LMP8640MK-F/NOPB National Semiconductor, LMP8640MK-F/NOPB Datasheet - Page 12

• Manufacturer Part Number: LMP8640MK-F/NOPB
• Description: IC CURRENT SENSE AMP 50V 6-TSOT
• Manufacturer: National Semiconductor

Specifications of LMP8640MK-F/NOPB

• Amplifier Type: Current Sense
• Number Of Circuits: 1
• Slew Rate: 1.8 V/µs
• Gain Bandwidth Product: 450kHz
• Current - Input Bias: 13µA
• Voltage - Input Offset: 900µV
• Current - Supply: 2.3mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 12 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-6, TSOT-6
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Output Type: -
• Current - Output / Channel: -
• -3db Bandwidth: -
• Other names: LMP8640MK-F/NOPBTR

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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
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 in-
verting input. In this way the voltage drop across R
V
relation:
flows entirely in the internal gain resistor R
voltage drop equal to
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…).
where G=R
gain options.
SELECTION OF THE SHUNT RESISTOR
The value chosen for the shunt resistor, R
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 ap-
plications where a small current is sensed, a bigger value of
R
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 loss-
es in R
S
SENSE
S
is selected to minimize the error in the proportional output
develops a voltage drop equal to V
V
RG
. A current proportional to I
S
= I
can be significant so a smaller value of R
G
G
/R
*R
IN
FIGURE 1. Current monitor
I
G
G
= 10V/V, 25V/V, 50V/V, according to the
= (V
= V
V
OUT
SENSE
SENSE
= 2*(R
/R
/R
IN
IN
) *R
= R
S
S
*I
according to the following
S
S
G
)*G,
*I
= ((R
S
SENSE
/R
IN
S
S
, depends on the
,
*I
G
across R
S
)/R
developing a
IN
IN
)*R
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S
matches
is sug-
S
G
. The
12
gested. In this condition is required to take in account also the
power rating of R
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.
DRIVING ADC
The input stage of an Analog to Digital converter can be mod-
elled with a resistor and a capacitance versus ground. So if
the voltage source doesn't have a low impedance an error in
the amplitude's measurement will occur. In this case a buffer
is needed to drive the ADC. The LMP8640 has an internal
output buffer able to drive a capacitance load up to 30 pF or
the input stage of an ADC. If required an external low pass
RC filter can be added at the output of the LMP8640 to reduce
the noise and the bandwidth of the current sense.
DESIGN EXAMPLE
For example in a current monitor application is required to
measure the current sunk by a load (peak current 10A) with
a resolution of 10mA and 0.5% of accuracy. The 10bit analog
to digital converter accepts a max input voltage of 4.1V. More-
over in order to not burn much power on the shunt resistor it
needs to be less than 10mΩ. In the table below are summa-
rized the other working condition.
First step – LMP8640 / LMP8640HV selection
The required common mode voltage of the application implies
that the right choice is the LMP8640HV (High common mode
voltage up tp 76V).
Second step – Gain option selection
We can choose between three gain option (20V/V, 50V/V,
100V/V). considering the max input voltage of the ADC
Working Condition
Supply Voltage
Common mode Voltage
Temperature
Signal BW
FIGURE 2. LMP8640 to ADC interface
S
resistor. The low input offset of the
Min
5V
48V
0°C
Value
Max
5.5V
70V
70°C
50kHz
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