ISL4221E_07 Datasheet, PDF (6/13 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +2.7V to +5.5V, 150nA, 250kBps, RS-232 Transmitters/Receivers

ISL4221E, ISL4223E
voltage doubler and inverter functions. 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.
All 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.
The 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 900kBps.
Transmitter inputs float if left unconnected, and may cause
I_CC increases. Connect unused inputs to GND for the best
performance.
Receivers
All the ISL422XE devices contain standard inverting receivers
that three-state via the EN control line. All the receivers
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.
Receivers driving a powered down UART must be disabled to
prevent current flow through, and possible damage to, the
UART’s protection diodes (see Figures 2 and 3). This can be
accomplished on the ISL422XE by driving the EN input high
whenever the UART powers down. Figure 3 also shows that
the INVALID output can be used to determine when the UART
should be powered down. When the RS-232 cable is
disconnected, INVALID switches low indicating that the UART
is no longer needed. Reconnecting the cable drives INVALID
back high, indicating that the UART should be powered up.
V_CC
R_XIN
-25V ≤ V_RIN ≤ +25V 5kΩ
R_XOUT
GND ≤ V_ROUT ≤ V_CC
GND
FIGURE 1. INVERTING RECEIVER CONNECTIONS
Low Power Operation
These 3V devices require a nominal supply current of
0.3mA, even at V_CC = 5.5V, during normal operation (not in
powerdown mode). This is considerably less than the 5mA to
11mA current required by comparable 5V RS-232 devices,
allowing users to reduce system power simply by switching to
this new family.
Powerdown Functionality
The already low current requirement drops significantly when
the device enters powerdown mode. In powerdown, supply
current drops to 150nA, because the on-chip charge pump
turns off (V+ collapses to V_CC, V- collapses to GND), and the
transmitter outputs three-state. Receiver outputs are
unaffected by powerdown; refer to Table 2 for details. This
micro-power mode makes the ISL422XE ideal for battery
powered and portable applications.
Software Controlled (Manual) Powerdown
The ISL422XE family provides pins that allow the user to force
the IC into the low power, standby state.
The ISL422XE utilize a two pin approach where the
FORCEON and FORCEOFF inputs determine the IC’s
V_CC
V_CC
V_CC
CURRENT
FLOW
Rx
POWERED
DOWN
UART
Tx
GND
V_OUT = V_CC
SHDN = GND
OLD
RS-232 CHIP
FIGURE 2. POWER DRAIN THROUGH POWERED DOWN
PERIPHERAL
V_CC
TRANSITION
DETECTOR
TO
WAKE-UP
LOGIC
V_CC
ISL422XE
INVALID
R_X
POWERED
DOWN
UART _TX
V_OUT = HI-Z
R_OUT
T_IN
EN = V_CC
R_IN
T_OUT
FIGURE 3. DISABLED RECEIVERS PREVENT POWER DRAIN
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FN6045.2
April 13, 2007