LMP8640 datasheet(12/22 Pages) TI1

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+IN

-IN

RG = 2*RIN

VOUT

LMP8640

RIN

VSENSE

LMP8640

LMP8640HV

SNOSB28D – AUGUST 2010 – REVISED NOVEMBER 2011

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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 RS develops a voltage drop equal to VSENSE across

RS. 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 RIN matches VSENSE. A current proportional to IS according to the following relation:

IG = VSENSE/RIN = RS*IS/RIN ,

(1)

flows entirely in the internal gain resistor RG developing a voltage drop equal to

VRG = IG *RG = (VSENSE/RIN) *RG = ((RS*IS)/RIN)*RG

(2)

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…).

VOUT = 2*(RS*IS)*G,

(3)

where G=RG/RIN = 10V/V, 25V/V, 50V/V, according to the gain options.

Figure 25. Current Monitor

SELECTION OF THE SHUNT RESISTOR

The value chosen for the shunt resistor, RS, 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 RS 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 RS can be significant so a smaller value of RS is

suggested. In this condition is required to take in account also the power rating of RS 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.

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