Refer to appendix "Light load

losses evaluation" for the calcula-

an estimate of power dissipation

and wake-up time.

The circuit of fig. 7c is active only

during the start-up period, there-

fore it reduces dramatically the

power dissipated. Once the sys-

tem is running, the start-up MOS

is turned off and the only signifi-

cant consumption is on the 2M

Ω

resistor (less than 20mW in a

wide-range

mains

application).

The components in the dashed

box can be omitted, provided the

18V or more, so that the gate-to-

source voltage of the MOS is be-

low the turn-on threshold.

This circuit can deliver several mA, thus the system will wake up very quickly. On the other hand, it re-

quires several additional parts (mainly, a small high voltage MOS) and therefore has an impact on the

overall cost that needs to be evaluated by the designer.

µA max.) to achieve a reasonably low consumption even

with circuits a) or b), whose main merit is to be inexpensive.

Table 2 compares the worst case consumption of the start-up circuits of fig. 7a) and 7b), at minimum

and maximum mains voltage. The table has been compiled assuming that the IC’s work in a system that

10 ms, at full load. The consumption of the gate drive (see "Power MOSFET") is assumed to be 5mA

Device

Start-up

circuit

110V

±20%

220V

±20%

Universal Mains

(k

Ω)

(k

Ω)

(k

Ω)

min.

max.

min.

max.

min.

max.

L5991

33

µF

a)

215

55

140

430

130

300

215

55

600

b)

55

55

130

130

105

250

55

55

590

UC3842A

47

µF

a)

110

110

270

220

250

580

110

110

1170

b)

27

110

270

68

200

470

27

110

1190

fying and filtering the voltage generated by an auxiliary winding of the flyback transformer (see fig. 7).

The power delivered by such circuit amounts at:

(25),

t

voltage generated by

the self-supply circuit

WAKE-UP TIME

Figure 8. L5991 supply voltage at start-up.

AN1049 APPLICATION NOTE

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