KA7541
7
For example, the soft start capacitor of 0.2µF 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, V
CS
fsw (pre)
fsw (nor)
2V
tss
V
R4
Vdd
R
4
R
1
R
2
R
3
2R
f
×++
2
------------------------------------------ R
4
++
---------------------------------------------------------------------
×=
400≅
15KΩ
180KΩ
330KΩ 680KΩ+
2
--------------------------------------------- 15KΩ++
---------------------------------------------------------------------------------------------
×
V
3
V
2
R
3
R
2
R
3
+
--------------------
× 200V≅=
