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MAX3218EAP+T 其他数据使用手册 - Maxim Integrated（美信）
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Maxim Integrated（美信）
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接口芯片
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MAX3218EAP+T数据手册
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Figure 2. Because it is inefficient, the use of three charge pumps for generating RS-232-compliant
transmitter-output voltages is not suitable for low-power RS-232 devices.
Though suitable for higher supply voltages, this topology is not suitable for low-power RS-232 devices
operating from low voltages. The first disadvantage is poor efficiency. The input current is 42mA,
assuming a required transmitter current of 5mA for a 120kbps data rate, a standard load of 3kΩ/2500pF
in parallel, and a typical charge-pump efficiency of 80% in each section. Though only a first-order
estimation, that input current shows that the required power level (105mW) does not offer much
improvement over a MAX3218.
A triple-charge-pump IC eliminates the switched DC-DC converter, but its chip size is large because the
power MOSFETs (for charging and discharging capacitors in the third charge-pump section) require a lot
of die area. A larger package and higher price would be the consequences. Moreover, the part would
require two more external capacitors than does the double-charge-pump approach. For those reasons,
Maxim takes another path in realizing low-voltage, low-power, serial interface devices.
Table 1. Electrical specifications for the EIA/TIA-562 serial-interface standard (compare with Table
1sb in Sidebar)
Parameter Conditions Value
Data Rate 1000pF capacitive load 60kbps
Transmitter output range, minimum
3...7k_ load, 0V offset, 0 level +3.7V
3...7k_ load, 0V offset, 1 level -3.7V
Transmitter output range, maximum
Open circuit, 0 level +13.2V
Open circuit, 1 level -13.2V
Transmitter short circuit current, maximum 60mA 60mA
Maxim offers a variety of serial-interface ICs operating with supply voltages down to 2.25V. These
devices target palmtop computers, handheld instruments, PDAs and cellphones. All are compatible with
the RS-232 interface standard and compliant with EIA/TIA-562 (transmitter output amplitudes are below
5V but above 3.7V). All operate with dual charge pumps. No DC-DC converters or triple charge pumps
are necessary with this topology, so the result is much lower operating current. Other attributes of these
devices include high ESD protection and small packages.
When designed with a MAX3316E transceiver, the 120kbpsec serial-interface circuit above draws only
15mA from a 2.5V supply. The resulting power consumption (37.5mW) is only 30% of that with a
MAX3218 transceiver, even though the load (2500pF) represents a pretty long cable! Most portable
devices are not connected to a PC with 15-meter cables, so a more reasonable assumption for cable
length is two meters or less. The capacitive load for such cables is 500pF maximum, which further
reduces the supply current to 8mA (Figure 3), resulting in power consumption of only 20mW. The rule is
simple: short serial-data cables draw less current from the supply voltage. With short cables, the
transmitter output voltage (well above +4V and below –4V) is in line with the EIA/TIA-562 specification
and RS-232 compatible.
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