KA7540 Datasheet, PDF (7/14 Pages) Samsung semiconductor – Simple Dimming Ballast Control IC

KA7540
Vct
(Pin #2)
Voutput
(Pin #6, 7)
Vhigh
Vlow
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 / TS = 25kHz
The explicit equation calculating the value 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 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, VCS
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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