AN1523 APPLICATION NOTE

6/24

the circuit efficiency is always high and the input power is lower than 1W with twice the specified stand-

by load. In figure 7 the input power as a function of the 5V current, without load on the 12V is represented. The

only shortcoming is the 12V variation: the 12V increases above its limit when the +5V current exceeds 50mA,

due to coupling between the transformer windings. A bit heavier bleeder on the 12V solves this problem very

easily. Decreasing the R8 to 1.2k

tion to 1.06W @220Vac delivering 5V@100mA, or to 0.69W@220Vac delivering 5V@50mA.

At the oppo-

site, accepting an higher voltage variation of the 12V, it decreases the input power significantly: increasing R8

to 10K

mise between the bleeder resistors and the residual loads can be easily found giving the best results in stand-

by. In fact, if a stable load is present on the 5V and we remove the 5V bleeder (R8), delivering

5V@100mA the

consumption becomes 0.886W@220Vac.

In figure 8 there are the waveforms relevant to the L6590 during standby operation: it is easy to recognize that

the switching frequency has decreased from the initial value to about 22KHz. This feature is very important to

5V

12V

220Vac

Vout

@Iout

Vout

@Iout

Pin

η

[V]

[mA]

[V]

[mA]

[W]

[W]

5.02

10

11.95

0

0.050

0.330

15.2%

5.02

30

12.34

0

0.151

0.474

31.8%

5.02

50

12.66

0

0.251

0.627

40.0%

5.02

80

13.06

0

0.402

0.842

47.7%

5.02

100

13.28

0

0.502

0.986

50.9%

Figure 7. Input power @stand-by

F igure 8 . L6 590 sign als @ I+5V= 50m A -I+12V=0

Vin = 220 Vrms - 50 Hz

CH1:

VPIN1 – DRAIN VOLTAGE

CH2:

VPIN4 - COMP

CH3:

VPIN3 - Vcc

INPUT POWER @LOW LOAD

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1 0

30

50

80

100

Iout +5 V

Pin @220Vac [W]

Pin @115Vac [W]