TA9687GN datasheet(7/14 Pages) O2MICRO

7 / 14 page

ta9687

CONFIDENTIAL

ta9687-DS-1.0

Page 6

FUNCTIONAL DESCRIPTION

1. Power Conversion

The ta9687 controller supports full-bridge power

conversion topology and provides symmetrical

drive pulses to the tank circuit that includes the

transformer(s),

output

capacitors

and

the

CCFL/panel load that yields quasi-sinusoidal

CCFL voltage and current waveforms. High

efficiency operation of the ta9687 yields lower

heat dissipation for the inverter system resulting

in higher reliability.

To illustrate the controller functions, refer to

Figures 1 and 2 on pages 5 and 8, respectively

for the following sections.

2. Enable

Applying a voltage level greater than 2V to ENA

(pin 16) enables the IC. A voltage less than 1V

will disable the IC.

3. Soft-Start (SST)

Utilizing a patented multi-task technique, the soft-

start function and the loop compensation function

are combined to provide a good start-up

characteristic. Connecting an external capacitor

to SSTCMP (pin 2) provides the functions. In the

start-up mode, current charges capacitor C21

connected to SSTCMP. The voltage at the

capacitor controls the gradual increase in power

to the transformer and subsequently to the output

load. This reduces in-rush current and provides

reliable operation to the CCFL.

4. Ignition

When the VDDA voltage exceeds the under-

voltage lockout threshold, the IC is enabled and

internal striking timer is activated.

During ignition, the striking frequency for the

CCFL(s) can be chosen by the user to optimize

the inverter operation.

The approximate striking frequency is calculated

by the following equation.

()

]

[

]

[

]

[

]

[

RCT

RRT

KHz

STRK

Ω

5. Aged CCFL Ignition

ta9687 provides a striking timer function to

ensure that any aged, slow-turn-on CCFL is

provided with sufficient voltage and time to ignite.

The transformer output voltage is sensed at

VSEN (Pin 1).

When the voltage at VSEN

reaches a threshold of approximately 3.0V, the IC

regulates the output voltage at the transformer

secondary. If the lamps are not ignited when the

voltage at TIMER (pin 12) reaches a threshold of

approximately 3V, the IC will shutdown and latch

The approximate striking time is calculated by the

following equation.

]

[

]

[

TIMER

STRK

To resume normal operation, toggle the ENA

signal or reset VDDA.

6. Normal Operation

Once the lamps are ignited and the voltage at

ISEN (pin 14) is > 0.7V, the IC enters the normal

operation mode and the PWM dimming control is

activated.

The operating frequency is determined by

resistor (R15) and capacitor (C22) connected to

CT (pin 3). The control loop regulates the

average current through the lamps by adjusting

the overlap of the output drives. Constant

frequency operation eliminates interference with

the inverter and LCD panel that often occurs in a

variable frequency inverter system.

The peak

and valley of the CT waveform are 2V and 0V

respectively.

The

approximate

operating

frequency

calculated by the following equation.

]

[

]

[

]

[

KHz

Ω

7. Open Lamp Protection

If a CCFL is removed or damaged during normal

operation, the voltage at SSTCMP (pin 2) rises

rapidly because there is no current feedback at

ISEN (pin 14). When the voltage at SSTCMP

reaches a threshold of approximately 2V and

ISEN is less than 0.7V, a current source charges

the capacitor (C23) connected to TIMER (pin 12).

Once the voltage level at the TIMER pin reaches

a threshold of approximately 3V, the drive

outputs shut down and latch.

The shutdown delay feature avoids inverter

shutdown due to a VIN transient or if a lamp has

a positive impedance characteristic.

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