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

SP3203E_00_2080

DESCRIPTION

The

SP3203Eisa3-driver/2-receiverdevice

ideal for portable or handheld applications.

The

SP3203E transceivers meet the EIA/

TIA-232andITU-TV.28/V.24communication

protocolsandcanbeimplementedinbattery-

powered, portable, or handheld applications

such as notebook or palmtop computers,

PDA'sandcellphones. The

SP3203Edevice

features

Exar's proprietary and patented

(U.S.-- 5,306,954) on-board charge pump

circuitry that generates ±5.5V RS-232 volt-

age levels from a single +3.0V to +5.5V

power supply. The

SP3203E can operate at

a minimum data rate of 250kbps.

THEORY OF OPERATION

The

SP3203E is made up of four basic

circuit blocks:

. Drivers, 2. Receivers, 3. The Exar propri-

etary charge pump, and 4. V

Drivers

The drivers are inverting level transmitters

thatconvertTTLorCMOSlogiclevelsto5.0V

EIA/TIA-232 levels with an inverted sense

relativetotheinputlogiclevels. Typically,the

RS-232 output voltage swing is +5.4V with

no load and +5V minimum fully loaded. The

driver outputs are protected against infinite

short-circuits to ground without degradation

in reliability. These drivers comply with the

EIA-TIA-232F and all previous RS-232 ver-

sions. The driver output stages are turned

off (High Impedance) when the device is in

shutdown mode.

The drivers can guarantee output data rates

fully loaded with 3kΩ in parallel with 1000pF,

ensuring compatibility with PC-to-PC com-

munication software.

The slew rate of the driver output is internally

limited to a maximum of 30V/µs in order

to meet the EIA standards (EIA RS-232D

2..7, Paragraph 5). The transition of the

loaded output from HIGH to LOW also

meets the monotonicity requirements of

the standard.

Figure 2 shows a loopback test circuit used

to test the RS-232 Drivers. Figure 3 shows

the test results with all drivers active at

20kbpswithtypicalRS-232loadsinparallel

with a 000pF capacitors. Figure 4 shows

the test results where one driver was active

at 250kbps and all three drivers loaded with

an RS-232 receiver in parallel with a 000pF

capacitor.The transmitter inputs do not have

pull-up resistors. Connect unused inputs to

ground or V

Receivers

The receivers convert ±5.0V EIA/TIA-232

levels to TTL or CMOS logic output levels.

Receivers are disabled when in shutdown.

The truth table logic of the SP3203E driver

and receiver outputs can be found in Table

.

Since receiver input is usually from a trans-

mission line where long cable lengths 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. Should an input be left unconnected,

an internal 5kΩ pull-down resistor to ground

will commit the output of the receiver to a

HIGH state.

Charge Pump

The charge pump is a patented design (U.S.

5,306,954) and uses a unique approach

compared to older less–efficient designs.

The charge pump still requires four external

capacitors, but uses a four–phase voltage

shifting technique to attain symmetrical

5.5V power supplies. The internal power

supply consists of a regulated dual charge

pump that provides output voltages of

+/-5.5V regardless of input voltage (V

over the +3.0V to +5.5V range. This

is important to maintain compliant RS-

232 levels regardless of power supply

fluctuations.