FAN7544

7

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

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.

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.

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.

Rst

Vin ac

()

2Vth st

() max

,

–

×

Ist max

,

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

<

85

2 12.5

–

×

0.2 10

3

–

×

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

539k

Ω

=

=

Vin ac_max

()

2Vcc

–

⋅

()

2

------------------------------------------------------------------------ 0.5W

≤

=

Vin ac_max

()

2Vcc

–

⋅

()

2

×

≥

≥

≤≤

∴

DC 400V

Rectifier

Output

Cs

Rst

G1

G2

Vcc

M1

M2

CQ

D1

D2

ZD1

Figure 10. Start-up Circuit

Figure 11. CT & Output Waveforms

VCT

OUT1

OUT2

Charging

Period(

∆T

Discharging

Period(

∆T

I

CH

I

3.7V

Vref

I

CH

I

DIS

CT

Dead Time