ISL4238E Datasheet, PDF (9/17 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/1Mbps, RS-232 Transceivers with Enhanced Automatic Power-down

ISL4238E, ISL4244E, ISL4245E
TABLE 2. POWER-DOWN LOGIC TRUTH TABLE
RCVR OR
XMTR
EDGE
WITHIN 30
SEC?
FORCEOFF FORCEON TRANSMITTER RECEIVER R_OUTB
INPUT
INPUT
OUTPUTS OUTPUTS OUTPUTS
RS-232
LEVEL
PRESENT
AT
RECEIVER
INPUT?
INVALID
OUTPUT
MODE OF OPERATION
NO
H
H
Active
Active
Active
No
L
Normal Operation (Enhanced
NO
H
H
Active
Active
Active
Yes
H
Auto Power-down Disabled)
YES
H
L
Active
Active
Active
No
L
Normal Operation (Enhanced
YES
H
L
Active
Active
Active
Yes
H
Auto Power-down Enabled)
NO
H
L
High-Z
Active
Active
No
L
Power-down Due to Enhanced
NO
H
L
High-Z
Active
Active
Yes
H
Auto Power-down Logic
X
L
X
High-Z
High-Z
Active
No
L
Manual Power-down
X
L
X
High-Z
High-Z
Active
Yes
H
INVALID DRIVING FORCEON AND FORCEOFF (EMULATES AUTOMATIC POWER-DOWN)
X
Note 8
Note 8
Active
Active
Active
Yes
H
Normal Operation
X
Note 8
Note 8
High-Z
High-Z
Active
No
L
Forced Auto Power-down
NOTE:
8. Input is connected to INVALID Output.
GND for the best performance. ISL4238E transmitter inputs
incorporate an active positive feedback resistor that
maintains the last input state in the absence of a forcing
signal, so unused transmitter inputs may be left
unconnected.
Receivers
ISL4238E, ISL4244E, ISL4245 devices contain standard
inverting receivers which can tristate via the FORCEOFF
control line. Additionally, they 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.
V_CC
R_XIN
-25V ≤ V_RIN ≤ +25V
5kΩ
GND
R_XOUT
GND ≤ V_ROUT ≤ V_CC
FIGURE 1. INVERTING RECEIVER CONNECTIONS
The ISL4238E, ISL4244E, ISL4245 inverting receivers
disable during forced (manual) power-down, but not during
automatic power-down (see Table 2). Conversely, the
monitor receiver remains active even during manual power-
down 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
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
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
power-down 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.
9
FN8038.5
May 13, 2010