KA7541

7

For example, the soft start capacitor of 0.2

µF makes the soft start time (tss) to be 1.28sec.

Figure 3. Frequency & Soft start capacitor voltage variation during soft start

No Lamp Protection

When the voltage at pin #4 is lower than 2V, the gate drive output is off state, so the external power MOSFET stops switching.

In no lamp protection circuit the dc link voltage is divided by a couple of resistors including both lamp filaments, and The

divided voltage is applied to the pin #4 before the MOSFETs start switching.

When in normal mode the average voltage of the V3 is the half of the dc link voltage (Vdd). So, in order to make stable start

condition, the resistors are designed to make the voltage of V3 to be the half of the dc link voltage.

tss

Cs

V

×

i

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

=

0.2

10

6

–

2

×

×

313

10

9

–

×

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

1.28s

=

=

fsw (pre)

fsw (nor)

2V

tss

V

R4

Vdd

R

4

R

1

R

2

R

3

2R

f

×

++

2

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

R

4

++

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

×

=

400

≅

15K

Ω

180K

Ω 330KΩ 680KΩ

+

2

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

15K

Ω

++

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

×

V

3

V

2

R

3

R

2

R

3

+

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

×

200V

≅

=