LMH6639_14 Datasheet, PDF (15/26 Pages) National Semiconductor (TI) – LMH6639 190MHz Rail-to-Rail Output Amplifier with Disable

LMH6639
www.ti.com
APPLICATION NOTES
SNOS989G – JANUARY 2002 – REVISED MARCH 2013
INPUT AND OUTPUT TOPOLOGY
All input / output pins are protected against excessive voltages by ESD diodes connected to V+ and V- rails (see
Figure 44). These diodes start conducting when the input / output pin voltage approaches 1V_be beyond V+ or V-
to protect against over voltage. These diodes are normally reverse biased. Further protection of the inputs is
provided by the two resistors (R in Figure 44), in conjunction with the string of anti-parallel diodes connected
between both bases of the input stage. The combination of these resistors and diodes reduces excessive
differential input voltages approaching 2V_be. The most common situation when this occurs is when the device is
put in shutdown and the LMH6639’s inputs no longer follow each other. In such a case, the diodes may conduct.
As a consequence, input current increases, and a portion of signal may appear at the Hi-Z output. Another
possible situation for the conduction of these diodes is when the LMH6639 is used as a comparator (or with little
or no feedback). In either case, it is important to make sure that the subsequent current flow through the device
input pins does not violate the Absolute Maximum Ratings of the device. To limit the current through the
protection circuit extra series resistors can be placed. Together with the build in series resistors of several
hundred ohms this extra resistors can limit the input current to a safe number depending on the used application.
Be aware of the effect that extra series resistors may impact the switching speed of the device. A special
situation occurs when the part is configured for a gain of +1, which means the output is directly connected to the
inverting input, see Figure 45. When the part is now placed in shutdown mode the output comes in a high
impedance state and is unable to keep the inverting input at the same level as the non-inverting input. In many
applications the output is connected to the ground via a low impedance resistor. When this situation occurs and
there is a DC voltage offset of more than 2 volt between the non-inverting input and the output, current flows
from the non-inverting input through the series resistors R via the bypass diodes to the output. Now the input
current becomes much bigger than expected and in many cases the source at the input cannot deliver this
current and will drop down. Be sure in this situation that no DC current path is available from the non-inverting
input to the output pin, or from the output pin to the load resistor. This DC path is drawn by a curved line and can
be broken by placing one of the capacitors C_IN or C_OUT or both, depending on the used application.
V+
V+
R
IN+
V+
R
IN-
V-
V-
Figure 44.
5V
2
7
-
6
SD
+
8
COUT
CIN
3
4
1k
Figure 45. DC path while in shutdown
Copyright © 2002–2013, Texas Instruments Incorporated
Product Folder Links: LMH6639
Submit Documentation Feedback
15