KA7540 데이터 시트보기 (PDF) - Fairchild Semiconductor

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KA7540

Simple Dimming Ballast Control IC

Fairchild Semiconductor 

KA7540

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.

When 2 Lamp

VR4 = Vdd × -R----1----4----+------R---------1------5----------+----------R-----R-----1----1--8-----9-----+----------2----------×---------R---------f---+-----R-----1---9--

2

≡ 400 × 1----8---0----k---Ω------+-----3--------3------0--------k----1--Ω----5--------k+2------Ω----6-------8-------0-------k-------Ω---------+-----1---5----k---Ω---

V3

=

V2

×

---------R-----1---8-----------

R15 + R18

≡

200V

VR4 > 2V

When 1 Lamp

VR4

=

Vdd

×

-----------------------------------R-----1---9-------------------------------------

R14 + R15 + R18 + 2Rf + R19

≡ 400 × 1----8---0----k---Ω------+-----3----3---0----k--1-Ω--5----k+---Ω--6---8----0---k----Ω------+-----1---5----k---Ω---

VR4 > 2V

When No Lamp

VR4 = 0V( <2V) ⇒ Stop 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.

DC Link Voltage (Vdd), PFC_OUT

R14

V2

Rfilament

Rfilament

R16

Rfilament

To pin #4 VR4

R19

R17

C13

R15

Rfilament

V3

R18

Figure 4. Lamp detection resistor network

8

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