KA7541

6

Figure 2. Oscillator sawtooth & Output gate drive waveform

As a result, the switching frequency is as following

Ts = 2

× (∆Tch + ∆Tdis) = 15.29µ

The explicit equation calculating the size of the timing capacitor for a certain switching frequency is written below.

Soft Start

The switching frequency is linearly decreasing from the pre-heating frequency to the normal switching frequency.

In KA7541, the initial pre-heating frequency can be adjusted depending on the types of the lamps used. During the pre-heating

timing capacitor charging current (50

µA). The rising and falling slope of the triangular waveform are increased due to this

added current.

Rising slope: dv / dt = i / C = (50

Falling slope: dv / dt = i / C = 7

following equation.

The dead time ratio during pre-heating mode is maintained to be constant as well as in normal mode.

(on duty: dead time = 7:1 )

(Vth(st)), the soft start capacitor start to be charged by the current source (313nA). The switching frequency decreases linearly

can be acquired by the following formula.

Vct

(Pin #2)

Voutput

(Pin #6, 7)

Charging

Period(

∆Tch)

Discharging

Period(

∆Tdis)

2.86V

1.0V

14V

0V

C

t

11.76

10

6

–

×

f

sw

---------------------------------

=

fsw pre

()

50

10

6

–

0.33

Rs

-----------

+

×

Ct

4.25

×

-------------------------------------------

=