SP3223EB datasheet(12/25 Pages) SIPEX

12 / 25 page

Date: 6/2/04

SP3223EB/3243EB +3.0V to +5.5V RS-232 Transceivers

Receivers are active when the AUTO ON-LINE®

circuitry is enabled or when in shutdown.

During the shutdown, the receivers will continue

to be active. If there is no activity present at the

receivers for a period longer than 100

µs or when

SHUTDOWN is enabled, the device goes into a

standby mode where the circuit draws 1

µA.

Driving EN to a logic HIGH forces the outputs of

the receivers into high-impedance. The truth

table logic of the SP3223EB and SP3243EB driver

and receiver outputs can be found in Table 2.

The SP3243EB includes an additional non-in-

verting receiver with an output R2OUT. R2OUT

is an extra output that remains active and moni-

tors activity while the other receiver outputs are

forced into high impedance. This allows Ring

Indicator (RI) from a peripheral to be monitored

without forward biasing the TTL/CMOS inputs

of the other devices connected to the receiver

outputs.

Since receiver input is usually from a transmis-

sion line where long cable lengths and system

interference can degrade the signal, the inputs

have a typical hysteresis margin of 300mV. This

ensures that the receiver is virtually immune to

noisy transmission lines. Should an input be left

unconnected, an internal 5K

Ω pulldown resistor

to ground will commit the output of the receiver

to a HIGH state.

Charge Pump

The charge pump is a Sipex–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 5.5V regardless of the

input voltage (VCC) over the +3.0V to +5.5V

range. This is important to maintain compliant

RS-232 levels regardless of power supply

fluctuations.

The charge pump operates in a discontinuous

mode using an internal oscillator. If the output

voltages are less than a magnitude of 5.5V, the

charge pump is enabled. If the output voltages

exceed a magnitude of 5.5V, the charge pump is

disabled. This oscillator controls the four phases

of the voltage shifting. A description of each

phase follows.

Phase 1

— VSS charge storage — During this phase of

the clock cycle, the positive side of capacitors

C1 and C2 are initially charged to VCC. Cl+ is

then switched to GND and the charge in C1– is

transferred to C2–. Since C2+ is connected to

VCC, the voltage potential across capacitor C2 is

now 2 times VCC.

Phase 2

— VSS transfer — Phase two of the clock

connects the negative terminal of C2 to the VSS

storage capacitor and the positive terminal of C2

to GND. This transfers a negative generated

voltage to C3. This generated voltage is

regulated to a minimum voltage of -5.5V.

Simultaneous with the transfer of the voltage to

C3, the positive side of capacitor C1 is switched

to VCC and the negative side is connected to

GND.

Phase 3

— VDD charge storage — The third phase of the

clock is identical to the first phase — the charge

transferred in C1 produces –VCC in the negative

terminal of C1, which is applied to the negative

side of capacitor C2. Since C2+ is at VCC, the

voltage potential across C2 is 2 times VCC.

Phase 4

— VDD transfer — The fourth phase of the clock

connects the negative terminal of C2 to GND,

and transfers this positive generated voltage

across C2 to C4, the VDD storage capacitor. This

voltage is regulated to +5.5V. At this voltage,

the internal oscillator is disabled. Simultaneous

with the transfer of the voltage to C4, the

positive side of capacitor C1 is switched to VCC

and the negative side is connected to GND,

allowing the charge pump cycle to begin again.

The charge pump cycle will continue as long as

the operational conditions for the internal

oscillator are present.

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