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:

I

flows entirely in the internal gain resistor R

voltage drop equal to

V

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

where G=R

gain options.

30071403

FIGURE 1. Current monitor

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

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.

30071461

FIGURE 2. LMP8640 to ADC interface

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 10m

Ω. In the table below are summa-

rized the other working condition.

Working Condition

Value

Min

Max

Supply Voltage

5V

5.5V

Common mode Voltage

48V

70V

Temperature

0°C

70°C

Signal BW

50kHz

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

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