UCC3973PWTRG4 datasheet(7/24 Pages) TI1

Teilenummer	UCC3973PWTRG4

Bauteilbeschribung	BiCMOS Cold Cathode Fluorescent Lamp Driver Controller

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7

UCC1972/3

UCC2972/3

UCC3972/3

frequencies, the UCC3972/3 synchronizes the buck fre-

quency to the frequency of the push-pull stage. The tra-

ditional 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.

Lamp current is sensed directly with RS and a parallel di-

ode on each half cycle. The resulting voltage across the

sense resistor RS is kept at a 1.5V average by the error

amplifier, which in turn controls the duty cycle of SBUCK.

The buck converter typically operates in continuous cur-

rent mode but can operate with discontinuous current as

the CCFL is dimmed.

Design Procedure

A notebook computer backlight circuit will be presented

here to illustrate a design based on the UCC3972/3 con-

troller. The converter will be designed to drive a single

cold cathode fluorescent lamp (CCFL) with the following

specifications:

Input Voltage Range:

The notebook computer will be powered by a 4 cell Lith-

ium-Ion battery pack with an operational voltage range of

10V to 16.8V. When the pack is being charged, the back

light converter is powered from an AC adapter whose DC

output voltage can be as high as 22V.

Resonant Tank and Output Circuit

The selection of components to be used in the resonant

tank of the converter is critical in trading off the electrical

and optical efficiencies of the system. The value of the

output circuit’s ballast capacitor plays a key role in this

trade-off. The voltage across the ballast capacitor is a

function of the resonant frequency and secondary lamp

current:

V

I

CF

CB

LAMP

BALLAST

RESONANT

=

··

·

2 p

(1)

A voltage drop across CBALLAST many times the lamp

voltage will make the secondary current insensitive to

distortions caused by the non-linear behavior of the

lamp, providing a high impedance sinusoidal current

source with which to drive the CCFL. This approach im-

proves the optical efficiency of the system, as capacitive

leakage effects are minimized due to reduced harmonic

content in the voltage waveforms. Unfortunately, from an

electrical efficiency standpoint, an increased tank voltage

produces increased flux losses in the transformer and in-

creased circulating currents in the tank. In practice, the

voltage drop across the ballast capacitor is selected to

be approximately twice the lamp voltage (750V in our

case) at rated lamp current. Assuming a 50kHz resonant

frequency and 5mA operating current, a ballast capaci-

tance of 22pF is selected. Since the lamp and ballast ca-

pacitor impedance are 90 degrees out of phase, the

vector sum of lamp and capacitor voltages determine the

secondary voltage on the transformer.

() (

)

VV

V

SEC

CB

LAMP

=+

22

(2)

The resulting secondary voltage at rated lamp current is

820V. Since the capacitor dominates the secondary im-

pedance, the lamp current maintains a sinusiodal shape

despite the non-linear behavior of the lamp. As the CCFL

is dimmed, lamp voltage begins to dominate the second-

ary impedance and current becomes less sinusiodal.

Transformer secondary voltage is reduced, however, so

high frequency capacitive losses are less pronounced.

The value of ballast capacitor has no effect on current

regulation since the average lamp current is sensed di-

rectly by the controller.

Once the ballast capacitor is selected, the resonant fre-

quency of the push-pull stage can be determined from

the transformer’s inductance (L), turns ratio (N), and the

selection of resonating capacitor (CRES).

()

F

LC

N

C

RESONANT

PRIMARY

RES

BALLAST

=

+·

æ

èç

ö

ø÷

1

2

p

(3)

Output distortion is minimized by keeping the independ-

ent resonant frequencies of the primary and secondary

circuits equal. This is achieved by making the resonant

capacitor equal to the ballast capacitance times the turns

ratio squared:

()

CN

C

pF

F

RES

BALLAST

=·

=

·

=

2

67

22

0 1

. m

(4)

The resulting resonant frequency is about 50kHz, this

frequency will vary depending upon the lamp load and

amount of stray capacitance in the system. Since the

UCC3972/3 has an internal oscillator, it is important that

APPLICATION INFORMATION (cont.)

Lamp Length

250mm (10”)

Lamp Diameter

6mm

Striking Voltage (20

°C)

1000V (PEAK)

Operating Voltage (5mA)

375V (RMS)

Full Rated Current

5mA

Full Rated Power

1.9W

Table 1. Lamp Specifications

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