
7
Receiver ENABLE Control
This device also features an EN input to control the receiver 
outputs. Driving EN high disables all the inverting (standard) 
receiver outputs placing them in a high impedance state. 
This is useful to eliminate supply current, due to a receiver 
output forward biasing the protection diode, when driving the 
input of a powered down (V
CC
 = GND) peripheral (see 
Figure 2). The enable input has no effect on transmitter nor 
monitor (R
OUTB
) outputs.
MegaBaud Selection
In normal operating mode (MBAUD = GND), the ICL3237 
transmitters guarantee a 250kbps data rate with worst-case 
loads of 3k
 in parallel with 1000pF. This provides 
compatibility with PC-to-PC communication software, such 
as Laplink.
For higher speed serial communications, the ICL3237 
features MegaBaud operation. In MegaBaud operating 
mode (MBAUD = V
CC
), the ICL3237 transmitters guarantee 
a 1Mbps data rate with worst-case loads of 3k
 in parallel 
with 250pF for 3.0V < V
CC
 < 4.5V. For 5V 
±
10% operation, 
the ICL3237 transmitters guarantee a 1Mbps data rate into 
worst-case loads of 3k
 in parallel with 1000pF.
Capacitor Selection
The charge pumps require 0.1
μ
F capacitors for 3.3V 
operation. For other supply voltages refer to Table 3 for 
capacitor values. 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. C
2
, C
3
, and C
4
 can be 
increased without increasing C
1
’s value, however, do not 
increase C
1
 without also increasing C
2
, C
3
, and C
4
 to 
maintain the proper ratios (C
1
 to the other capacitors).
When using minimum required capacitor values, make sure 
that capacitor values do not degrade excessively with 
temperature. If in doubt, use capacitors with a larger nominal 
value. The capacitor’s equivalent series resistance (ESR) 
usually rises at low temperatures and it influences the 
amount of ripple on V+ and V-.
Power Supply Decoupling
In most circumstances a 0.1
μ
F bypass capacitor is 
adequate. In applications that are particularly sensitive to 
power supply noise, decouple V
CC
 to ground with a 
capacitor of the same value as the charge-pump capacitor C
1
. 
Connect the bypass capacitor as close as possible to the IC.
Transmitter Outputs when Exiting 
Powerdown
Figure 4 shows the response of two transmitter outputs 
when exiting powerdown mode. As they activate, the two 
transmitter outputs properly go to opposite RS-232 levels, 
with no glitching, ringing, nor undesirable transients. Each 
transmitter is loaded with 3k
 
in parallel with 2500pF. Note 
that the transmitters enable only when the magnitude of the 
supplies exceed approximately 3V.
High Data Rates
The ICL3237 maintains the RS-232 
±
5V minimum 
transmitter output voltages even at high data rates. Figure 5 
FIGURE 2. POWER DRAIN THROUGH POWERED DOWN 
PERIPHERAL 
OLD
V
CC
POWERED
DOWN
UART
GND
SHDN = GND
V
CC
Rx
Tx
V
CC
CURRENT
FLOW
V
OUT 
=
V
CC
RS-232 CHIP
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
ICL3237
TRANSITION
DETECTOR
R
X
T
X
R1
OUTB
R1
OUT
T1
IN
V
CC
V
CC
TO
R1
IN
T1
OUT
V
OUT 
=
HI-Z
POWERED
DOWN
UART
SHDN = GND, EN = V
CC
WAKE-UP
LOGIC
TABLE 3. REQUIRED CAPACITOR VALUES
V
CC 
(V)
C
1 
(
μ
F)
C
2
, C
3
, C
4 
(
μ
F)
3.0 to 3.6
0.22
0.22
3.15 to 3.6
0.1
0.1
4.5 to 5.5
0.047
0.33
3.0 to 5.5
0.22
1.0
ICL3237
Laplink is a registered trademark of Traveling Software.