5
FN6017.3
March 15, 2005
All the receivers convert RS-232 signals to CMOS output
levels and accept inputs up to
±30V while presenting the
required 3k
to 7k input impedance (see Figure 1) even if
the power is off (VCC = 0V). The receivers’ Schmitt trigger
input stage uses hysteresis to increase noise immunity and
decrease errors due to slow input signal transitions.
Low Power Operation
This 3V device requires a nominal supply current of 0.3mA,
even at VCC = 5.5V, during normal operation (not in
powerdown mode). This is considerably less than the 11mA
current required by comparable 5V RS-232 devices, allowing
users to reduce system power simply by replacing the old
style device with the ISL83384E.
Low Power, Pin Compatible Replacement
Pin compatibility with existing 5V products (e.g., MAX222),
coupled with the wide operating supply range, make the
ISL83384E a potential lower power, higher performance
drop-in replacement for existing 5V applications. As long as
the
±5V RS-232 output swings are acceptable, and
transmitter pull-up resistors aren’t required, the ISL83384E
should work in most 5V applications.
When replacing a device in an existing 5V application, it is
acceptable to terminate C3 to VCC as shown on the “Typical
Operating Circuit”. Nevertheless, terminate C3 to GND if
possible, as slightly better performance results from this
configuration.
Powerdown Functionality
The already low current requirement drops significantly
when the device enters powerdown mode. In powerdown,
supply current drops to 1
A, because the on-chip charge
pump turns off (V+ collapses to VCC, V- collapses to GND),
and the transmitter and receiver outputs three-state. This
micro-power mode makes these devices ideal for battery
powered and portable applications.
Software Controlled (Manual) Powerdown
The ISL83384E may be forced into its low power, standby
state via a simple shutdown (SHDN) pin (see Figure 4).
Driving this pin high enables normal operation, while driving
it low forces the IC into its powerdown state. The time
required to exit powerdown, and resume transmission is less
than 50
s. Connect SHDN to V
CC if the powerdown function
isn’t needed.
RXOUT
GND
≤ V
ROUT ≤ VCC
5k
RXIN
-25V
≤ VRIN ≤ +25V
GND
VCC
FIGURE 1. INVERTING RECEIVER CONNECTIONS
FIGURE 2. POWER DRAIN THROUGH POWERED DOWN
PERIPHERAL
OLD
VCC
POWERED
GND
SHDN = GND
VCC
Rx
Tx
VCC
CURRENT
VOUT = VCC
FLOW
RS-232 CHIP
DOWN
UART
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
ISL83384E
TRANSITION
DETECTOR
RX
TX
VCC
TO
VOUT = HI-Z
WAKE-UP
LOGIC
POWERED
DOWN
UART
V-
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
PWR
SHDN
CPU
I/O
ISL83384E
MGT
LOGIC
UART
ISL83384E