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-re-

ferred output.

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 external gain resistor developing a 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 LMP8645

with other ICs (ADC,

μC…).

V

30071603

FIGURE 1. Current monitor

SELECTION OF THE SHUNT RESISTOR

The accuracy of the current measurement strictly depends on

the value of the shunt resistor R

application and it is a compromise between small-signal ac-

curacy and maximum permissible voltage loss in the mea-

surement section. High values of R

at lower currents by minimizing the effects of offset, while low

values of R

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

gain resistor R

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.

The following equations explain how to select the gain resistor

for various range of the input common mode voltage.

Case 1

−2V < V

The max voltage at the RG pin is given by the following in-

equality V

*Gm

where Vout_max is the maximum allowable output voltage

according to the Electrical Tables.All the gain resistors

(R

graphical representation in Figure 2 helps in the selection; all

the combinations (V

lowed.

30071604

FIGURE 2. Allowed Gains for CASE 1

As a consequence once selected the gain (R

V

quired a Gain of 10, R

range 10 mV to 100 mV.

Case 2

1.8V < V

In this case the max voltage at the R

common mode voltage and V

which respect the following inequalities are allowed:

V

R

G

where

V

allowable output voltage according to the Electrical Tables.

The graphical representation in Figure 3 helps in the selec-

tion; all the combinations (V

given V

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