AN2870 datasheet(37/41 Pages) STMICROELECTRONICS

37 / 41 page

AN2870

Designing with L6585DE

37/41

The maximum deviation of the mean voltage of the lamp, Vlamp,EOL, depends on the lamp

type (e.g. is 15 V for T5-54W lamp).

The following relation can be used to calculate the correct value of the divider's resistors

(REhi and RElo)

Equation 56

The value of filtering capacitor should be calculated in order to have a cutoff frequency

equal to at least one hundreds of run frequency.

IC power supply design

L6585DE can be supplied by means of either external source, auxiliary winding on PFC

choke or charge pump connected to the middle point of half bridge. The most used method

is the charge pump connected to the middle point of the half bridge.

The charge pump must be able to deliver the correct current to the IC. ICC depends on both

half bridge and PFC driver switching activities. Typical values of ICC are reported on

Figure 20

The current is delivered by the capacitor during the edges of the middle point of the half

bridge. The slope of these edges is also related to the recovery time of the body diode of the

MOSFETs and to the capacitor itself. Assuming a linear slope the instantaneous current

delivered by the capacitor will be:

Equation 57

Equation 58

The diode connected to ground is a Zener diode (15 V is the suggested value): it limits the

voltage across the Vcc pin avoiding an extra stress of the internal active clamp. In order to

limit the current flowing into this diode, when it is directly biased, a low value resistor is

placed in series with the capacitor.

A bigger capacitor (>1 µF) and a 100 nF ceramic capacitor placed near the IC are needed to

filter the Vcc voltage.

At start up the current is sunk from rectified mains and delivered to the IC through a resistor

path. This resistor is chosen in order to guarantee the minimum quiescent current required

by L6585DE (370 µA). Its value influences also the start-up time because it had to charge

the electrolytic capacitor connected to Vcc.

(

)

Ehi

Elo

Ehi

EOL

lamp

EOL

lamp

max

EOL

RElo

Ehi

EOL

lamp

max

⋅

−

⋅

−

⋅

−

rise

OUT

run

rise

≥

⋅

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