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 0.1F 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 0.1F 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 3kin 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
GND
SHDN
= GND
V
CC
Rx
Tx
V
CC
CURRENT
V
OUT
=
V
CC
FLOW
RS-232 CHIP
DOWN
UART
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
ICL3237
TRANSITION
R
X
T
X
R1
OUTB
R1
OUT
T1
IN
V
CC
V
CC
TO
R1
IN
T1
OUT
V
OUT
=
HI-Z
POWERED
SHDN
= GND, EN = V
CC
DETECTOR
DOWN
UART
WAKE-UP
LOGIC
TABLE 3. REQUIRED CAPACITOR VALUES
V
CC
(V) C
1
(F) C
2
, C
3
, C
4
(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.
