, portable instrumenta

tion, automotive displays, and retail terminals. Fluorescent lamps provide superior light output effi

ciency, making their use ideal for power sensitive porta

ble applications where the backlight circuit can consume a significant portion of the battery’s capacity. The

backlight converter must produce the high voltage needed to strike and operate the lamp. Although CCFLs can be operated with a DC voltage, a symmetrical AC operating voltage is recommended to maintain the rated life of the lamp. Sinusiodal voltage and current lamp waveforms are also recommended to achieve optimal electrical to light conversion and to reduce high voltage electromagnetic interference (EMI). A topology that pro

vides these requirements while maintaining efficient op

eration is presented below.

APPLICATION INFORMATION

UCC1972/3 UCC2972/3 UCC3972/3

Circuit Operation

A current fed push-pull topology is used to power the CCFL backlight shown in Fig. 1. This topology accommo

dates a wide input voltage and dimming range while re

taining sinusoidal operation of the lamp. The converter consists of a resonant push-pull stage, a high voltage output stage, and a buck pre-stage used to regulate cur

rent in the converter.

Referring to Fig. 1, the push-pull stage consists of C

RES

Q1, Q2, R

, and T1’s primary and auxiliary windings.

The output stage consists of C

BALLAST

, the lamp, the

current sense resistor R

, and T1’s secondary. The reso

nant frequency of the tank is set by the primary induc

tance of T1, along with the resonant capacitor (C

RES

), and the reflected secondary impedance. The secondary impedance includes the lamp, the ballast capacitor (C

BALLAST

), the distributed winding capacitance of T1, and the stray capacitance which forms between the lamp, lamp wires, and the backlight reflector. Since the lamp impedance is nonlinear with operating current, the tank resonant frequency will vary slightly with load (typically 1.5:1).

The resonant tank consisting of C

RES

and T1 produces

sinusoidal currents (I

RES

) and voltages and is fed by a

controlled DC current (I

BUCK

) from the buck stage. Note

that the BUCK node voltage is ½ the primary tank volt

age, as VBAT is located at the center tap of the trans

former. The high turns ratio transformer (T1) amplifies the sinusoidal tank voltage to produce a sinusoidal sec

ondary voltage that is divided between the lamp and bal

last capacitor.

Transistors Q1 and Q2 are driven out of phase at 50 per

cent duty cycle with an auxiliary winding on T1. The winding provides a floating AC voltage source at the res

onant frequency that is used to drive the transistor bases alternately on and off. One leg of the auxiliary winding is tied to the input voltage through base resistor R

, which

is sized to provide sufficient base current to the transis

tors. The transistors channel the buck inductor current into opposing ends of the tank at the resonant frequency, supplying energy for the lamp and system losses.

The buck power stage consists of inductor L

BUCK

MOSFET switch S

BUCK

, and flyback diode D

BUCK

.Inor-

der to prevent interactions between multiple switching

APPLICATION INFORMATION (cont.)

RES

VBAT

RES

VBAT

AUXILIARY

RESONANT PUSH-PULL STAGE

T1 PRIMARY

BALLAST

VBAT

LAMP

T1 SECONDARY

CCFL

OUTPUT STAGE

BUCK

OUT

VBAT

BUCK

BUCK STAGE

VBUCK

GND

VBAT

Q1ONQ2ONQ1ONQ2

BUCK

Figure 1. Push-pull, output, and buck stages.

UDG-98157

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