Capacitor C1 sets the oscillator frequency and is selected

from

Figure 1.

Capacitor C2 serves as a compensation capacitor for syn-

chronous operation and a value of 10 to 50 pF should be suf-

ficient for most applications.

A minimum value for an ideal output capacitor C3, could be

calculated as C = I

the transistor on time (typically 0.4/f

and

∆Visthe

peak-to-peak output voltage ripple. A larger output capacitor

than this theoretical value should be used since electrolytics

have poor high frequency performance. Experience has

shown that a value from 5 to 10 times the calculated value

should be used.

For good efficiency, the diodes must have a low forward volt-

age drop and be fast switching. 1N5819 Schottky diodes

work well.

Transformer selection should be picked for an output transis-

tor “on” time of 0.4/f

enough to prevent the output transistor switch from ramping

higher than the transistor’s rating of 750 mA. Pulse Engi-

neering (San Diego, Calif.) and Renco Electronics, Inc.

(Deer Park, N.Y.) can provide further assistance in selecting

the proper transformer for a specific application need. The

transformer used in

Figure 23 was a Pulse Engineering

PE-64287.

DS008711-13

9V

≤ V

R5 = 270

C1 = 1820 pF

C2 = 220 µF

C3 = 20 pF

C4 = 0.0022 µF

R1 = 110k

L1 = 220 µH

R2 = 10k

D1, D2 = 1N5819

R3 = 0.15

Q1 = D44

R4 = 220k

FIGURE 22. Buck-Boost Regulator

DS008711-14

R4 = 0.15

Ω

C1 = 820 pF

C2 = 10 pF

C3 = 220 µF

R1 = 10 k

Ω

D1, D2, D3 = 1N5819

R2 = 240 k

Ω

T1 = PE-64287

R3 = 240 k

Ω

FIGURE 23. RS-232 Line Driver Power Supply

15

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