OZ960 datasheet(7/12 Pages) O2MICRO

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OZ960

OZ960-DS-1.6

Page 7

FUNCTIONAL INFORMATION

1. Steady-State Operation

Refer to the schematic shown in Figure 1, the

OZ960 drives a full-bridge power train where the

transformer couples the energy from the power

source

the

secondary

CCFL load. The

switches in the bridge denoted as QA, QB, QC

and QD are configured such that QA and QB, QC

and QD are turned on complementarily. The

duration of QA and QD, QB and QC turn on

simultaneously determines an amount of energy

put into the transformer which in turn delivers to

the CCFL. The current in CCFL is sensed via

resistor R9 and regulated through the adjustment

of the turn-on time for both diagonal switches.

This is accomplished through an error amplifier in

the current feedback loop. A voltage loop is also

established to monitor the output voltage so that

a programmable striking voltage is achieved. The

OVP represents the peak-detect signal of the

voltage on the output of the transformer. A soft-

start circuit ensures a gradual increase in the

input and output power. The soft-start capacitor

determines the rate of rise of the voltage on SST

pin where the voltage level determines the on-

time duration of QA and QD, QB and QC

diagonal switches. This minimizes the surge

impacts in circuit designs.

Apply enable signal to the ENA pin of the IC after

the bias voltage applied to VDDA initiates the

operation of the circuit. The output drives, include

PDR_A, NDR_B, PDR_C and NDR_D put out a

complementary square pulse. The frequency is

determined by R4 and C5 where they are

connected to RT and CT pins respectively.

Initially, the energy converted from the power

source to the CCFL is low due to the soft start

function. It increases as soft start capacitor

voltage increases linearly with time. The voltage

at the secondary side of the transformer T1

increases

correspondingly.

This

process

continues until the CCFL current is detected and

reaches a regulated value. The output of the

error amplifier, CMP, follows the feedback signal,

commands a proper switching among the four

output drives to maintain current regulation. The

operations of the four switches are implemented

with zero-voltage-switching to provide a high-

efficiency power conversion.

In the case of open-lamp condition, the OZ960

provides

programmable

striking-frequency

intelligence to optimize the ignition scheme. This

is implemented through resistor R5. Effectively,

R5 is in parallel with R4 to yield a required

striking frequency. In addition, the striking time is

also programmable through the capacitor C8.

Striking voltage, or the open-lamp voltage, is

regulated through a voltage feedback loop where

output voltage is monitored. The signal, being

sent to the OVP pin, commands the output drives

to provide the desired output voltage. This design

provides

high

degree

flexibility

while

maintaining

OZ960

very

high

integration

device.

One protection feature needed is removing the

lamp

during

normal

operation.

The

OZ960

senses the missing current signal through current

amplifier, it shuts off the output drives and stay in

the

latched

mode.

This

differentiated

intelligently with turning on the inverter while

CCFL is not connected. Recycle of the IC power

is necessary to resume normal operation.

Dimming control: dimming control of the inverter

is implemented by adjusting the amount of

energy processed and delivered to the CCFL. A

PWM burst-mode scheme is internally generated

which provides 0% to 100% wide dimming

control. An input analog voltage signal is fed into

DIM pin and determines the dimming level of the

CCFL.

The

burst-mode

frequency

programmable through a capacitor C10 as shown

in the schematic.

The OZ960 inverter operates in a constant

frequency mode. This eliminates any undesired

interference between inverter and LCD panels

where the interference is usually associated with

variable-frequency designs.

Symmetrical drive to the power transformer gives

a very dynamic choice of selecting transformers.

This vulnerable design offers flexibility to the

system designers to choose transformer sources.

There is no limitation to the gap-less transformer.

2. CCFL Ignition Time

Ignition time for CCFLs varies with CCFL length,

diameter, module package and temperature. The

OZ960 provides a flexible design where a

capacitor is connected to CTIMR pin to determine

the necessary striking time. An approximate of

the timing calculation is:

T[second] = C[uF]

This capacitor remains reset at no charge if lamp

is connected and at normal operation.

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