Datasheet

+IN
-IN
V
+
R
S
V
-
R
GAIN
V
OUT
LMP8645
R
IN
R
IN
I
S
-+
+
-
+
-
L
o
a
d
V
SENSE
R
G
R
IN
=1/Gm
LMP8645, LMP8645HV
SNOSB29F –NOVEMBER 2009–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
GENERAL
The LMP8645 and LMP8645HV are single supply high side current sense amplifiers with variable gain selected
through an external resistor and a common mode voltage Range of -2V to 42V or -2V to 76V depending on the
grade.
The sense voltage is amplified by a user-selected gain and level shifted from the positive power supply to a
ground-referred output.
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
S
′ = V
SENSE
/R
IN
= R
S
*I
S
/R
IN
, where R
IN
= 1/Gm (1)
flows entirely in the external gain resistor developing a voltage drop equal to
V
G
= I
S
′ *R
GAIN
= (V
SENSE
/R
IN
) *R
GAIN
= ((R
S
*I
S
)/R
IN
)*R
GAIN
(2)
This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of
the LMP8645 with other ICs (ADC, μC…).
V
OUT
= (R
S
*I
S
)*G, where G = R
GAIN
/R
IN
(3)
Figure 16. Current monitor
SELECTION OF THE SHUNT RESISTOR
The accuracy of the current measurement strictly depends on the value of the shunt resistor R
S
. Its value
depends on the application and it is a compromise between small-signal accuracy and maximum permissible
voltage loss in the measurement section. High values of R
S
provide better accuracy at lower currents by
minimizing the effects of offset, while low values of R
S
minimize voltage loss in the supply section. For most
applications, best performance is obtained with an R
S
value that provides a full-scale shunt voltage Range of 100
mV to 200 mV.
SELECTION OF THE GAIN RESISTOR
In the LMP8645 and LMP8645HV the gain is selected through an external resistor connected to the R
G
pin.
Moreover the gain resistor R
GAIN
determines the voltage of the output buffer which is related to the supply
voltage and to the common mode voltage of the input signal. The gain resistor must be chosen such that the
max output voltage does not exceed the LMP8645 max output voltage rating for a given common mode voltage.
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