Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-7000 • Fax (50)668-707 • www.exar.com

SP232A_00_05308

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DESCRiPTion

DETAiLED DESCRiPTion

The SP232A transceiver is a two driver and two

receiver device that meets the EIA/TIA- 232 and

V.28 serial communication protocol. The device

is pin-to-pin compatible with popular industry

standard pinouts. The SP232A offers 20kbps

data rate, 0V/

charge pump that operates from a single 5V

supply. The proprietary on-board charge pump

generates a regulated output of +/-6.5V for RS-

232 compliant voltage levels.

Theory of operation

The SP232A is made up of three basic circuit

blocks: . Driver, 2. Receiver, and 3. charge

pump.

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. Typically, the RS-232 output

voltage swing is +/-6.5V with no load and +/-

5.0V minimum when fully loaded. The driver out-

puts are protected against infinite short-circuits

to ground without degradation in reliability.

The drivers can guarantee output data rates of

20Kbps under worst case loading of 3k ohms

and 2500pF.

The driver output Slew Rate is internally limited

to 30V/

μs in order to meet the EIA standards

(EIA-232F). Additionally, the driver output LOW

to HIGH transition meet the monotonicity output

requirements of the standard.

Receivers

The receivers convert EIA/TIA-232 signal levels

to TTL or CMOS logic output levels. Since the

input is usually from a transmission line, where

long cable length and system interference can

degrade the signal, the receiver inputs have a

typical hysteresis margin of 500mV. This en-

sures that the receiver is virtually immune to

noisy transmission lines. Should a input be left

unconnected, an internal 5Kohm pull-down re-

sistor to ground will commit the output of the re-

ceiver to a logic HIGH state. The input voltage

range for the SP232A Receiver is +/-5V.

Charge pump

The charge pump is a patented design and

uses a unique approach compared to older less

efficient designs. The charge pump requires 4

external capacitors and uses a four phase volt-

age shifting technique. The internal power sup-

ply consists of a regulated dual charge pump

that provides output voltages of +/-6.5V. This

is important to maintain compliant RS-232 lev-

els regardless of power supply fluctuations.

The charge pump operates in a discontinous

mode using an internal oscillator. If the voltages

are less than a magnitude of 6.5V, the charge

pump is enabled. If the output voltage exceed

a magnitude of 6.5V then the charge pump is

disabled. The internal oscillator controls the four

phases of the voltage shifting. A description of

each phase follows:

Phase 1

Vss charge store and double: The positive ter-

minals of capacitors C and C2 are charged

from Vcc with their negative terminals initially

connected to ground. C+ is then connected

to ground and the stored charge from C- 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 inverted

(V-) voltage onto C4. Meanwhile, capacitor C

is charged from Vcc in order to prepare it for its

next phase.

Phase 3

Vdd charge store and double: Phase three is

identical to the first phase. The positive termi-

nals of C and C2 are charged from Vcc with

their negative terminals initially connected to

ground. C+ is then connected to ground and

the stored charge from C- is superimposed

onto C2-. Since C2+ is still connected to Vcc

the voltage potential across capacitor C2 is now

2 x Vcc.