KA7540 参数 Datasheet PDF下载

KA7540

Vct

(Pin #2)

Vhigh

Vlow

Voutput

(Pin #6, 7)

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)

= 40µs

fsw = 1 / T

S

= 25kHz

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

11.76

×

10

-

Ct

= --------------------------------

fsw

–

6

Soft Start

The switching frequency is decreasing linearly from the pre-heating frequency to the normal switching frequency. In KA7540,

the normal timing capacitor charging current is increased by 25µA during the pre-heating mode. This addition of the charging

current sets the pre-heating frequency to be 1.33 times the switching frequency at the normal mode.

fsw, V

CS

fsw (pre)

fsw (dim)

fsw (nor)

3.5V

2.7V

2V

0.95

ts/s

td1

td2

Figure 3. Frequency & Soft start capacitor voltage variation during soft start and dimming mode

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