AN993 datasheet(6/27 Pages) STMICROELECTRONICS

6 / 27 page

L6574 ballast driver

AN993

6/27

Doc ID 5656 Rev 10

Figure 3.

Connection of a typical application

AM01311v1

Cpre

Rcs

L_ballast

Cres

LAMP

Rign.

410

L6574

Rpre.

HVBus

CSupply

CBoot

CSnub

Ref.

Rgl

Rgh

Table 1.

Description of device pins

Number

Name

Function

Cpre

Preheat timing capacitor. The capacitor Cpre sets the preheating and the frequency

shift time, according to the relations: tpre= Kpre· Cpre and tSH= KFS· Cpre (typ. Kpre=

1.5 s/

∝F, K

FS= 0.15s/∝F). This feature is obtained by charging Cpre with two different

currents. During tpre, this current is independent of the external components, so Cpre

is charged up to 3.5 V (preheat timing comparator threshold). During tSH, the current

depends on the value of Rpre (that is, on the difference between fpre and fign). In this

way tSH is always set at 0.1tpre. In steady state the voltage at pin 1 is 5 V (see

Figure 5).

Rpre

Maximum oscillation frequency setting. The resistance connected between this pin

and ground sets the fpre value, fixing the difference between fpre and fign(fpre> fign). At

the end of the start-up procedure, the effect current drown from Rpre is over. The

voltage at this pin is fixed at VREF =2 V.

Oscillator frequency setting. The capacitor CF, along with to Rpre and Rign, sets fpre

and fING. In normal operation this pin shows a triangular wave.

Rign

Minimum oscillation frequency setting. The resistance connected between this pin

and ground sets the fign value. The voltage at this pin is fixed at VREF =2 V.

OPout

Out of the operational amplifier. To implement a feedback control loop this pin can be

connected to the Rign pin by means of appropriate circuitry.

OPin-

Inverting input of the operational amplifier.

OPin+

Non-inverting Input of the operational amplifier.

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