ICL3221EM_14 Datasheet, PDF (9/12 Pages) Intersil Corporation – ±15kV ESD Protected, +3V, 1μA, 250kbps, RS-232 Transmitters/Receivers

ISL3221EM
high impedance state. This is useful to eliminate supply
current, due to a receiver output forward biasing the
protection diode, when driving the input of a powered
down (V_CC = GND) peripheral (see Figure 2).
Capacitor Selection
The charge pumps require 0.1µF capacitors for 3.3V
operation. For other supply voltages, refer to Table 3 for
capacitor values. 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. C₂, C₃, and C₄ can
be increased without increasing C₁’s value, however, do
not increase C₁ without also increasing C₂, C₃, and C₄ to
maintain the proper ratios (C₁ to the other capacitors).
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-.
TABLE 3. REQUIRED CAPACITOR VALUES
V_CC
(V)
C₁
(µF)
C₂, C₃, C₄
(µF)
3.0 to 3.6
0.1
0.1
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 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.
Operation Down to 2.7V
The ICL3221EM transmitter outputs meet RS-562 levels
(±3.7V), at full data rate, with V_CC as low as 2.7V.
RS-562 levels typically ensure interoperability with RS-
232 devices.
Transmitter Outputs when
Exiting Power-down
Figure 8 shows the response of two transmitter outputs
when exiting power-down 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 2500pF. Note that the transmitters enable only
when the magnitude of the supplies exceed
approximately 3V.
5V/DIV
FORCEOFF
T1
2V/DIV
T2
V_CC = +3.3V
C1 - C4 = 0.1µF
TIME (20µs/DIV)
FIGURE 8. TRANSMITTER OUTPUTS WHEN EXITING
POWER-DOWN
High Data Rates
The ICL3221EM maintains the RS-232 ±5V minimum
transmitter output voltages even at high data rates.
Figure 9 details a transmitter loopback test circuit, and
Figure 10 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 11 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 ⁺
+
C₁
+
C₂
C1+
V_CC
V+
C1- ICL3221EM
C2+
V-
C2-
T_IN
T_OUT
R_OUT
R_IN
+
C₃
+C⁴
1000pF
EN
5k
V_CC
SHDN OR
FORCEOFF
FIGURE 9. TRANSMITTER LOOPBACK TEST CIRCUIT
9
FN7552.0
December 17, 2009