AN2300 datasheet(13/21 Pages) STMICROELECTRONICS

13 / 21 page

AN2300

Modified Buck converter

Rev1

13/21

2.3

Higher output power

Higher output power levels could be required in some applications. Typical values are 50mA

on the 12V output and 100mA on the -5V output, as listed in Table 5.

The proposed Buck converters can provide such current values adjusting the value of the

bleeder resistor, R1. In fact, in order to maintain the regulation when out1 is in open load

condition, (2) has to be verified:

Since Iout2 = 100mA, we can set VR1/R1 = 120mA, resulting in:

VR1/R1≈12/R1=120mA, therefore R1=100Ω for schematic A (see Figure 7.);

VR1/R1≈7/R1=120mA, therefore R1 = 56Ω for schematic B (see Figure 8. ).

Thus, the R1 value is lower than in the previous case. Of course, this results in higher power

dissipation across the bleeder.

In Table 6. the part list of the modified components is given.

Figure 18. Buck converter: conducted

emission @ 230VAC, full load: Phase

Figure 19. Buck converter: conducted

emission @ 230VAC, full load:

Neutral

Table 5.

Higher output power requirements

AC input voltage VIN

185

÷265VAC

Outputs

Vout1=12V; Iout1=50mA

Vout2=-5V; Iout2=100mA

Total output power

1.1W

----------

ou t2

Dz2

(2)

AN2300 Datasheet(PDF) 13 Page - STMicroelectronics