FAN7544 Ver la hoja de datos (PDF) - Fairchild Semiconductor

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Fabricante

FAN7544

Simple Ballast Controller

Fairchild Semiconductor 

FAN7544

Application Information

Start-up Circuit

The 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 output voltage.

The following equations can be used to calculate the size of Rst

R

st

<

-V----i--n---(---a---c----)---×---------2-----–-----V----t--h----(--s---t---)--,-m------a----x-

Ist,max

= -8---5-----×---------2-----–----1---2----.-5-- = 539kΩ

0.2 × 10–3

PRSt=

(---V----i--n----(--a----c---_----m----a----x---)----⋅-------2-----–----V-----c---c---)---2- ≤ 0.5W

RSt

RSt ≥ 2 × (Vin(ac_max) ⋅ 2 – Vcc)2

RSt ≥ 260K

∴260K ≤ RSt ≤ 539K

The size of supply capacitor (Cs) is normally decided in

terms of the start up time and operating current built up

by auxiliary operating current source.

The turn off snubber capacitor (CQ) and two diodes

(D1, D2) constitute the auxiliary operating current

source for the IC. The charging current through the CQ

flows into the IC and also charges the supply capacitor.

If the size of CQ is increased, the VCC voltage on the

Cs is also increased.

Under Voltage Lock Out(UVLO)

UVLO mode of the FAN7544 is designed to maintain

an ultra low supply current of less than 140uA, and to

guarantee that the IC is fully functional before two out-

put drivers are activated.

DC 400V

Rectifier

Output

Rst

D2

Vcc

M1

G1

CQ

Cs

ZD1

D1

G2

M2

Figure 10. Start-up Circuit

Oscillator

The gate drive output frequency is as half as that of the triangular waveform on timing capacitor (CT) at pin #2. In normal

operating mode, the timing capacitor charging current is 4×Irt(=Vref/RT). The discharging current is 5.25 times of the charg-

ing current (5.25×4×Ιrt). The charging period of the timing capacitor is the on time of the MOSFET. During the discharging

period, both of the MOSFETs are off.

Charging

Period(∆TCH)

ICH

Discharging

Period(∆TDIS)

IDIS=6.25*ICH

VCT

3.7V

Vref

ICH

IDIS

CT

OUT1

OUT2

Dead Time

Figure 11. CT & Output Waveforms

7

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