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MAX3320TEAP

P1-P3P4-P6P7-P9P10-P12
MAX3320A/B/L/T
3V to 5.5V, up to 250kbps True RS-232 Transceiver
with 4µA AutoShutdown Plus and Power-On Reset
_______________________________________________________________________________________ 7
reducing supply current to 4µA. This occurs if the RS-232
cable is disconnected or the connected peripheral trans-
mitters turn off. The system turns on again when a valid
transition occurs at any RS-232 receiver or transmitter
input. As a result, the system saves power without
changes to the existing BIOS or operating system. The
INVALID output is high when the receivers are active.
Since INVALID indicates the receiver inputs’ condition, it
can be used in any mode (Figure 2).
Tables 1 and 2 and Figure 2 summarize the MAX3320’
s
operating modes. FORCEON and FORCEOFF override
the automatic circuitry and force the transceiver into its
normal operating state or into its low-power standby
state. When neither control is asserted, the IC enters
AutoShutdown Plus mode and selects between these
states automatically, based on the last receiver or
transmitter input edge received.
When shut down, the device’s charge pumps turn off,
V+ decays to V
CC
, V- decays to ground, and the trans-
mitter outputs turn off (high impedance). The time
required to exit shutdown is typically 25µs (Figure 3a).
Software-Controlled Shutdown
If direct software control is desired, use INVALID to indi-
cate DTR or Ring Indicator signal. Connect FORCEOFF
and FORCEON together to bypass the AutoShutdown
Plus feature so the line acts like a SHDN input.
Power-On Reset
In addition to issuing a reset to the microprocessor (µP)
during power-up, power-down, and brownout condi-
tions, the MAX3320 is relatively immune to short-
duration, negative-going V
CC
transitions (glitches).
Typically, a V
CC
transient that goes 100mV below the
reset threshold and lasts 20µs or less does not cause a
reset pulse (see
Typical Operating Characteristics
).
Additional bypass capacitance mounted as close as
possible to the V
CC
pin provides additional transient
immunity.
Table 2. INVALID Truth Table
MAX3320
Figure 2a. INVALID Functional Diagram, INVALID Low
Figure 2b. INVALID Functional Diagram, INVALID High
Figure 2c. AutoShutdown Plus Logic
Figure 2d. Power-Down Logic
Yes
RS-232 SIGNAL PRESENT AT
RECEIVER INPUT
No L
H
INVALID
OUTPUT
MAX3320A/B/L/T
3V to 5.5V, up to 250kbps True RS-232 Transceiver
with 4µA AutoShutdown Plus and Power-On Reset
8 _______________________________________________________________________________________
__________Applications Information
Ensuring a Valid Reset Output
Down to V
CC
= 0V
When V
CC
falls below 1V, RESET no longer sinks cur-
rent: it becomes an open circuit. Therefore, high-
impedance CMOS logic inputs connected to RESET
can drift to undefined voltages. This presents no prob-
lem in most applications, since most µPs and other cir-
cuitry is inoperative with V
CC
below 1V. However, in
applications where RESET must be valid down to 0V,
add a pull-down resistor to ground, holding RESET low
(Figure 4). R1’s value is not critical; 100kΩ is large
enough not to load RESET, and small enough to pull it
to ground.
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; use either polarized or nonpolarized
capacitors. The charge pump requires 0.1µF capaci-
tors for 3.3V operation. For other supply voltages, refer
to Table 3 for required capacitor values. Do not use val-
ues smaller than those listed in Table 3. Increasing the
capacitor values (e.g., by a factor of 2) reduces ripple
on the transmitter outputs and slightly reduces power
consumption. C2, C3, and C4 can be increased without
changing C1's value. However, do not increase C1
without also increasing the values of C2, C3, C4,
and C
BYPASS
, to maintain the proper ratios (C1 to
the other capacitors).
Figure 3b. Receiver Positive/Negative Thresholds for INVALID
Figure 3a. AutoShutdown Plus and INVALID Timing Diagram
V
CC
0
V+
V-
V
CC
0
INVALID
OUTPUT
* ALL RECEIVER/TRANSMITTER INPUTS INACTIVE
** ANY ONE RECEIVER/TRANSMITTER INPUT BECOMES ACTIVE FORCEON = GND, FORCEOFF = V
CC
TRANSMITTER
INPUTS
VOLTAGE
RECEIVER
INPUTS
VOLTAGE
TRANSMITTER
OUTPUTS
VOLTAGE
t
AUTOSHDN
INVALID
REGION
* **
t
WU
t
WU
I
NVL
t
AUTOSHDN
MAX3320A/B/L/T
3V to 5.5V, up to 250kbps True RS-232 Transceiver
with 4µA AutoShutdown Plus and Power-On Reset
_______________________________________________________________________________________ 9
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperatures outside the 0°C to +70°C
range. If in doubt, use capacitors with a larger nominal
value. The capacitor's equivalent series resistance
(ESR), which usually rises at low temperatures, influ-
ences the amount of ripple on V+ and V-.
Transmitter Outputs
when Exiting Shutdown
Figure 5 shows two transmitter outputs when exiting
shutdown mode. As they become active, the two trans-
mitter outputs go to opposite RS-232 levels (one trans-
mitter input is high; the other is low). Each transmitter is
loaded with 3kΩ in parallel with 2500pF. The transmitter
outputs display no ringing or undesirable transients as
they exit shutdown. Note that the transmitters are
enabled only when the magnitude of V- exceeds
approximately 3V.
High Data Rates
The MAX3320 maintains the RS-232 ±5V minimum
transmitter output voltage even at high data rates.
Figure 6 shows a transmitter loop-back test circuit.
Figure 7 shows a loop-back test result at 120kbps, and
Figure 8 shows the same test at 250kbps. For Figure 7,
both transmitters were driven simultaneously at
120kbps into RS-232 loads in parallel with 1000pF. For
Figure 8, a single transmitter was driven at 250kbps,
and both transmitters were loaded with an RS-232
receiver in parallel with 1000pF.
Interconnection with 3V and 5V Logic
The MAX3320 can directly interface with various 5V
logic families, including ACT and HCT CMOS. See
Table 4 for more information on possible combinations
of interconnections.
Figure 5. Transmitter Outputs when Exiting Shutdown or
Powering Up
Figure 6. Loop-Back Test Circuit
Figure 4. Ensuring RESET Valid to V
CC
= 0V
0.1 0.473 to 5.5
0.047 0.334.5 to 5.5
C1
(µF)
0.1
C2, C3, C4, C
BYPASS
(µF)
0.1
V
CC
(V)
3 to 3.6
Table 3. Required Minimum
Capacitance Values
P1-P3P4-P6P7-P9P10-P12

MAX3320TEAP

Mfr. #:
Buy MAX3320TEAP
Manufacturer:
Maxim Integrated
Description:
RS-232 Interface IC 3V to 5.5V, Up to 250kbps True RS-232 Transceiver with 4A AutoShutdown Plus and Power On Reset
Lifecycle:
New from this manufacturer.
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