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FAN7532

Ballast Control IC

Fairchild Semiconductor 

Application Information

1. Start-up Circuit

The start-up current is supplied to the IC through the

start-up resistor (Rst). To reduce the power dissipation in

Rst, Rst is connected to the full-wave, rectified output

voltage. The size of Rst can be determined by equations

(1) and (2).

DC 400V

3. Oscillator

The gate drive output frequency is half that of the trian-

gular waveform on timing capacitor (CT) at pin #6. In

normal operating mode, the timing capacitor charging

current is 4•Irt (=Vref/RT). The discharging current is 6.2

times the charging current. During the charging period of

the timing capacitor (CT), the MOSFET alternatively

turns on. During the discharging period of the timing

capacitor (CT), both MOSFETs are off.

Rectifier

Output

Rst

D2

Vcc

M1

G1

CQ

Cs

ZD1

D1

G2

M2

FAN7532 Rev. 00

Figure 15. Start-up Circuit

Charging

Period (ΔTCH)

ICH

Discharging

Period (ΔTDIS)

IDIS=6.2∗ICH

VCT

3.45V

LO

Vref

ICH

IDIS

CT

HO

Dead Time

FAN7532 Rev. 00

Figure 16. CT & Output Waveforms

Rst

=

Vin(ac)

×

2 -Vth(st).max

Ist,max

(1)

=

85 ×

0.18

2 -12.4

× 10 -3

=599

[kΩ]

Rst

=

(Vin(ac,max) ×

Rst

2 -Vcc )2

≤ 0.5

[W]

(2)

Rst ≥ 2 × (Vin(ac,max) × 2 − Vcc )2

≥ 260 [kΩ]

∴ 260[kΩ] ≤ Rst ≤ 599[kΩ]

The size of supply capacitor (Cs) is normally determined

by the start-up time and the operating current which is

built up by the 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 charges the supply capacitor. If the size of CQ is

increased, the VCC voltage on the Cs is also increased.

2. Under-Voltage Lockout (UVLO)

The UVLO mode of the FAN7532 is designed to maintain

an ultra low supply current of less than 120µA, and to

guarantee that the IC is fully functional before two output

drivers are activated.

The FAN7532 has three operating modes according to

VCPH, as shown in Figure 17.

Vcc

Vth(st)

VCPH

5V

2.9V

f

fpre

frun

t

ICPHL=1.5mA

ICPHH=10mA

t

fpre

fign

frun

preheating

tph

ignition

tsw

t

run

FAN7532 Rev. 00

Figure 17. Operating Modes

© 2006 Fairchild Semiconductor Corporation

FAN7532 Rev. 1.0.2

9

www.fairchildsemi.com

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