MAX3311E Datasheet, PDF (5/8 Pages) Maxim Integrated Products – ±15kV ESD-Protected, 460kbps, 1μA, RS-232-Compatible Transceivers in μMAX

±15kV ESD-Protected, 460kbps, 1µA,
RS-232-Compatible Transceivers in µMAX
Detailed Description
Single Charge-Pump Voltage Converter
The MAX3311E/MAX3313E internal power supply has a
single inverting charge pump that provides a negative
voltage from a single +5V supply. The charge pump
operates in a discontinuous mode and requires a flying
capacitor (C1) and a reservoir capacitor (C2) to gener-
ate the V- supply.
RS-232-Compatible Driver
The transmitter is an inverting level translator that con-
verts CMOS-logic levels to EIA/TIA-232 compatible lev-
els. It guarantees data rates up to 460kbps with
worst-case loads of 3kΩ in parallel with 1000pF. When
SHDN is driven low, the transmitter is disabled and put
into tri-state. The transmitter input does not have an
internal pullup resistor.
RS-232 Receiver
The MAX3311E/MAX3313E receiver converts RS-232
signals to CMOS-logic output levels. The MAX3311E
receiver will remain active during shutdown mode. The
MAX3313E INVALID indicates when an RS-232 signal
is present at the receiver input, and therefore when the
port is in use.
The MAX3313E INVALID output is pulled low when no
valid RS-232 signal level is detected on the receiver
input.
MAX3311E Shutdown Mode
In shutdown mode, the charge pump is turned off, V- is
pulled to ground, and the transmitter output is disabled
(Table 1). This reduces supply current typically to 1µA.
The time required to exit shutdown is less than 25ms.
Applications Information
Capacitor Selection
The capacitor type used for C1 and C2 is not critical for
proper operation; either polarized or nonpolarized
capacitors are acceptable. If polarized capacitors are
used, connect polarity as shown in the Typical
Operating Circuit. The charge pump requires 0.1µF
capacitors. Increasing the capacitor values (e.g., by a
factor of 2) reduces power consumption. C2 can be
Table 1. MAX3311E Shutdown Logic
Truth Table
SHDN
L
H
TRANSMITTER
OUTPUT
High Z
Active
RECEIVER
OUTPUT
Active
Active
CHARGE
PUMP
Inactive
Active
increased without changing C1’s value. However, do
not increase C1’s value without also increasing the
value of C2 and C_BYPASS to maintain the proper ratios
(C1 to the other capacitors).
When using the minimum 0.1µF capacitors, make sure
the capacitance does not degrade excessively with
temperature. If in doubt, use capacitors with a larger
nominal value. The capacitor’s equivalent series resis-
tance (ESR) usually rises at low temperatures and influ-
ences the amount of ripple on V-.
To reduce the output impedance at V-, use larger
capacitors (up to 10µF).
Bypass V_CC to ground with at least 0.1µF. In applica-
tions sensitive to power-supply noise generated by the
charge pump, decouple V_CC to ground with a capaci-
tor the same size as (or larger than) charge-pump
capacitors C1 and C2.
Transmitter Output when Exiting
Shutdown
Figure 1 shows the transmitter output when exiting
shutdown mode. The transmitter is loaded with 3kΩ in
parallel with 1000pF. The transmitter output displays no
ringing or undesirable transients as the MAX3311E
comes out of shutdown. Note that the transmitter is
enabled only when the magnitude of V- exceeds
approximately -3V.
High Data Rates
The MAX3311E/MAX3313E maintain RS-232-compati-
ble ±3.7V minimum transmitter output voltage even at
5V/div
0
1.5V/div
0
TIN = GND
SHDN
TOUT
TIN = V_CC
10µs/div
Figure 1. Transmitter Output when Exiting Shutdown or
Powering Up
_______________________________________________________________________________________ 5