UCC1973 datasheet(12/26 Pages) TI2

12 / 26 page

UCC1972/3

UCC2972/3

UCC3972/3

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. 7, the transformer

turns ratio provides a voltage gain and the output circuit

(whose impedance includes the lamp and ballast capaci-

tor) converts the voltage into a current. The current

sense resistor produces a voltage on each half cycle,

leaving the error amplifier as the final gain block.

Loop gain is greatest at minimum lamp current and maxi-

mum input voltage. With a 22V input, a 2V saw-tooth,

and 1:67 turns transformer, the low frequency voltage

gain of the PWM, RLC filter, and Transformer is 1500.

With a 375V lamp and 1mA of lamp current (using a

22pF ballast capacitor and 50kHz switching frequency)

the secondary impedance is 400k

W.RSENSE at 1mA is

W (equation 12), resulting in a low frequency power

loop gain of 7.5. The error amplifier is configured as an

integrator, giving a single pole roll-off and a high gain at

DC. A 68k resistor and 33nF capacitor give a 70Hz

crossover frequency for the feedback network, yielding a

maximum crossover frequency of 500Hz for the total loop

avoiding stability problems with the Q of the resonant

tank. For 5mA of lamp current with a 22V input the total

loop crossover is 200Hz, for low frequency dimming ap-

plications CFB can be reduced to 6.8nF with no instability

(1kHz crossover).

Striking the Lamp

Before the lamp is struck, the lamp presents an imped-

ance much larger than the ballast capacitor and the full

output voltage of the transformer secondary is across the

lamp. Since the buck converter must reverse the

volt-seconds on the buck inductor, the average tank volt-

age at the primary can be no greater than the DC input

voltage. This constraint along with the turns ratio of the

push-pull transformer sets the peak voltage available to

strike the lamp:

STRIKE

S P

INPUT

=· ·

(15)

The Coiltronics transformer has a 67:1 turns ratio, giving

2100 peak volts available to strike the lamp with the mini-

mum 10V input. In our example this is more than suffi-

cient for the 1000V required to strike the lamp. With the

22V maximum charger input, the available striking volt-

age could theoretically reach 5000V! The possibility of

breaking down the transformer’s secondary insulation

becomes a real concern at this voltage.

Voltage Clamp Circuit (UCC3972)

An external voltage clamp circuit consisting of D4, Q4,

R7, R8, and R9 can be added to the typical application

circuit as shown in Fig. 8. This circuit limits the maximum

transformer voltage during startup, allowing an extended

time period for striking the lamp while protecting the

transformer from over voltage. For fixed input voltage de-

signs, this circuit is optional since the transformer turns

can be optimized at one voltage.

–

PWM

MODULATOR

2POLE

RLC

FILTER

TRANSFORMER

SECONDARY

IMPEDANCE

1.5V

0.5RS

2NS

VBAT

VSAW

RLAMP

2+X

ERROR

AMPLIFIER

Figure 7. Current control loop block diagram.

APPLICATION INFORMATION (cont.)

UCC1973 Datasheet(PDF) 12 Page - Texas Instruments

UCC1973 Datasheet(HTML) 12 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL