460kbps, 1碌A(chǔ) Supply Current,
RS-232-Compatible Transceivers in 碌MAX
Detailed Description
Single Charge-Pump Voltage Converter
The MAX3311/MAX3313 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.
factor of 2) reduces power consumption. C2 can be
increased without changing C1鈥檚 value. However, do
not increase C1鈥檚 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 0.1碌F capacitors, make sure
the capacitance does not degrade excessively with
temperature. If in doubt, use capacitors with a larger
nominal value. The capacitor鈥檚 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 10碌F).
Bypass V
CC
to ground with at least 0.1碌F. 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.
MAX3311/MAX3313
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 3k鈩?in parallel with 1000pF. When
SHDN
is driven low, the transmitter is disabled and put
into three state. The transmitter input does not have an
internal pullup resistor.
RS-232 Receiver
The MAX3311/MAX3313 receiver converts RS-232 sig-
nals to CMOS-logic output levels. The MAX3311 receiv-
er will remain active during shutdown mode. The
MAX3313
INVALID
indicates when an RS-232 signal is
present at the receiver input, and therefore when the
port is in use.
The MAX3313
INVALID
output is pulled low when no
valid RS-232 signal level is detected on the receiver
input.
Transmitter Output when Exiting
Shutdown
Figure 1 shows the transmitter output when exiting
shutdown mode. The transmitter is loaded with 3k鈩?in
parallel with 1000pF. The transmitter output displays no
ringing or undesirable transients as the MAX3311
comes out of shutdown. Note that the transmitter is
enabled only when the magnitude of V- exceeds
approximately -3V.
MAX3311 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 1碌A(chǔ).
The time required to exit shutdown is typically less than
100碌s.
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 0.1碌F
capacitors. Increasing the capacitor values (e.g., by a
5V/div
0
TIN = GND
SHDN
1.5V/div
0
TOUT
TIN = V
CC
Table 1. MAX3311 Shutdown Logic Truth
Table
SHDN
L
H
TRANSMITTER
OUTPUT
High Z
Active
RECEIVER
OUTPUT
Active
Active
CHARGE
PUMP
Inactive
Active
10碌s/div
Figure 1. Transmitter Output when Exiting Shutdown or
Powering Up
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Maxim
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