RS-232 Datasheet, PDF (1/2 Pages) Grayhill, Inc – COMMUNICATION CONVERTERS

RS-232 Line Driver Power
Supply
National Semiconductor
Application Brief 30
April 1998
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
supply into a separate ±12V supply for powering the inter-
face line drivers.
CIRCUIT DESCRIPTION
The power supply, shown in Figure 1, operates from an input
voltage as low as 4.2V, and delivers an output of ±12V at
±40 mA with an efficiency of better than 70%. The circuit
provides a load regulation of ±1.25% (from 10% to 100% of
full load) and a line regulation of ±0.08%. Other notable fea-
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 kΩ for R1;
R2 = (V_OUT −1V)/45.8 µA = 240 kΩ
R3 = (|V_OUT| +1V)/54.2 µA = 240 kΩ
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 kΩ). 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 =
0.13Ω. A value of 0.1Ω, used here, will trip the current limit at
1A peak. A more conservative design would use 0.15Ω 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 used a 1 nF (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_OUT. In most cases,
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_OUT/100 mA, where
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 0.4Ω.
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.
© 1998 National Semiconductor Corporation AN008756
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