ICL3225 Datasheet, PDF (7/18 Pages) Intersil Corporation – 1 Microamp, +3V to +5.5V, 1Mbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3225, ICL3245
Detailed Description
These ICL32XX interface ICs operate from a single +3V to
+5.5V supply, guarantee a 1Mbps 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 1Mbps data rate for full load
conditions (3kΩ and 250pF), 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 1.4Mbps. Transmitter skew is
extremely low on these devices, and is specified at the
receiver input trip points (1.4V), rather than the arbitrary 0V
crossing point typical of other RS-232 families.
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 ICL3245 receivers can three-state, via
the FORCEOFF control line. Additionally, the ICL3245
includes 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’
7
Schmitt trigger input stage uses hysteresis to increase noise
immunity and decrease errors due to slow input signal
transitions.
The ICL3245 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.
V_CC
R_XIN
-25V ≤ V_RIN ≤ +25V 5kΩ
R_XOUT
GND ≤ V_ROUT ≤ V_CC
GND
FIGURE 1. INVERTING RECEIVER CONNECTIONS
V_CC
V_CC
V_CC
CURRENT
FLOW
POWERED
DOWN
UART
GND
V_OUT = V_CC
Rx
Tx
SHDN = GND
OLD
RS-232 CHIP
FIGURE 2. POWER DRAIN THROUGH POWERED DOWN
PERIPHERAL
V_CC
TRANSITION
DETECTOR
TO
WAKE-UP
LOGIC
V_CC
R2_OUTB
R_X
POWERED
DOWN
UART
T_X
V_OUT = HI-Z
R2_OUT
T1_IN
FORCEOFF = GND
ICL3245
R2_IN
T1_OUT
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN