KA7540D Просмотр технического описания (PDF) - Fairchild Semiconductor
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KA7540D Datasheet PDF : 14 Pages
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KA7540
Dimming Control
The lighting output of the lamp can be controlled by varying the switching frequency of the ballast circuit. In voltage source
series resonant type converter, the output power is inversely proportional to the switching frequency. As a result, in order to
make the lamp lighting output less bright (so called “dimming”), the switching frequency should be increased compared to that
of the normal full lighting output frequency.
With KA7540, the switching frequency can be controlled by the voltage level at the pin #3 (Vdim). Since the IC starts to
operate, the voltage level at the dimming pin doesn’t affect the oscillator frequency until the time of td1 in figure 3. At the time
td1, the switching frequency starts to ramp up to the dimming switching frequency level that is determined by the voltage level
at the pin #3 (Vdim). In dimming mode, the timing capacitor charging current is increased by the following amount of the
dimming current (Id).
Id = 25uA × (Vref - Vd) / Vref
Vd = Vdim / 5
So, the equations for the dimming frequency are as following.
d----V--- = -5---0----u---A------+-----I--d-
dt
Ct
dTch(dim) = ---------------------------d---V------×-----C----t--------------------------
50uA + 2----5----u---A-----(-V-V----rr--e-e---f-f---–----V----d-----)
dTdis(dim) = -------------------------------d----V------×----C-----t-------------------------------
7
×
50uA
+
2----5----u---A-----(-V-V----rr--e-e---ff----–----V-----d----)
Ts(dim) = 2 × (Tch(dim) + Tdis(dim))
fSW(dim)
=
----------1-----------
Ts(dim)
If the dimming pin is open, the dimming pin voltage becomes 10V due to the internal 100µA current source, which is
equivalent to the normal full lighting output case.
Dimming Control can be reallized by simple voltage source and current source of variable resistor at pin #3.
9
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