AD7306
–6–
REV. C
GENERAL DESCRIPTION
The AD7306 drivers/receivers provide an interface which is
compatible with RS-232/RS-422 standard interfaces. As both
standards are widely accepted it is often necessary to provide an
interface which is compatible with both. The AD7306 is ideally
suited to this type of application as both standards may be met
using a single package. This part contains two RS-232 drivers,
one RS-422 driver, one RS-232 receiver, and one receiver path
which can be configured as either RS-232 or RS-422. This re-
ceiver is configured using the 232/
422 SEL pin.
This part also contains an internal charge pump voltage con-
verter which facilitates operation using a single +5 V power
supply.
Charge Pump DC-DC Voltage Generator
The charge pump voltage generator uses a switched capacitor
technique to develop ±10 V levels from an input +5 V supply. A
highly efficient charge pump design coupled with a high fre-
quency internal oscillator permit operation using four 0.1 µF
capacitors.
±10V
POWER SUPPLY
GENERATOR
+5V
INPUT
SUPPLY
+10V
OUTPUT
–10V
OUTPUT
C3
0.1µF
C4
0.1µF
C1+
C1– C2+ C2–
V–
V+
C1
0.1µF
C2
0.1µF
V
CC
Figure 7. Charge Pump Voltage Generator
Capacitors C1 and C2 act as charge storage capacitors while C3
and C4 provide output smoothing. For correct operation all four
capacitors must be included. Either polarized or nonpolarized
capacitors may be used for C1–C4. If a polarized type is used,
then the correct polarity should be observed. This may be ig-
nored with nonpolarized type capacitors.
The charge pump output voltages, V+ and V–, are used inter-
nally to power the RS-232 transmitters. This permits RS-232
output levels to be developed on the RS-232 transmitter out-
puts. The charge pump output voltages may also be used to
power external circuitry if the current requirements are small.
Please refer to the Typical Performance Characteristics.
The generator rise time after power up is 200 µs typical. This
time is necessary to completely charge the storage capacitors in
the charge pump. Therefore, RS-232 data transmission should
not be initiated until this time has elapsed after switch on. This
will ensure that valid data is always transmitted.
RS-232 Drivers
The RS-232 drivers in the AD7306 meet the EIA RS-232
specifications. The drivers are inverting level shifters which
convert TTL/CMOS levels into RS-232 output levels. The in-
put switching threshold is typically 1.3 V. With a typical
RS-232 load, the output levels are ±7.5 V. Under worst case
load conditions, the drivers are guaranteed to provide ±5 V
which meets the minimum RS-232 requirement. The output
slew rate is internally limited to <30 V/µs without the need for
an external slew limiting capacitor. Short circuit protection is
also provided which prevents damage in the event of output
fault conditions. Active current limiting is used which limits the
output short circuit current to less than 12 mA in the event of
an output fault. This type of current limiting does not degrade
the output voltage swing under normal loading conditions as
would be the case with conventional passive limiting.
The powered-down output impedance is typically 10 MΩ. This
is considerably larger than the 300 Ω minimum value required
by the RS-232 specification. It provides additional protection
under fault conditions where another powered-up transmitter
output is inadvertently shorted to the powered-down device.
RS-232 Receivers
The receivers are inverting level shifters which accept RS-232
input levels (±3 V to ±15 V) and translates them into 5 V
TTL/CMOS levels. The input switching thresholds are 0.8 V
minimum and 2.4 V maximum which are well within the
RS-232 requirement of ±3 V. Internal 5 kΩ pull-down resistors
to GND are provided on the receiver inputs. This ensures that
an unconnected input will be interpreted as a low level giving a
Logic “1” on the TTL/CMOS output. Excellent noise immu-
nity is achieved by the use of hysteresis and internal filtering
circuitry. The filter rejects noise glitches of up to 0.5 µs in
duration.
RS-422 Driver
The RS-422 driver on the AD7306 accepts a TTL/CMOS in-
put and translates it into a differential RS-422 level signal. The
input switching threshold is typically 1.3 V. The unloaded out-
put differential voltage is typically ±5 V (see Typical Perfor-
mance Characteristics). Short circuit protection is provided on
the output which limits the current to less than 150 mA.
RS-422 Receiver
The RS-422 receiver on the AD7306 accepts a differential in-
put signal and translates it into a TTL/CMOS output level.
The input resistance on both differential inputs is 5 kΩ typical.
With the receiver inputs unconnected (floating), internal bias-
ing ensures that the receiver output is a Logic “1.”
Excellent noise immunity and high transmission speed is
achieved using the differential configuration.
