SP505AN Datasheet, PDF (9/30 Pages) Sipex Corporation – Multi-Protocol Serial Transceivers

The SP505, SP506 and SP507 can be easily designed
to support this type of configuration. Figure 6 shows
a typical circuit illustrating two SP506 devices
connected in a half-duplex configuration. The top
circuit is dedicated to DTE and the bottom SP506 is
dedicated to DCE. Note that only one device is active
at any given time. For DTE, the decoder for the DCE
device should be off (0000), and vice versa. During
the shutdown or off state of the SP506, the driver
output typically draws 100µA of leakage current.
Even with the maximum SP506 leakage current of
500µA, the receiver input impedance would only
change by 500Ω. This is important for RS-232 since
the input voltage range can be up to 15V and the
typical RS-232 receiver input impedance is 5kΩ.
For V.11 differential receivers, the maximum range
is +7V and typical input impedance is 10kΩ. Thus for
V.28 receivers, the drivers would be effectively
driving into 5kΩ in parallel with the disabled
receiver with 10kΩ input impedance. The resultant
impedance is 3.3kΩ. For V.11 mode, the drivers will
drive into either a terminated receiver of 120Ω or
unterminated receiver at 3.9kΩ. These two values in
parallel with the disabled 10kΩ receiver will yield
118Ω and 2.8kΩ, respectively, and will not degrade
the V.11 driver performance. The receiver outputs are
typically at 1µA when disabled.
The SP505, SP506 and SP507 adds convenience
by incorporating the V.11 and V.35 termination
resistors inside the device. For this type of 2-chip
DTE/DCE configuration, the termination resistors
would need to be disabled along with the receivers.
A "0000" code into the SP505 and SP506 will
automatically disable all termination networks as
well as the transceivers. A "111" code into the SP507
performs the same function. In the shutdown mode,
the IC will draw less than 10mA of supply current.
Adding Additional Transceivers
To support additional signals, the SP522 can easily
attach onto the SP505, SP506 or SP507 charge pump
outputs, V_DD and V_SS. The SP522 adds two drivers
and two receivers for supporting other signals such
as RI and RL. In Figure 7, the SP522 is hardwired for
RS-423 or ITU-T V.10 mode. This allows for the
support of RI and RL in RS-449 or V.35 modes if
necessary.
Schottky Diode on the SP50x
Sipex requires the installation of a Schottky rectifier
placed between the V_CC and V_DD pins of the SP50x
charge pump, where the anode is connected to V_CC
and the cathode is connected to V_DD. It is required to
bootstrap the charge pump's internal circuitry during
power off conditions in presence of signals or voltages
through the receiver inputs or driver outputs.
When placed in parallel with the charge pump
capacitor, the diode will allow some of the V_CC
current to flow into the V_DD regions of the device,
which will partially bias the V_DD charged regions
before the device charge pump is fully functioning.
This prevents biasing of V_DD from other sources
such as through the driver outputs or receiver inputs,
typical of serial port connections to other powered-on
equipment. Once the charge pump oscillator starts up
and becomes functional, current flows from V_DD back
into V_CC through the capacitor, ensuring that a
rapidly rising V_DD does not rise too quickly above the
V_CC regions before the V_CC regions have become
fully charged.
The main characteristics of the Schottky diode
necessary for this application is the forward
voltage. The V_F of the 1N5819 type, which is the
diode recommended, is 0.6V @ 1A. Surface mount
versions are available from Motorola. The
MBRS130T3 from Motorola is used with our SP505,
SP506, and SP507 evaluation boards. Other options
are MBRS140T3 or MBRS130LT3, which are all in a
"403A-03 SMB" package. The end-to-end length is
5.40mm typical and the width is 3.55mm typical.
Motorola also offers the Powermite^™ line, which
offers the Schottky rectifiers in a 1.1mm height,
3.75mm length, and 1.90mm width surface mount
package. The part numbers recommended are
MBRM120LT3, MBRM120ET3, and MBRM140T3.
Specifics can be found in Motorola Semiconductor's
web site (http://mot-sps.com/products/index.html).
The Schottky rectifiers can be found in the discrete
rectifier section and datasheets can be downloaded
after searching for the part number.
Powermite^™is a trademark of Motorola.
SP505/6/7APN/03
SP505, SP506, SP507 Application Note
9
© Copyright 2000 Sipex Corporation