KA7540

KA7540

KA7540

KA7540

6

6

6

6

Start-up Circuit

Start-up Circuit

Start-up Circuit

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.

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

Figure 1. Start-up circuit

Figure 1. Start-up circuit

Figure 1. Start-up circuit

Figure 1. Start-up circuit

Oscillator

Oscillator

Oscillator

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 50

µA. 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 = 50

µA / 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

R

st

Vin ac

()

2Vth st

() max

,

–

×

lst max

,

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

≤

85

2

13.5

–

×

0.3

10

3

–

×

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

356K

Ω

=

=

P

Rst

Vin ac_max

()

2V

CC

–

⋅

()

2

Rst

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

1W

≤

=

Rst

Vin ac_max

()

2V

CC

–

⋅

()

2

≥

Rst

130K

Ω

≥

130K

Ω Rst 356KΩ

≤≤

∴

Q1

Q2

Cq2

D1

D2

Rectifier

Output

Rst

Cst

+

−

(Pin 8)