8
FN4876.10
March 1, 2006
Detailed Description
These ICL32XX interface ICs operate from a single +3V to
+5.5V 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: 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 0.1µF 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.
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.
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 1Mbps.
Transmitter inputs float if left unconnected, and may cause
I
CC
increases. Connect unused inputs to GND for the best
performance.
Receivers
All the ICL32XX devices contain standard inverting
receivers, but only the ICL3238 and ICL3244 receivers can
tristate, via the FORCEOFF
control line. Additionally, the
ICL3238 and ICL3244 include a noninverting (monitor)
receiver (denoted by the R
OUTB
label) that is always active,
regardless of the state of any control lines. Both receiver
types 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.
The ICL3238 and ICL3244 inverting receivers disable during
forced (manual) powerdown, but not during automatic
powerdown (see Table 2). Conversely, the monitor receiver
remains active even during manual powerdown making it
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.
R
XOUT
GND ≤ V
ROUT
≤ V
CC
5kΩ
R
XIN
-25V ≤ V
RIN
≤ +25V
GND
V
CC
FIGURE 1. INVERTING RECEIVER CONNECTIONS
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
ICL3238/44
TRANSITION
R
X
T
X
R2
OUTB
R2
OUT
T1
IN
FORCEOFF = GND
V
CC
V
CC
TO
R2
IN
T1
OUT
V
OUT
=
HI-Z
POWERED
DETECTOR
DOWN
UART
WAKE-UP
LOGIC
ICL3224, ICL3226, ICL3238, ICL3244
