True RS232 Transceivers MAX3232CPE Integrated Circuit Chip Using
Four 0.1レF External Capacitors
3.0V to 5.5V, Low-Power, up to 1Mbps, True RS-232
Transceivers Using Four 0.1µF External Capacitors
General Description
The MAX3222/MAX3232/MAX3237/MAX3241 transceivers have a proprietary
low-dropout transmitter output stage enabling true RS-232
performance from a 3.0V to 5.5V supply with a dual charge pump. The
devices require only four small 0.1µF external chargepump
capacitors. The MAX3222, MAX3232, and MAX3241 are guaranteed to run
at data rates of 120kbps while maintaining RS-232 output levels.
The MAX3237 is guaranteed to run at data rates of 250kbps in the
normal operating mode and 1Mbps in the MegaBaud™ operating mode,
while maintaining RS-232 output levels. The MAX3222/MAX3232 have 2
receivers and 2 drivers.
The MAX3222 features a 1µA shutdown mode that reduces power
consumption and extends battery life in portable systems. Its
receivers remain active in shutdown mode, allowing external devices
such as modems to be monitored using only 1µA supply current. The
MAX3222 and MAX3232 are pin, package, and functionally compatible
with the industry-standard MAX242 and MAX232, respectively. The
MAX3241 is a complete serial port (3 drivers/ 5 receivers) designed
for notebook and subnotebook computers. The MAX3237 (5 drivers/3
receivers) is ideal for fast modem applications. Both these devices
feature a shutdown mode in which all receivers can remain active
while using only 1µA supply current. Receivers R1 (MAX3237/MAX3241)
and R2 (MAX3241) have extra outputs in addition to their standard
outputs. These extra outputs are always active, allowing external
devices such as a modem to be monitored without forward biasing the
protection diodes in circuitry that may have VCC completely
removed. The MAX3222, MAX3237, and MAX3241 are available in
space-saving TSSOP and SSOP packages.
__Detailed Description Dual Charge-Pump Voltage Converter
The MAX3222/MAX3232/MAX3237/MAX3241’s internal power supply
consists of a regulated dual charge pump that provides output
voltages of +5.5V (doubling charge pump) and -5.5V (inverting
charge pump), regardless of the input voltage (VCC) over the 3.0V
to 5.5V range. The charge pumps operate in a discontinuous mode; if
the output voltages are less than 5.5V, the charge pumps are
enabled, and if the output voltages exceed 5.5V, the charge pumps
are disabled. Each charge pump requires a flying capacitor (C1, C2)
and a reservoir capacitor (C3, C4) to generate the V+ and V-
supplies.
High Data Rates
The MAX3222/MAX3232/MAX3241 maintain the RS-232 ±5.0V minimum
transmitter output voltage even at high data rates. Figure 5 shows
a transmitter loopback test circuit. Figure 6 shows a loopback test
result at 120kbps, and Figure 7 shows the same test at 235kbps. For
Figure 6, all transmitters were driven simultaneously at 120kbps
into RS-232 loads in parallel with 1000pF. For Figure 7, a single
transmitter was driven at 235kbps, and all transmitters were loaded
with an RS-232 receiver in parallel with 1000pF. The MAX3237
maintains the RS-232 ±5.0V minimum transmitter output voltage at
data rates up to 1Mbps. Figure 8 shows a loopback test result at
1Mbps with MBAUD = VCC. For Figure 8, all transmitters were loaded
with an RS-232 receiver in parallel with 250pF.
