KA7540

KA7540

KA7540

KA7540

8

8

8

8

No Lamp Protection

No Lamp Protection

No Lamp Protection

No Lamp Protection

When the voltage at pin #4 is lower than 2V, the gate drive output is off-state, so the external power MOSFETs stop

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.

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

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

Figure 4. Lamp detection resistor network

Figure 4. Lamp detection resistor network

Figure 4. Lamp detection resistor network

Figure 4. Lamp detection resistor network

When 2 Lamp

V

R4

Vdd

R19

R14

R15

R18

2

Rf

×

++

2

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

R19

++

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

×

=

400

15k

Ω

180k

Ω 330kΩ 680kΩ

+

2

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

15k

Ω

++

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

×

≡

V3

V2

R18

R15

R18

+

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

200V

≡

×

=

V

R4

2V

>

When 1 Lamp

V

R4

Vdd

R19

R14

R15

R18

2Rf

R19

++

+

+

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

×

=

400

15k

Ω

180k

Ω 330kΩ 680kΩ 15kΩ

++

+

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

×

≡

V

R4

2V

>

When No Lamp

V

R4

0V <2V

()

Stop switching

⇒

=

DC Link Voltage (Vdd), PFC_OUT

R14

R15

R18

V3

C13

R19

V2

Rfilament

Rfilament

Rfilament

Rfilament

To pin #4

R16

R17

VR4