ISL4260E Datasheet, PDF (6/11 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +3V to +5.5V, 150nA, 250kbps, RS-232 Transmitters/Receivers with Separate Logic Supply

ISL4260E
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 1.25Mbps.
The transmitter input threshold is set by the voltage applied
to the V_L pin. Transmitter inputs float if left unconnected
(there are no pull-up resistors), and may cause I_CC
increases. Connect unused inputs to GND for the best
performance.
TABLE 2. POWERDOWN TRUTH TABLE
SHDN TRANSMITTER RECEIVER
INPUT OUTPUTS OUTPUTS MODE OF OPERATION
L
High-Z
High-Z Manual Powerdown
H
Active
Active Normal Operation
Receivers
The ISL4260E contains standard inverting receivers that
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. Receiver outputs swing
from GND to V_L, and tristate in powerdown.
V_L
R_XIN
-25V ≤ V_RIN ≤ +25V
5kΩ
GND
R_XOUT
GND ≤ V_ROUT ≤ V_L
FIGURE 1. RECEIVER CONNECTIONS
Low Power Operation
This 3V device requires 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 11mA
current required by comparable 5V RS-232 devices, allowing
users to reduce system power simply by replacing the old
style device with the ISL4260E in new designs.
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 and receiver outputs tristate. This micro-
power mode makes these devices ideal for battery powered
and portable applications.
Software Controlled (Manual) Powerdown
The ISL4260E may be forced into its low power, standby
state via a simple shutdown (SHDN) pin (see Figure 2).
Driving this pin high enables normal operation, while driving
it low forces the IC into its powerdown state. The time
required to exit powerdown, and resume transmission is less
than 100μs. Connect SHDN to V_CC if the powerdown
function isn’t needed.
PWR
MGT
LOGIC
SHDN
I/O CHIP
POWER SUPPLY
V_L ISL4260E
V_CC
CPU
I/O
UART
FIGURE 2. CONNECTIONS FOR MANUAL POWERDOWN
V_L Logic Supply Input
Unlike other RS-232 interface devices where the CMOS
outputs swing between 0 and V_CC, the ISL4260E features a
separate logic supply input (V_L; 1.8V to 5V, regardless of
V_CC) that sets V_OH for the receiver outputs. Connecting V_L
to a host logic supply lower than V_CC, prevents the
ISL4260E outputs from forward biasing the input diodes of a
logic device powered by that lower supply. Connecting V_L to
a logic supply greater than V_CC ensures that the receiver
output levels are compatible even with the CMOS input V_IH
of AC, HC, and CD4000 devices. Note that the V_L supply
current increases to 100μA with V_L = 5V and V_CC = 3.3V
(see Figure 11). V_L also powers the transmitter and logic
inputs, thereby setting their switching thresholds to levels
compatible with the logic supply. This separate logic supply
pin allows a great deal of flexibility in interfacing to systems
with different logic supplies. If logic translation isn’t required,
connect V_L to the ISL4260E V_CC.
Capacitor Selection
The ISL4260E charge pumps only require 0.1μF capacitors
for the full operational voltage range. Table 3 lists other
acceptable capacitor values for various supply voltage
ranges. Do not use values smaller than those listed in
Table 3. Increasing the capacitor values (by a factor of 2)
reduces ripple on the transmitter outputs and slightly
reduces power consumption.
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