ISL3241E Datasheet, PDF (12/16 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +2.7V to +3.6V, 250kbps, RS-232 Transmitters/Receivers with Separate Logic Supply Pin

ISL3241E, ISL3243E
transmitters power-down, thereby reducing supply current to
less than 1µA. Invalid receiver levels occur whenever the
driving peripheral’s outputs are shut off (powered down) or
when the RS-232 interface cable is disconnected. The
ISL3243E powers back up whenever it detects a valid
RS-232 voltage level on any receiver input. This automatic
power-down feature provides additional system power
savings without changes to the existing operating system.
Automatic power-down operates when the FORCEON input
is low, and the FORCEOFF input is high. Tying FORCEON
high disables automatic power-down, but manual
power-down is always available via the overriding
FORCEOFF input. Table 2 summarizes the automatic
power-down functionality.
The time to recover from automatic power-down mode is
typically 20µs.
Capacitor Selection
The charge pumps require 0.1µF, or greater, capacitors for
proper operation. Increasing the capacitor values (by a
factor of 2) reduces ripple on the transmitter outputs and
slightly reduces power consumption.
When using minimum required capacitor values, make sure
that capacitor values do not degrade excessively with
temperature. If in doubt, use capacitors with a larger nominal
value. The capacitor’s equivalent series resistance (ESR)
usually rises at low temperatures and it influences the
amount of ripple on V+ and V-.
Power Supply Decoupling
In most circumstances a 0.1µF bypass capacitor is
adequate. In applications that are particularly sensitive to
power supply noise, decouple V_CC and V_L to ground with a
capacitor of the same value as the charge-pump capacitor C₁.
Connect the bypass capacitor as close as possible to the IC.
Transmitter Outputs when Exiting
Power-down
Figure 12 shows the response of two transmitter outputs
when exiting powerdown mode. As they activate, the two
transmitter outputs properly go to opposite RS-232 levels,
with no glitching, ringing, nor undesirable transients. Each
transmitter is loaded with 3kΩ in parallel with 1000pF. Note
that the transmitters enable only when the magnitude of V+
and V- exceeds approximately 3V.
5V/DIV.
FORCEOFF
T1
2V/DIV.
T2
V_CC = +3.3V, V_L = 1.8V
C1 TO C4 = 0.1µF
TIME (20µs/DIV.)
FIGURE 12. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN
Operation Down to 2.7V
ISL324xE transmitter outputs meet RS-562 levels (±3.7V), at
the full data rate, with V_CC as low as 2.7V. RS-562 levels
typically ensure inter operability with RS-232 devices.
High Data Rates
The ISL324xE maintain the RS-232 ±5V minimum
transmitter output voltages even at high data rates.
Figure 13 details a transmitter loopback test circuit, and
Figure 14 illustrates the loopback test result at 120kbps. For
this test, all transmitters were simultaneously driving RS-232
loads in parallel with 1000pF, at 120kbps. Figure 15 shows
the loopback results for a single transmitter driving 1000pF
and an RS-232 load at 250kbps. The static transmitters were
also loaded with an RS-232 receiver.
V_CC
0.1µF
+
0.1µF ⁺
V_L
+
C₁
+
C₂
C1+
V_CC VL _V+
C1-
ISL324xE
C2+
V-
C2-
EN
T_IN
T_OUT
R_OUT
R_IN
+
C₃
+C⁴
1000pF
FORCEON
5kΩ
V_L
FORCEOFF OR SHDN
FIGURE 13. TRANSMITTER LOOPBACK TEST CIRCUIT
12
FN6768.1
May 13, 2010