DESCRꢁPTꢁꢀꢂ
Receivers
DETAꢁLED DESCRꢁPTꢁꢀꢂ
The receivers convert EIA/TIA-232 signal lev-
els to inverted TTL or CMOS logic output lev-
els. Since the input is usually from a transmis-
sion line, where long cable length and system
interference can degrade the signal, the inputs
have a typical hysteresis margin of 500mV. This
ensures that the receiver is virtually immune to
noisy transmission lines. The input thresholds
are 0.8V minimum and 2.8V maximum, again
well within the +/-3V RS-232 requirements.
Should an input be left unconnected, an internal
5kohm pull-down resistor to ground will commit
the output of the receiver to a HIGH state.
The SP202E, SP232E, SP233E, SP3ꢀ0E and
SP3ꢀ2A devices are a family of line driver and
receiver pairs that meet the EIA/TIA-232 and
V.28 serial communication protocols. The ESD
tolerance has been improved on these devices
to over +/-ꢀ5kV for Human Body Model. These
devices are pin-to-pin compatible with Exar's
232A, 233A, 3ꢀ0A and 3ꢀ2A as well as popular
industry standards. This family of parts offer a
ꢀ20kbps data rate, ꢀ0V/μs slew rate and an on-
board charge pump that operates from a single
5V supply using 0.ꢀμF ceramic capacitors.
The SP202E, 232E, 233E, 3ꢀ0E and 3ꢀ2E de-
vices have internal charge pump voltage con-
verters which allow them to operate from a sin-
gle +5V supply. The charge pumps will operate
with polarized or non-polarized capacitors rang-
ing from 0.ꢀ to ꢀ0µF and will generate the +/-6V
needed to generate the RS-232 output levels.
In actual system applications, it is quite pos-
sible for signals to be applied to receiver inputs
before power is applied to the receiver circuitry.
This occurs, for example, when a PC user at-
tempts to print, only to realize that the printer
wasn't turned on. In this case an RS-232 signal
from the PC will appear on the receiver input
at the printer. When the printer power is turned
on, the receiver will operate normally. All of
these devices are fully protected.
The SP233E design offers internal charge pump
capacitors. The SP3ꢀ0E provides an ON/OFF
input that simultaneously disables the internal
charge pump circuit and puts all transmitter and
receiver outputs into a high impedance state.
The SP3ꢀ2E is identical to the SP3ꢀ0E but with
seperate tri-state and shutdown inputs
Charge pump
The charge pump is an Exar patented design
and uses a unique approach compared to
older less efficiant designs. The charge pump
requires 4 external capacitors and uses a four
phase voltage shifting technique. The internal
power supply consists of a dual charge pump
that provides a driver output voltage swing of
+/-6V. The internal oscillator controls the four
phases of the voltage shifting. A description of
each phase follows:
Theory ꢀf ꢀperation
The SP202E, SP232E, SP233E, SP3ꢀ0E and
SP3ꢀ2E devices are made up of three basic
circuit blocks: 1. Drivers, 2. Receivers, and 3.
charge pump. Each block is described below.
Drivers
The drivers are inverting level transmitters that
convert TTL or CMOS logic levels to EIA/TIA-
232 levels with an inverted sense relative to
the input logic levels. The typical driver output
voltage swing is +/-6V. Even under worst case
loading conditions of 3k ohms and 2500pF, the
driver output is guaranteed to be +/-5.0V mini-
mum, thus satisfying the RS-232 specification.
The driver outputs are protected against infinite
short-circuits to ground without degradation in
reliability.
Phase 1
Vss charge store and double: The positive ter-
minals of capacitors Cꢀ and C2 are charged
from Vcc with their negative terminals initially
connected to ground. Cꢀ+ is then connected
to ground and the stored charge from Cꢀ- is
superimposed onto C2-. Since C2+ is still con-
nected to Vcc the voltage potential across C2
is now 2 x Vcc.
Phase 2
Vss transfer and invert: Phase two connects
the negative terminal of C2 to the Vss storage
capacitor and the positive terminal of C2 to
ground. This transfers the doubled and invert-
ed (V-) voltage onto C4. Meanwhile, capacitor
The slew rate of the driver output is internally
limited to 30V/μs in order to meet the EIA stan-
dards (EIA-232F). Additionally, the driver out-
puts LOW to HIGH transition meets the mon-
tonic output requirements of the standard.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (5ꢀ0)668-7000 • www.exar.com SP202E,232E,233E, 3ꢀ0E, 3ꢀ2E_ꢀ00_ꢀꢀ0608
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EXAR [ EXAR CORPORATION ]
