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SP505_00_08308

+4V to +6V during an open-circuit (no

load). The charge pump would need to

be regulated down from +0V to +5V.

A typical +0V charge pump would require

external clamping such as 5V zener diodes

on V

hassymmetricallevelsasinthe+0Voutput.

The+5Visusedinthefollowingmodeswhere

RS-423 (V.0) are used: RS-449, EIA-530,

EIA-530A and V.36.

Phase 1 (±10V)

—V

theclockcycle,thepositivesideofcapacitors

C

+

is

then switched to ground and the charge on

C



–

is transferred to C

2

–

. Since C

2

+

is con-

nected to +5V, the voltage potential across

capacitor C

Phase 1 (±5V)

— V

— With the C

chargedto+5V,C

l

+

isthenswitchedtoground

andthechargeonC



–

istransferredtotheV

SS

storage capacitor. Simultaneously the C

2

–

is

switched to ground and 5V charge on C

2

+

is

transferred to the V

FEATURES…

The SP505 is a highly integrated serial

transceiver that allows software control of

its interface modes. Similar to the SP504,

the SP505 offers the same hardware inter-

face modes for RS-232 (V.28), RS-422A

(V.), RS-449, RS-485, V.35, EIA-530 and

includes V.36 and EIA-530A. The interface

mode selection is done via a 4–bit switch

for the drivers and receivers. The SP505

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 SP505 is ideally suited for wide area

network connectivity based on the interface

modes offered and the driver and receiver

configurations. The SP505 has seven (7)

independent drivers and seven (7) indepen-

dent receivers. In V.35 mode, the SP505

includes the necessary components and

termination resistors internal within the de-

vice for compliant V.35 operation.

THEORY OF OPERATION

The SP505 is made up of five separate

circuit blocks — the charge pump, drivers,

receivers,decoderandswitchingarray. Each

of these circuit blocks is described in more

detail below.

Charge–Pump

The SP505 charge pump is based on the

SP504designwhereExar'spatentedcharge

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

voltageshiftingtechniquetoattainsymmetri-

cal 0V power supplies. The charge pump

still requires external capacitors to store the

charge. In addition the SP504 charge pump

supplies +0V or +5V on V

pending on the mode of operation. There is

afree–runningoscillatorthatcontrolsthefour

phases of the voltage shifting. A description

of each phase follows.

The SP505 charge pump is used for RS-

232 where the output voltage swing is

typically +0V and also used for RS-423.

However, RS-423 requires the voltage

swing on the driver output be between

–5V

–5V

+5V

+

+

+

–

–

–

+

–

Figure 46. Charge Pump Phase  for +5V.

+5V

+

+

+

–

–

–

+

–

–5V

Figure 45. Charge Pump Phase  for +0V.