appears across C6 when the switch is closed. When the

switch opens, C6 is referenced to the 8V output minus a

diode drop, which raises the voltage at C6 with respect to

ground to about 24V. Hence, in the fourth stage, C7 is

charged to 24V when the switch is open. From the first stage

to the last, there are three diode drops that make the output

voltage closer to 24 - 3xVDIODE (about 22.8V if a 0.4V

forward drop is assumed).

TABLE 1. Components For Circuits in

Figure 3

Component

600 kHz

1.3 MHz

L

10µH

4.7µH

COUT1

10µF

22µF

COUT2

10µF

NOT USED

CC

3.9nF

1.5nF

CFB1

0.1µF

15nF

CFB2

NOT USED

560pF

CIN

10µF

22µF

C1

4.7µF

4.7µF

Component

600 kHz

1.3 MHz

C2

0.1µF

0.1µF

C4

1µF

1µF

C5

1µF

1µF

C6

1µF

1µF

C7

1µF

1µF

RFB1

40.2k

Ω

91k

Ω

RFB2

7.5k

Ω

18k

Ω

RC

5.1k

Ω

10k

Ω

D1

MBRM140T3

MBRM140T3

D2

BAT54S

BAT54S

D3

D4

BAT54S

BAT54S

D5

D6

BAT54S

BAT54S

D7

600 kHz Operation

10127331

FIGURE 4. 600 kHz operation

www.national.com

13