ICL3221EM, ICL3221EF
6
FN7552.1
October 13, 2016
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Detailed Description
ICL3221EM and ICL3221EF interface ICs operate from a single
+3V supply, guarantee a 250kbps minimum data rate, require
only four small external 0.1µF capacitors, feature low power
consumption, and meet all ElA RS-232C and V.28 specifications.
The circuit is divided into three sections: charge pump,
transmitters, and receivers.
Charge-Pump
Intersil’s ICL3221EM and ICL3221EF utilize regulated on-chip
dual charge pumps as voltage doublers, and voltage inverters to
generate ±5.5V transmitter supplies from a V
CC
supply as low as
3.0V. This allows these devices to maintain RS-232 compliant
output levels over the ±10% tolerance range of 3.3V powered
systems. The efficient on-chip power supplies require only four
small, external 0.1µF capacitors for the voltage doubler and
inverter functions at V
CC
= 3.3V. See “Capacitor Selection” on
page 9 and Table 3 on page 9 for capacitor recommendations for
other operating conditions. The charge pumps operate
discontinuously (i.e., they turn off as soon as the V+ and V-
supplies are pumped up to the nominal values), resulting in
significant power savings.
Transmitters
The transmitters are proprietary, low dropout, inverting drivers
that translate TTL/CMOS inputs to EIA/TIA-232 output levels.
Coupled with the on-chip ±5.5V supplies, these transmitters
deliver true RS-232 levels over a wide range of single supply
system voltages.
The transmitter output disables and assumes a high impedance
state when the device enters the power-down mode (see Table 2
on page 7). These outputs may be driven to ±12V when disabled.
All devices guarantee a 250kbps data rate for full load conditions
(3kΩ and 1000pF), V
CC
≥ 3.0V, with one transmitter operating at
full speed. Under more typical conditions of V
CC
≥ 3.3V,
R
L
=3kΩ, and C
L
= 250pF, one transmitter easily operates at
900kbps.
Transmitter inputs float if left unconnected, and may cause I
CC
increases. Connect unused inputs to GND for the best
performance.
Receivers
The ICL3221EM and ICL3221EF devices contain standard
inverting receivers that three-state via the EN
or FORCEOFF
control lines. The receivers 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 2
) 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.
The ICL3221EM’s and ICL3221EF’s inverting receivers are
disabled only when EN
is driven high (see Table 2 on page 7).
Standard receivers driving powered down peripherals must be
disabled to prevent current flow through the peripheral’s
protection diodes (see Figures 3
and 4 on page 7). This renders
them useless for wake up functions, but the corresponding
monitor receiver can be dedicated to this task as shown in
Figure 4
.
Low Power Operation
These 3V devices require a nominal supply current of 0.3mA,
during normal operation (not in power-down mode). This is
considerably less than the 5mA to 11mA current required by
comparable 5V RS-232 devices, allowing users to reduce system
power simply by switching to this new family.
Pin Compatible Replacements for 5V
Devices
The ICL3221EM and ICL3221EF are pin compatible with existing
5V RS-232 transceivers - see the “Features” section on page 1
for
details.
This pin compatibility coupled with the low I
CC
and wide operating
supply range, make the ICL3221EM and ICL3221EF potential
lower power, higher performance drop-in replacements for existing
5V applications. As long as the ±5V RS-232 output swings are
acceptable, and transmitter input pull-up resistors aren’t required,
the ICL3221EM, ICL3221EF should work in most 5V applications.
When replacing a device in an existing 5V application, it is
acceptable to terminate C
3
to V
CC
as shown on the “Typical
Operating Circuit” on page 3. Nevertheless, terminate C
3
to GND if
possible, as slightly better performance results from this
configuration.
Power-Down Functionality
The already low current requirement drops significantly when the
device enters power-down mode. In power-down, 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 outputs
three-state. Inverting receiver outputs may or may not disable in
power-down; refer to Table 2 for details. This micro-power mode
makes these devices ideal for battery powered and portable
applications.
Software Controlled (Manual) Power-Down
The ICL3221EM and ICL3221EF devices provide a pin that allows
the user to force the IC into the low power, standby state. Driving
this pin high enables normal operation, while driving it low forces
the IC into its power-down state. Connect FORCEOFF
to V
CC
if the
power-down function isn’t needed. Note that all the receiver
outputs remain enabled during shutdown (see Table 2
). For the
lowest power consumption during power-down, the receivers
should also be disabled by driving the EN
input high (see
Figures 3
and 4).
FIGURE 2. INVERTING RECEIVER CONNECTIONS
R
XOUT
GND ≤ V
ROUT
≤ V
CC
5kΩ
R
XIN
-25V ≤ V
RIN
≤ +25V
GND
V
CC
