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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 integrating

such features as a feedback controlled PWM driver stage, open lamp pro

tection, startup delay and synchronization circuitry between the buck and

push-pull stages. The UCC3972/3 include an internal shunt regulator, al

lowing 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)

SLUS252C - OCTOBER 1998 - REVISED MARCH 2005

VBAT

VDD

GND

MODE

COMP

BUCK

OUT

0.1µF

D1 R10

R11

C5 0.1µF

68µH

R6 75Ω

R2 1k

C6 27pF

LAMP

R3 68k

R5 10k

R4 750

ANALO G

DIMMING

LF D

68k

0V-5V LOW FREQUENCY CONTROL S IGNAL

C4 33nF

LOW FREQUENCY DIMMING

1µF

1kΩ

6.8µF

S YS TEM VO LTAG E

(4.5V TO 25V)

1µF

UCC3972

UCC3973

NO INTER NAL VO LTAG E C LAMP

INTE R NAL VO LTAG E C LAMP LIMITS TR ANS FO R MER

VO LTAG E AT S TART-UP O R DUR ING F AULT

UCC3972/3

TYPICAL APPLICATION CIRCUIT

UDG-99154

Summary of content (24 pages)

PAGE 1
UCC1972/3 UCC2972/3 UCC3972/3 BiCMOS Cold Cathode Fluorescent Lamp Driver Controller FEATURES DESCRIPTION • 1mA Typical Supply Current Design goals for a Cold Cathode Fluorescent Lamp (CCFL) converter used in a notebook computer or portable application include small size, high efficiency, and low cost. The UCC3972/3 CCFL controllers provide the necessary circuit blocks to implement a highly efficient CCFL backlight power supply in a small footprint 8 pin TSSOP package.
PAGE 2
UCC1972/3 UCC2972/3 UCC3972/3 CONNECTION DIAGRAMS ABSOLUTE MAXIMUM RATINGS VBAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +27V VDD Maximum Forced Current . . . . . . . . . . . . . . . . . . . . 30mA Maximum Forced Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 17V BUCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –5V to VBAT MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 4.
PAGE 3
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; TA=TJ; VDD=VBAT=VBUCK=12V; MODE=OPEN. For any tests with VBAT>17V, place a 1k resistor from VBAT to VDD.
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UCC1972/3 UCC2972/3 UCC3972/3 PIN DESCRIPTIONS BUCK: Senses the voltage on the top side of the inductor 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 below 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. GND: Ground reference for the IC.
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UCC1972/3 UCC2972/3 UCC3972/3 BLOCK DIAGRAM VBAT VDD 8 18V + TO S 3 UVLO 4.0V/3.8V – UVLO=1 OP EN LAMP DETECT COMP ARATOR 7.0V S2 + VREF 3V REF + 3 BIT UP -DOWN COUNTER OSCILLATOR – + 1.0V – + S YNC UVLO 0.2V Q + 7 OUT 6 GND 4 FB VDD UVLO R + P WM – S Q OUTP UT OFF (FROM MODE S ELECT) FROM CLAMP COMP VDD 20µA S1 5 BUCK R S MODE 1 ZERO DETECT COMP ARATOR FROM MODE S ELECT RAMP OUT 2 9.
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UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) Circuit Operation The resonant tank consisting of CRES and T1 produces sinusoidal currents (IRES) and voltages and is fed by a controlled DC current (IBUCK) from the buck stage. Note that the BUCK node voltage is ½ the primary tank voltage, as VBAT is located at the center tap of the transformer.
PAGE 7
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) frequencies, the UCC3972/3 synchronizes the buck frequency to the frequency of the push-pull stage. The traditional buck topology is inverted to take advantage of the lower RDS(on) characteristics of an N-Channel MOSFET switch (SBUCK). With a sinusoidal voltage across the tank, the resulting output of the buck stage (VBUCK) becomes a full-wave rectified voltage referenced to VBAT as shown in Fig. 1.
PAGE 8
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) the operating frequencies of a particular design are within the synchronizable frequencies of the controller. half cycles (i.e. only ½ of the primary winding sees the buck current depending upon which transistor is on). MaxiComponent Selection for the Resonant Tank and Out - mum resonant current is equal to: V 820 put Circuit (6) I RES = PRIMARY = = 600 mA LPRIMARY 44 Since high efficiency is a primary goal of the backlight 67· 0 .
PAGE 9
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) The current sense resistor RS provides direct control of lamp current. Since the current sense resistor voltage is controlled to a 1.5V reference, its power loss is inversely proportional to its value at a given lamp current. VBUCK _ AVE = VBAT = VBAT - Synchronizing the Stages I BUCK = • The topology is inverted using a ground referenced N-Channel MOSFET rather than a VDD referenced P-Channel.
PAGE 10
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) Dimming Techniques Analog Dimming: A control circuit that implements analog dimming with a potentiometer (RADJ) is shown in Fig. 2. When the secondary has a positive polarity current, D1 is reversed biased and lamp current is sensed directly through RL and RADJ. When the current reverses direction, D1 conducts and the voltage on the sense node VX is clamped to the forward drop of the diode.
PAGE 11
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) Referring to Fig. 5, at time t0 the control signal is brought low and the voltage in the resonant tank begins to build. At time t1 there is sufficient voltage for the lamp to strike and the feedback loop controls the lamp at rated current using a fixed current sense resistor. When the LFD signal is brought low at time T2, the COMP output is low and the OUT pin stops switching. The resonant tank voltage decays until the lamp extinguishes.
PAGE 12
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) C FB – + ERROR AMP LIFIER 1.5V P WM MODULATOR VBAT VS AW 2 P OLE RLC FILTER TRANS FORMER 2NS NP S ECONDARY IMP EDANCE R LAMP 2 +XC 2 R FB 0.5R S Figure 7. Current control loop block diagram. The resulting gain of the filter is unity below the 15kHz corner frequency, peaking up at the corner frequency with Q, and rolling off with a 2-pole response above the corner frequency. As shown in Fig.
PAGE 13
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) VBAT 2 T1 LAMP C5 R9 Q2 DCLAMP Q1 R7 R8 VBUCK D4 R3 Q4 2N3906 D2 R4 UCC3972 EXTERNAL VOLTAGE CLAMP L1 4 FB UDG-99161 Figure 8. Optional voltage clamp circuit. For UCC3972. (Not required for UCC3973) An optional zener diode DCLAMP can be added to either UCC3972 or UCC3973 designs as shown in Fig. 8.
PAGE 14
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) ond blank time if a true open lamp existed. If the open lamp voltage is increased, the peak clamp circuit voltage (equation 16) would need to be increased accordingly. A peak VBAT-Vbuck voltage of 10.5V has been set for open lamp detection in this example. (R10 = 2k, R11 = 1k).
PAGE 15
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) Low Current Shutdown Circuit: Since the shunt regulator circuitry needs to remain active, even when the MODE pin is less than 1V and the output is not switching, a low current shutdown is not provided in the UCC3972/3. The following is a simple on/off control requiring only two transistors with internal bias resistors to disconnect VIN providing a low current shutdown.
PAGE 16
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) Since a fluorescent lamp is a pressurized gas filled tube (usually Argon and Mercury vapor), it shouldn’t be surprising that temperature plays a major role in the lamp characteristics. Fig. 13 depicts the variations in striking and operating voltage for a 150 x 3mm lamp over temperature, illustrating the importance of taking temperature effects into account when designing the converter.
PAGE 17
UCC1972/3 UCC2972/3 UCC3972/3 APPLICATION INFORMATION (cont.) In a practical backlight design, the physical spacing between the lamp and high voltage secondary wiring with respect to the foil reflector and LCD frame can be tight. With this tight spacing, distributed stray capacitance will form as shown in Fig. 17. The stray capacitance causes leakage currents from the high voltage secondary to circuit ground.
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE OPTION ADDENDUM www.ti.com 18-Oct-2013 LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device.
PAGE 20
PACKAGE MATERIALS INFORMATION www.ti.com 6-Jun-2013 TAPE AND REEL INFORMATION *All dimensions are nominal Device UCC2973PWTR Package Package Pins Type Drawing TSSOP PW 8 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) 2000 330.0 12.4 Pack Materials-Page 1 7.0 B0 (mm) K0 (mm) P1 (mm) 3.6 1.6 8.0 W Pin1 (mm) Quadrant 12.
PAGE 21
PACKAGE MATERIALS INFORMATION www.ti.com 6-Jun-2013 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) UCC2973PWTR TSSOP PW 8 2000 367.0 367.0 35.
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