LMH6645
,
LMH6646
,
LMH6647
SNOS970D –JUNE 2001–REVISED NOVEMBER 2014
www.ti.com
8.4 Device Functional Modes
The LMH6647 can be shutdown to save power and reduce its supply current to less than 50 μA ensured, by
applying a voltage to the SD pin. The SD pin is “active high” and needs to be tied to V−for normal operation. This
input is low current (< 20 μA, 4 pF equivalent capacitance) and a resistor to V−(≤20 kΩ) will result in normal
operation. Shutdown is ensured when SD pin is 0.4V or less from V+at any operating supply voltage and
temperature.
In the shutdown mode, essentially all internal device biasing is turned off in order to minimize supply current flow
and the output goes into Hi-Z (high impedance) mode. Complete device Turn-on and Turn-off times vary
considerably relative to the output loading conditions, output voltage, and input impedance, but is generally
limited to less than 1μs (see tables for actual data).
As seen in Figure 42 in shutdown, there may be current flow through the internal diodes shown, caused by input
potential, if present. This current may flow through the external 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.
The total input common mode voltage range, which extends from below V−to beyond V+, is covered by both an
NPN and a PNP stage. The NPN stage is switched on whenever the input is less than 1.2 V from V+and the
PNP stage covers the rest of the range. In terms of the input voltage, there is an overlapping region where both
stages are processing the input signal. This region is about 0.5 V from beginning to the end. As far as the device
application is concerned, this transition is a transparent operation. However, keep in mind that the input bias
current value and direction will depend on which input stage is operating (see Figure 29). For low distortion
applications, it is best to keep the input common mode voltage from crossing this transition point. Low gain
settling applications, which generally encounter larger peak-to-peak input voltages, could be configured as
inverting stages to eliminate common mode voltage fluctuations.
In terms of the output, when the output swing approaches either supply rail, the output transistor will enter a
quasi-saturated state. A subtle effect of this operational region is that there is an increase in supply current in this
state (up to 1 mA). The onset of Quasi-saturation region is a function of output loading (current) and varies from
100 mV at no load to about 1 V when output is delivering 20 mA, as measured from supplies. Both input
common mode voltage and output voltage level affect the supply current (see Figure 32).
With 2.7V supplies and a common mode input voltage range that extends beyond either supply rail, the
LMH664x family is well suited to many low voltage/low power applications. Even with 2.7 V supplies, the -3dB
BW (@ AV= +1) is typically 55 MHz with a tested limit of 45 MHz. Production testing guarantees that process
variations will not compromise speed.
This device family is designed to avoid output phase reversal. With input over-drive, the output is kept near the
supply rail (or as close to it as mandated by the closed loop gain setting and the input voltage). Figure 45, below,
shows the input and output voltage when the input voltage significantly exceeds the supply voltages.
The output does not exhibit any phase reversal as some op amps do. However, if the input voltage range is
exceeded by more than a diode drop beyond either rail, the internal ESD protection diodes will start to conduct.
The current flow in these ESD diodes should be externally limited.
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