SNOSB28F – AUGUST 2010 – REVISED APRIL 2013
APPLICATION INFORMATION
GENERAL
The LMP8640 and LMP8640HV are single supply high side current sense amplifiers with a fixed gain of 20V/V,
50V/V, 100V/V and a common mode voltage range of -2V to 42V or -2V to 76V depending on the grade.
THEORY OF OPERATION
As seen from the picture below, the current flowing through R
S
develops a voltage drop equal to V
SENSE
across
R
S
. The high impedance inputs of the amplifier doesn’t conduct this current and the high open loop gain of the
sense amplifier forces its non-inverting input to the same voltage as the inverting input. In this way the voltage
drop across R
IN
matches V
SENSE
. A current proportional to I
S
according to the following relation:
I
G
= V
SENSE
/R
IN
= R
S
*I
S
/R
IN
,
(1)
(2)
flows entirely in the internal gain resistor R
G
developing a voltage drop equal to
V
RG
= I
G
*R
G
= (V
SENSE
/R
IN
) *R
G
= ((R
S
*I
S
)/R
IN
)*R
G
This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of
the LMP8640 with other ICs (ADC, µC…).
V
OUT
= 2*(R
S
*I
S
)*G,
(3)
where G=R
G
/R
IN
= 10V/V, 25V/V, 50V/V, according to the gain options.
V
SENSE
I
s
+IN
R
IN
R
s
-IN
R
IN
LMP8640
L
o
a
d
V
I
G
+
SELECTION OF THE SHUNT RESISTOR
The value chosen for the shunt resistor, R
S
, depends on the application. It plays a big role in a current sensing
system and must be chosen with care. The selection of the shunt resistor needs to take in account the small-
signal accuracy, the power dissipated and the voltage loss across the shunt itself. In applications where a small
current is sensed, a bigger value of R
S
is selected to minimize the error in the proportional output voltage. Higher
resistor value improves the SNR at the input of the current sense amplifier and hence gives an accurate output.
Similarly when high current is sensed, the power losses in R
S
can be significant so a smaller value of R
S
is
suggested. In this condition is required to take in account also the power rating of R
S
resistor. The low input
offset of the LMP8640 allows the use of small sense resistors to reduce power dissipation still providing a good
input dynamic range. The input dynamic range is the ratio expressed in dB between the maximum signal that can
be measured and the minimum signal that can be detected, usually the input offset is the principal limiting factor.
+
-
+
-
G
R
G
= 2*R
IN
V
-
V
OUT
Figure 25. Current Monitor
12
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