Fo rc e -S e n s e S w it c h e s
/MAX456
Figure 6 shows how to switch a single voltage or cur-
rent source between two loads using the MAX4554 or
MAX4556. By interchanging loads and sources, the cir-
cuits can be reversed so that they switch two sources
to a single load. The two loads are electrically connect-
ed together at one point, but may be physically sepa-
rated. This means that one force wire does not need to
be switched, but the corresponding sense wires do.
would be close to (but not exactly equal to) the desired
value; this would not cause any damage to the device.
S w it c h in g 3 -Wire Gu a rd e d Circ u it s
Figure 7 shows how to switch a single guarded voltage
or c urre nt s ourc e b e twe e n two loa d s us ing the
MAX4554 or MAX4556. By interchanging loads and
sources, the circuits can be reversed to switch two
sources to a single load. If the loads have a common
connection, the switch to that node can be eliminated.
The MAX4554 has independent 3PST, NO switches dri-
ven out of phase by an external CMOS inverter, so that
one switch is on while the other is off. If both switches
were turned on at the same time, both loads would be
connected, and the resulting voltage at either load
Note that these circuits use sense (high-resistance)
switches to switch the common wire. This is permissible
only if the load currents are very low. If the currents are
high, the common connection should not be switched
unless another force switch is substituted.
V+
V-
VL
MAX4556
NC1
FORCE
SENSE
COM1
COM2
NO1
NC2
NO2
FEEDBACK
V
NC3
NO3
LOAD2
LOAD1
SENSE
FORCE
COM3
IN1
IN2
VOLTAGE/CURRENT SOURCE
GND
LOGIC
LOAD
IN
IN
0
IN3
1
2
1
V+
V-
VL
MAX4554
NOF1
NOF2
FORCE
SENSE
FCOM
SCOM
NOS1
NOS2
FEEDBACK
V
SENSE
FORCE
GCOM
IN1
LOAD2
LOAD1
NOG2
NOG1
GND
VOLTAGE/CURRENT SOURCE
LOGIC
LOAD
EN
IN
0
1
1
2
IN2
CMOS INVERTER
IN
Figure 6. Using the MAX4554/MAX4556 to Switch 4-Wire Force-Sense Circuits from One Source to Two Loads
______________________________________________________________________________________ 21
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