LMH6645 Datasheet, PDF (15/28 Pages) National Semiconductor (TI) – 2.7V, 650μA, 55MHz, Rail-to-Rail Input and Output Amplifiers with Shutdown Option

LMH6645, LMH6646, LMH6647
www.ti.com
SNOS970C – JUNE 2001 – REVISED APRIL 2013
As can be seen, 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.
LMH6647
MICRO-POWER SHUTDOWN
The LMH6647 can be shutdown to save power and reduce its supply current to less than 50μA guaranteed, 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 (<20μA, 4pF equivalent capacitance) and a resistor to V^− (≤20kΩ) will result in normal
operation. Shutdown is guaranteed 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).
During shutdown, the input stage has an equivalent circuit as shown below in Figure 48.
INVERTING
INPUT
R_S
200-400:
D4
D1
D3
D2
NON-INVERTING
INPUT
Figure 48. LMH6647 Equivalent Input in Shutdown Mode
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 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.
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 49 shows a circuit where a NPN bipolar is
used to keep the output near ground (∼80mV):
5V
-
LMH6647
V_IN
+
SD
V^-
V_OUT
SHUTDOWN
INPUT
Q1
R_S
10k
Figure 49. Active Pull-Down Schematic
Copyright © 2001–2013, Texas Instruments Incorporated
Submit Documentation Feedback
15
Product Folder Links: LMH6645 LMH6646 LMH6647