RS-232 Line Driver Power

Supply

INTRODUCTION

A large segment of today’s systems comply with the Elec-

tronic Industries Association (EIA) RS-232 specification for

the interface between data processing and data communica-

tions equipment. Because this specification calls for the use

of positive and negative signal levels, the designer quite of-

ten needs to add a dual supply to a board which can other-

wise operate from a single 5V supply. The LM1578A Switch-

ing Regulator can be used to convert the already existing

face line drivers.

CIRCUIT DESCRIPTION

The power supply, shown in

Figure 1, operates from an input

tures include a cycle-by-cycle current limit and an output

voltage ripple of less than 40 mVp-p.

A unique feature of this flyback regulator is its use of feed-

back from BOTH outputs. This dual feedback configuration

results in a sharing of the output voltage regulation by each

output so that one output is not left unregulated as in single

feedback systems. In addition, since both sides are regu-

lated, it is not necessary to use a linear regulator for output

regulation.

COMPONENT SELECTION

The following design procedure is provided for the user who

wishes to tailor the power supply circuit to fit their own spe-

cific converter application.

The feedback resistors, R2 and R3, may be selected as fol-

lows by assuming a value of 10 k

Ω for R1;

R2 = (V

R3 = (|V

Actually, the currents used to program the values for the

feedback resistors may vary from 40 µA to 60µA, as long as

their sum is equal to the 100 µA necessary to establish the

1V threshold across R1 (10 k

Ω). Ideally, these currents

should be equal (50 µA each) for optimal control. However,

as was done here, they may be mismatched in order to use

standard resistor values. This results in a slight mismatch of

regulation between the two outputs.

The current limit resistor, R4, is selected by dividing the cur-

rent limit threshold voltage (approximately 100 mV) by the

maximum peak current level in the output switch (750 mA

steady-state). For our purposes R4 = 100 mV/750 mA =

0.13

Ω. A value of 0.1Ω, used here, will trip the current limit at

1A peak. A more conservative design would use 0.15

Ω for

this resistor.

Capacitor C1 sets the oscillator frequency according to the

equation C1 = 80/f, where C1 is in nano-Farads and f is the

frequency of the oscillator in kHz. This application runs at

80 kHz and useda1nF (1000 pF) silver-mica capacitor. The

oscillator section provides a 10% deadtime each cycle to

protect the output transistor.

Capacitor C2 serves as a compensation capacitor for oper-

ating the circuit in the synchronous conduction mode. That

is, the output transistor will switch on each cycle, thereby

eliminating the random noise spikes which occur with non-

synchronous operation and are at best difficult to filter. This

capacitor is optional and may be omitted if desired. If used,

a value of 10 to 50 pF should be sufficient for most applica-

tions.

The choice for an output capacitor value depends primarily

on the allowed output ripple voltage,

∆V

the capacitor’s equivalent series resistance (ESR) at the

switching frequency produces more ripple voltage than does

the charging and discharging of the capacitor. The capacitor

should be chosen to have an ESR

≤∆V

100 mA is approximately the greatest ripple current pro-

duces by the transformer secondary. Higher-value capaci-

tors tend to have lower ESR; 1000 µF aluminum electrolytic

was used in this circuit to assure low ESR, under 0.4

Ω.

The input capacitors, C5 and C6, are used to reduce the

transients that may be fedback to the main supply. Capacitor

C5 is a 100 µF electrolytic and is bypassed by C6, a 0.1 µF

ceramic disc.

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

age drop and be fast switching. 1N5819 Schottky diodes

work well.

National Semiconductor

Application Brief 30

April 1998

© 1998 National Semiconductor Corporation

AN008756

www.national.com