6
Detailed Description
ICL3238E operates from a single +3V to +5.5V supply,
guarantees a 250kbps minimum data rate, requires only four
small external 0.1
µF (0.22µF for V
CC
= 3.0V) 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 ICL32XXE 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
µF 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 incorporate an active positive feedback
resistor that maintains the last driven input state in the
absence of a forcing signal. Unused transmitter inputs may
be left unconnected.
Receivers
The ICL3238E contains both standard inverting, three-state
receivers, and a single 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 inverting receivers disable during forced (manual)
powerdown, but not during automatic powerdown (see
Table 2). Conversely, the monitor receiver remains active
TABLE 2. POWERDOWN LOGIC TRUTH TABLE
RCVR OR
XMTR
EDGE
WITHIN 30
SEC?
FORCEOFF
INPUT
FORCEON
INPUT
TRANSMITTER
OUTPUTS
RECEIVER
OUTPUTS
R
OUTB
OUTPUT
RS-232
LEVEL
PRESENT
AT
RECEIVER
INPUT?
INVALID
OUTPUT MODE OF OPERATION
No H H Active Active Active No L Normal Operation (Enhanced
Auto Powerdown Disabled)
No H H Active Active Active Yes H
Yes H L Active Active Active No L Normal Operation (Enhanced
Auto Powerdown Enabled)
Yes H L Active Active Active Yes H
No H L High-Z Active Active No L Powerdown Due to Enhanced
Auto Powerdown Logic
No H L High-Z Active Active Yes H
X L X High-Z High-Z Active No L Manual Powerdown
X L X High-Z High-Z Active Yes H
INVALID
DRIVING FORCEON AND FORCEOFF (EMULATES AUTOMATIC POWERDOWN)
X Note 7 Note 7 Active Active Active Yes H Normal Operation
X Note 7 Note 7 High-Z High-Z Active No L Forced Auto Powerdown
NOTE:
7. Input is connected to INVALID
Output.
ICL3238E
