KA7541 Просмотр технического описания (PDF) - Fairchild Semiconductor

Номер в каталоге

Компоненты Описание

производитель

KA7541

Simple Ballast Controller

Fairchild Semiconductor 

KA7541

Vct

(Pin #2)

Voutput

(Pin #6, 7)

2.86V

1.0V

14V

0V

Charging

Period(∆Tch)

Discharging

Period(∆Tdis)

Figure 2. Oscillator sawtooth & Output gate drive waveform

As a result, the switching frequency is as following

Ts = 2 × (∆Tch + ∆Tdis) = 15.29µ

fsw = 1 / TS = 65KHz

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

Ct

=

1----1---.--7---6-----×-----1---0----–--6-

fsw

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

mode, a sixth of the soft start current (IS) which flows through the soft start resistor (RS) at pin #3 is added to the normal

timing capacitor charging current (50µA). The rising and falling slope of the triangular waveform are increased due to this

added current.

Soft start current (IS) = 2V / RS

Rising slope: dv / dt = i / C = (50µA + IS / 6) / Ct

Falling slope: dv / dt = i / C = 7 × (50µA + IS / 6) / Ct

So, once the value of RS and Ct are known, the pre-heating frequency can be calculated straightforward by using the

following equation.

50 × 10–6 + 0----.--3---3--

fsw(pre)

=

----------------------------------R-----s---

Ct × 4.25

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 )

The voltage of the soft start capacitor (CS) determines the soft start time (tss). When VCC voltage exceeds the start-up voltage

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

to fsw(nor) from fsw(pre) until the soft start capacitor voltage (VCS) touches 2V. Therefore the soft start duration time (tss)

can be acquired by the following formula.

6

Share Link: