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SP504

WAN Multi-Mode Serial Transceiver

Exar Corporation 

FEATURES…

The SP504 is a highly integrated serial trans-

ceiver that allows software control of its inter-

face modes. Similar to the SP503, the SP504

offers the same hardware interface modes

for RS-232 (V.28), RS-422A (V.11), RS-449,

RS-485, V.35, EIA-530 and includes V.36 and

EIA-530A. The interface mode selec-

tion is done via an 8–bit switch; four

(4) bits control the drivers and four (4)

bits control the receivers. The SP504

is fabricated using low power BiCMOS

process technology, and incorporates a

Exar patented (5,306,954) charge pump

allowing +5V only operation. Each device

is packaged in an 80–pin JEDEC Quad

FlatPack package.

The SP504 is ideally suited for wide area

network connectivity based on the interface

modes offered and the driver and receiver

configurations. The SP504 has seven (7)

independent drivers and seven (7) indepen-

dent receivers. In V.35 mode, the SP504

includes the necessary components and

termination resistors internal within the de-

vice for compliant V.35 operation.

Theory of Operation

The SP504 is made up of five separate

circuit blocks — the charge pump, drivers,

receivers, decoder and switching array. Each

of these circuit blocks is described in more

detail below.

Charge–Pump

The SP504's charge pump design is based

on the SP503 where Exar's patented charge

pump design (5,306,954) uses a four–phase

voltage shifting technique to attain sym-

metrical ±10V power supplies. In addition,

the SP504 charge pump incorporates a

"programmable" feature that produces

an output of ±10V or ±5V for VSS and VDD

depending on the mode of operation. The

charge pump still requires external capaci-

tors to store the charge. Figure 18a shows

the waveform found on the positive side of

capacitor C2, and Figure 18b shows the

negative side of capcitor C2. There is a

free–running oscillator that controls the four

phases of the voltage shifting. A description

of each phase follows.

The SP504 charge pump is used for RS-232

where the output voltage swing is typically

±10V and also used for RS-423. However,

RS-423 requires the voltage swing on the

driver output be between ±4V to ±6V during

an open circuit (no load). The charge pump

would need to be regulated down from ±10V

to ±5V. A typical ±10V charge pump would

require external clamping such as 5V zener

diodes on VDD and VSS to ground. The ±5V

output has symmetrical levels as in the ±10V

output. The ±5V is used in the following

modes where RS-423 levels are used: RS-

449, EIA-530, EIA-530A and V.36.

Phase 1 (±10V)

— VSS charge storage — During this phase

of the clock cycle, the positive side of capaci-

tors

The

CC1l+

and C2

is then

are initially charged

switched to ground

to +5V.

and the

charge on C1– is transferred to C2–. Since C2+

is connected to +5V, the voltage potential

across capacitor C2 is now 10V.

Phase 1 (±5V)

— VSS & VDD charge storage and transfer

— With the C1 and C2 capacitors initially

charged to +5V, Cl+ is then switched to

ground and the charge on C1– is transferred

toousthlyethVeSSC2s–toisrasgweitcchaepdactoitogrr.ouSndimaunltdanthee-

5V charge on C2+ is transferred to the VDD

storage capacitor.

VCC = +5V

+

C1 –

–5V

+5V

+

C2 –

–5V

C4

+ – VDD Storage Capacitor

– + VSSStorage Capacitor

C3

Figure 14a. Charge Pump Phase 1 for ±10V.

VCC = +5V

+

C1 –

–5V

+5V

+

C2 –

C4

+ – VDD Storage Capacitor

– + VSS Storage Capacitor

C3

Figure 14b. Charge Pump Phase 1 for ±5V.

Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

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