KA7541

5

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

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

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 50

µA. 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 = 50

µA / Ct

Falling slope: dv / dt = i / C = 7

× 50µA / Ct

For example, when the timing capacitor is 180pF,

∆Tch = 6.69µ

∆Tdis = 0.956µ

Rst

Vin ac

()

2Vth st

() max

,

–

×

Ist max

,

---------------------------------------------------------------------------

<

85

2

10.5

–

×

0.25

10

3

–

×

--------------------------------------

440k

Ω

=

=

P

RSt

Vin ac_max

()

2Vcc

–

⋅

()

2

R

St

------------------------------------------------------------------------

0.5W

≤

=

R

St

2

Vin ac_max

()

2Vcc

–

⋅

()

2

×

≥

R

St

260K

≥

260K

R

St

440K

≤≤

∴

Q1

Q2

Cq2

D1

D2

Rectifier

Output

Rst

Cst

+

−

(Pin 8)