ISL3241E_14 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
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 powerdown 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
3V/DIV.
T1_IN
5V/DIV.
T1_OUT
3V/DIV.
R1_OUT
V_CC = +3.3V, V_L = 1.8V
C1 TO C4 = 0.1µF
4µs/DIV.
FIGURE 14. LOOPBACK TEST AT 120kbps
12
FN6768.2
June 18, 2012