KA7541 Просмотр технического описания (PDF) - Fairchild Semiconductor
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KA7541 Datasheet PDF : 14 Pages
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KA7541
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 output voltage.
The following equation can be used to calculate the size of Rst
Rst
<
-V----i-n----(---a---c----)---×---------2-----–-----V----t--h----(--s----t--)--,--m-----a----x-
Ist,max
PRSt=
(---V----i--n----(--a----c---_---m-----a----x---)----⋅-------2-----–----V-----c---c---)---2-
RSt
≤
0.5
W
= 8----5-----×---------2-----–----1---0----.-5-- = 440kΩ
0.25 × 10–3
RSt ≥ 2 × (Vin(ac_max) ⋅ 2 – Vcc)2
RSt ≥ 260K
∴260K ≤ RSt ≤ 440K
The size of start up capacitor (Cst) is normally decided in terms of the start up time and operating current build up time with
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 size 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 gate drive output frequency 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). The charging period of the timing capacitor is the on duty of the gate drive. The discharging period is the off duty
of the gate drive.
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 180pF,
∆Tch = 6.69µ
∆Tdis = 0.956µ
5
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