LMH6645MF datasheet(15/19 Pages) NSC

15 / 19 page

Application Notes (Continued)

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 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:

20020256

FIGURE 2. LMH6647 Equivalent Input in Shutdown

Mode

20020264

FIGURE 3. Active Pull-Down Schematic

20020236

FIGURE 4. Output Held Low by Active Pull-Down

Circuit

20020258

FIGURE 5. 2:1 MUX Operating off a 2.7V Single Supply

LMH6645MF Datasheet(PDF) 15 Page - National Semiconductor (TI)