KA7540 데이터 시트보기 (PDF) - Fairchild Semiconductor
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KA7540 Datasheet PDF : 14 Pages
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KA7540
Start-up Circuit
Start-up current is supplied to the IC through the start-up resistor (Rst). In order to reduce the power dissipation in Rst, the Rst
is connected to the full-wave rectified AC line voltage.
The following equation can be used to calculate the value of Rst.
Rst
≤
-V----i--n---(---a---c----)---×---------2-----–-----V----t--h----(--s----t--)--,---m-----a----x-
lst, max
=
8----5-----×---------2-----–----1---3----.-5--
0.3 × 10–3
=
356KΩ
2
PRst
=
-(--V----i--n----(--a----c---_---m-----a----x---)----⋅-------2-----–----V-----C----C-----)----
Rst
≤
1W
2
Rst ≥ (Vin(ac_max) ⋅ 2 – VCC)
Rst ≥ 130KΩ
∴130KΩ ≤ Rst ≤ 356KΩ
The value of start-up capacitor (Cst) is normally determined in terms of the start-up time and operating current build up time
with the auxiliary operating current source.
The turn-off snubber capacitor (Cq2) and two diodes (D1, D2) constitute the auxiliary operating current source for the IC. The
charging current through the Cq2 flows into the IC and also charges the start-up capacitor. If the value of Cq2 is increased, the
VCC voltage of the Cst is also increased.
Q1
Rectifier
Output
Q2
To VCC
(Pin 8)
Rst
+
D2
− Cst
Cq2
D1
Figure 1. Start-up circuit
Oscillator
The frequency of the gate drive output is as half as that of the triangular waveform in timing capacitor (Ct) at pin #2. In normal
operating mode, the timing capacitor charging current is 50µA. The discharging current is seven times of the charging current
(7 × 50µA). During the charging period, one of the two MOSFETs remains ON state. On the contrary both of MOSFETs are
OFF during the discharging period.
The rising slope and falling slope of the triangular waveform are as following.
Rising slope: dv / dt = i / C = 50µA / Ct
Falling slope: dv / dt = i / C = 7 × 50µA / Ct
For example, when the timing capacitor is 470pF, ∆V(Vhigh - Vlow) = (2.86V - 1.0V) = 1.86V,
∆Tch = 17.5µs, ∆Tdis = 2.5µs
6
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