Detailed Description
The MAX4890/MAX4891/MAX4892 are high-speed ana-
log switches targeted for 10/100/1000 Base-T applica-
tions. In a typical application, the MAX4890/MAX4891/
MAX4892 switch the signals from two separate inter-
face transformers and connect the signals to a single
10/100/1000 Base-T Ethernet PHY (see the
Typical
Operating Circuit
). This configuration simplifies docking
station design by avoiding signal reflections associated
with unterminated transmission lines in a T configura-
tion. The MAX4891 and MAX4892 also include LED
switches that allow the LED output signals to be routed
to a docking station along with the Ethernet signals.
See the
Functional Diagrams.
The MAX4890/MAX4891/MAX4892 switches provide
an extremely low capacitance and on-resistance
to meet Ethernet insertion and return-loss specifica-
tions. The MAX4891/MAX4892 feature one and three
built-in LED switches, respectively.
The MAX4890/MAX4891/MAX4892 incorporate a unique
architecture design utilizing only n-channel switches
within the main Ethernet switch, reducing I/O capaci-
tance and channel resistance. An internal two-stage
charge pump with a nominal output of 7.5V provides the
high voltage needed to drive the gates of the n-channel
switches, while maintaining a consistently low R
ON
throughout the input signal range. An internal bandgap
reference set to 1.23V and an internal oscillator running
at 2.5MHz provide proper charge-pump operation.
Unlike other charge-pump circuits, the MAX4890/
MAX4891/MAX4892 include internal flyback capacitors,
reducing design time, board space, and cost.
Digital Control Inputs
The MAX4890/MAX4891/MAX4892 provide a single
digital control SEL. SEL controls the switches as well as
the LED switches as shown in Table 1.
MAX4890/MAX4891/MAX4892
10/100/1000 Base-T Ethernet LAN Switch
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