6
FN6034.1
November 19, 2004
(there are no pull-up resistors), and may cause I
CC
increases. Connect unused inputs to GND for the best
performance.
Receivers
The ISL83386E contains standard inverting receivers that
convert RS-232 signals to CMOS output levels and accept
inputs up to ±25V while presenting the required 3kΩ to 7kΩ
input impedance (see Figure 1) even if the power is off
(V
CC
= 0V). The receivers’ Schmitt trigger input stage uses
hysteresis to increase noise immunity and decrease errors
due to slow input signal transitions. Receiver outputs swing
from GND to V
L
, and tristate in powerdown.
Low Power Operation
This 3V device requires a nominal supply current of 0.3mA,
even at V
CC
= 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 ISL83386E in new designs.
Powerdown Functionality
The already low current requirement drops significantly
when the device enters powerdown mode. In powerdown,
supply current drops to 1µA, because the on-chip charge
pump turns off (V+ collapses to V
CC
, V- collapses to GND),
and the transmitter and receiver outputs tristate. This micro-
power mode makes these devices ideal for battery powered
and portable applications.
Software Controlled (Manual) Powerdown
The ISL83386E may be forced into its low power, standby
state via a simple shutdown (SHDN
) pin (see Figure 2).
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 100µs. Connect SHDN
to V
CC
if the powerdown
function isn’t needed.
V
L
Logic Supply Input
Unlike other RS-232 interface devices where the CMOS
outputs swing between 0 and V
CC
, the ISL83386E features
a separate logic supply input (V
L
; 1.8V to 5V, regardless of
V
CC
) that sets V
OH
for the receiver outputs. Connecting V
L
to a host logic supply lower than V
CC
, prevents the
ISL83386E outputs from forward biasing the input diodes of
a logic device powered by that lower supply. Connecting V
L
to a logic supply greater than V
CC
ensures that the receiver
output levels are compatible even with the CMOS input V
IH
of AC, HC, and CD4000 devices. Note that the V
L
supply
current increases to 100µA with V
L
= 5V and V
CC
=3.3V
(see Figure 11). V
L
also powers the transmitter and logic
inputs, thereby setting their switching thresholds to levels
compatible with the logic supply. This separate logic supply
pin allows a great deal of flexibility in interfacing to systems
with different logic supplies. If logic translation isn’t required,
connect V
L
to the ISL83386E V
CC
.
Capacitor Selection
The ISL83386E charge pumps only require 0.1µF capacitors
for the full operational voltage range. Table 3 lists other
acceptable capacitor values for various supply voltage
ranges. Do not use values smaller than those listed in Table
3. Increasing the capacitor values (by a factor of 2) reduces
ripple on the transmitter outputs and slightly reduces power
consumption.
TABLE 2. POWERDOWN TRUTH TABLE
SHDN
INPUT
TRANSMITTER
OUTPUTS
RECEIVER
OUTPUTS MODE OF OPERATION
L High-Z High-Z Manual Powerdown
H Active Active Normal Operation
R
XOUT
GND ≤ V
ROUT
≤ V
L
5kΩ
R
XIN
-25V ≤ V
RIN
≤ +25V
GND
V
L
FIGURE 1. RECEIVER CONNECTIONS
TABLE 3. REQUIRED CAPACITOR VALUES
V
CC
(V)
C
1
(µF)
C
2
, C
3
, C
4
(µF)
3.0 to 3.6 0.1 0.1
4.5 to 5.5 0.047 0.33
3.0 to 5.5 0.22 1
FIGURE 2. CONNECTIONS FOR MANUAL POWERDOWN
PWR
SHDN
CPU
I/O
ISL83386E
MGT
LOGIC
UART
V
L
I/O CHIP
POWER SUPPLY
V
CC
ISL83386E
