lmh6646-mdc National Semiconductor Corporation, lmh6646-mdc Datasheet - Page 15

• Manufacturer Part Number: lmh6646-mdc
• Description: 2.7v, 650?a, 55mhz Rail-to-rail Input And Output Amplifiers With Shutdown Option
• Manufacturer: National Semiconductor Corporation

Application Notes
During shutdown, the input stage has an equivalent circuit as
shown below in Figure 2
As can be seen above, in shutdown, there may be current
flow through the internal diodes shown, caused by input
potential, if present. This current may flow through the exter-
nal feedback resistor and result in an apparent output signal.
In most shutdown applications the presence of this output is
inconsequential. However, if the output is “forced” by another
device such as in a multiplexer, the other device will need to
conduct the current described in order to maintain the output
potential.
To keep the output at or near ground during shutdown when
there is no other device to hold the output low, a switch
(transistor) could be used to shunt the output to ground.
Figure 3 shows a circuit where a NPN bipolar is used to keep
the output near ground (∼80mV):
FIGURE 2. LMH6647 Equivalent Input in Shutdown
FIGURE 3. Active Pull-Down Schematic
Mode
(Continued)
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Figure 4 shows the output waveform.
If bipolar transistor power dissipation is not tolerable, the
switch could be by a N-channel enhancement mode MOS-
FET.
2.7V SINGLE SUPPLY RRIO 2:1 MUX
The schematic show in Figure 5 will function as a 2:1 MUX
operating on a single 2.7V power supply, by utilizing the
shutdown feature of the LMH6647:
FIGURE 5. 2:1 MUX Operating off a 2.7V Single Supply
FIGURE 4. Output Held Low by Active Pull-Down
Circuit
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