+IN

-IN

V

+

V

-

RG

LMP8646

RIN

RIN

+

-

+

VOUT

13

LMP8646

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SNOSC63B – FEBRUARY 2012 – REVISED DECEMBER 2014

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7 Detailed Description

7.1 Overview

The LMP8646 is a single-supply precision current limiter with variable gain selected through an external resistor

(RG) and a variable bandwidth selected through an external capacitor (CG) in parallel with RG. Its common-mode

of operation is –2 V to 76 V, and the LMP8646 has an buffered output to provide a low-output impedance. More

details of the LMP8646's functional description can be seen in the following subsections.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Theory of Operation

As seen from Figure 26, the sense current flowing through RSENSE develops a voltage drop equal to VSENSE. The

high impedance inputs of the amplifier does not conduct this current and the high open-loop gain of the sense

amplifier forces its noninverting input to the same voltage as the inverting input. In this way the voltage drop

across

RIN

matches

VSENSE.

The

current

IIN

flowing

through

RIN

has

the

following

equation:

IIN = VSENSE/ RIN = RSENSE*ISENSE/RIN

where

•

RIN = 1/Gm = 1/(200 µA/V) = 5 kOhm

(1)

IIN flows entirely across the external gain resistor RG to develop a voltage drop equal to:

VRG = IIN*RG = (VSENSE/RIN) *RG = [(RSENSE*ISENSE) / RIN]*RG

(2)

This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of

the LMP8646 with the feedback of many voltage regulators. This output voltage has the following equation:

VOUT = VRG = [(RSENSE*ISENSE) / RIN]*RG

(3)

VOUT = VSENSE* RG/RIN

(4)

VOUT = VSENSE* RG/(5 kOhm)

(5)

VOUT = VSENSE* Gain

where

•

Gain = RG/RIN

(6)