
6
Detailed Description
The ICL3237 operates from a single +3V to +5.5V supply, 
guarantees a 1Mbps minimum data rate, requires only four 
small external 0.1
μ
F capacitors, features low power 
consumption, and meets all EIA/TIA-232 and V.28 
specifications. The circuit is divided into three sections: The 
charge pump, the transmitters, and the receivers.
Charge-Pump
Intersil’s new ICL32XX family utilizes 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 0.1
μ
F capacitors for the 
voltage doubler and inverter functions at V
CC
 = 3.3V. See 
the “Capacitor Selection” section, and Table 3 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.
All transmitter outputs disable and assume a high impedance 
state when the device enters the powerdown mode (see Table 
2). These outputs may be driven to 
±
12V when disabled.
The ICL3237 guarantees a 1Mbps data rate (if MBAUD = 1) 
for full load conditions (3k
 and 250pF), V
CC
≥
 3.0V, with 
one transmitter operating at full speed. Under more typical 
conditions of V
CC
≥
 3.3V, C
1-4
 = 0.1
μ
F, R
L
 = 3k
, and 
C
L
= 250pF, one transmitter easily operates at 1.7Mbps.
Transmitter inputs float if left unconnected, and may cause 
I
CC
 increases. Connect unused inputs to GND for the best 
performance.
Receivers
The ICL3237 device contains standard inverting receivers 
that three-state only when the EN control line is driven high. 
Additionally, it includes a noninverting (monitor) receiver 
(denoted by the R
OUTB
 label) that is always active, 
regardless of the state of any control lines. All the receivers 
convert RS-232 signals to CMOS output levels and accept 
inputs up to 
±
25V while presenting the required 3k
 to 7k
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.
Monitor receivers remain active even during manual 
powerdown and forced receiver disable, making them 
extremely useful for Ring Indicator monitoring. Standard 
receivers driving powered down peripherals must be disabled 
to prevent current flow through the peripheral’s protection 
diodes (see Figures 2 and 3). This renders them useless for 
wake up functions, but the corresponding monitor receiver 
can be dedicated to this task as shown in Figure 3.
Powerdown Functionality
This 3V device requires a nominal supply current of 0.3mA 
during normal operation (not in powerdown mode). This is 
considerably less than the 5mA to 11mA current required of 
5V RS-232 devices. The already low current requirement 
drops significantly when the device enters powerdown 
mode. In powerdown, supply current drops to 1
μ
A, because 
the on-chip charge pump turns off (V+ collapses to V
CC
, V- 
collapses to GND), and the transmitter outputs three-state. 
This micro-power mode makes the ICL3237 ideal for battery 
powered and portable applications.
Software Controlled (Manual) Powerdown
On the ICL3237, the powerdown control is via a simple 
shutdown (SHDN) pin. Driving this pin high enables normal 
operation, while driving it low forces the IC into it’s 
powerdown state. Connect SHDN to V
CC
 if the powerdown 
function isn’t needed. Note that all the receiver outputs 
remain enabled during shutdown (see Table 2). For the 
lowest power consumption during powerdown, the receivers 
should also be disabled by driving the EN input high (see 
next section, and Figures 2 and 3). The time required to exit 
powerdown, and resume transmission is only 100
μ
s.
R
XOUT
GND 
≤
 V
ROUT
≤
 V
CC
5k
R
XIN
-25V 
≤
 V
RIN
≤
 +25V
GND
V
CC
FIGURE 1. INVERTING RECEIVER CONNECTIONS
TABLE 2. POWERDOWN AND ENABLE LOGIC TRUTH TABLE
SHDN INPUT
EN INPUT
TRANSMITTER OUTPUTS
RECEIVER OUTPUTS
R
OUTB
 OUTPUT
MODE OF OPERATION
L
L
High-Z
Active
Active
Manual Powerdown
L
H
High-Z
High-Z
Active
Manual Powerdown w/Rcvr. Disabled
H
L
Active
Active
Active
Normal Operation
H
H
Active
High-Z
Active
Normal Operation w/Rcvr. Disabled
ICL3237