STC International Limited
http://www.mcu-memory.com
STC232E Datasheet Rev.0.1, Dec.2005
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Test Circuits
Figure 1. Transition Slew-Rate Circuit
Detailed Description
Capacitor Selection
RS-232 Drivers
The drivers are inverting transmitters, which accept TTL or CMOS inputs and output the RS-232 signals with an
inverted sense relative to the input logic levels. Typically the RS-232 output voltage swing is 9V. Even under worst case
loading conditions of 3kO and 2500pF, the output is guaranteed to be 5V, which is consistent with the RS-232 standard
specifications. The transmitter outputs are protected against infinite short-circuits to ground without degradation in
reliability. The instantaneous slew rate of the transmitter output is internally limited to a maximum of 30V/µs in order to
meet the standards [EIA RS-232-D 2.1.7, Paragraph (5)]. However, the transition region slew rate of these enhanced
products is typically 10V/µs. The smooth transition of the loaded output from VOL to VOH clearly meets the
monotonicity requirements of the standard [EIA RS-232-D 2.1.7, Paragraphs (1) & (2)].
The STC232E consists of three sections: charge-pump voltage converters, drivers, and receivers. These E versions
provide extra protection against ESD. They survive ±15kV discharges to the RS-232 inputs and outputs, tested using the
Human Body Model. When tested according to IEC1000-4-2, they survive ±8kV contact-discharges and ±15kV air-gap
discharges. The rugged E versions are intended for use in harsh environments or applications where the RS-232
connection is frequently changed. The STC232E devices have internal charge pump voltage converters which allow
them to operate from a single +5V supply. The charge pumps will operate with polarized or non-polarized capacitors
ranging from 0.1 to 10 µF and will generate the ±9V needed to generate the RS-232 output levels.
The capacitor type used for C1–C4 is not critical for proper operation. The STC232E require 0.1 µF capacitors, although
capacitors up to 10 µF can be used without harm. Ceramic dielectrics are suggested for the 0.1 µF capacitors. When
using the minimum recommended capacitor values. If in doubt, use capacitors with a large (e.g.,2X) nominal value. The
effective series resistance (ESR) of capacitors, which usually rises at low temperatures, influences the amount of ripple
on V+ and V-. Use larger capacitors (up to 10 X) to reduce the output impedance at V+ and V-. Bypass VCC to ground
with at least 0.1 µF. In applications sensitive to power-supply noise generated by the charge pumps, decouple VCC to
ground with a capacitor the same size as (or larger than) the charge-pump capacitors (C1–C4).
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