LT6106

7

Rev. C

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APPLICATIONS INFORMATION

Introduction

TheLT6106highsidecurrentsenseamplifier(Figure1)pro-

videsaccuratemonitoringofcurrentthroughauser-selected

sense resistor. The sense voltage is amplified by a user-

selected gain and level shifted from the positive power sup-

ply to a ground-referred output. The output signal is analog

and may be used as is, or processed with an output filter.

Theory of Operation

An internal sense amplifier loop forces –IN to have the

same potential as +IN. Connecting an external resistor,

RIN, between –IN and V+ forces a potential across RIN

that is the same as the sense voltage across RSENSE. A

corresponding current, VSENSE/RIN, will flow through RIN.

The high impedance inputs of the sense amplifier will not

conduct this current, so it will flow through an internal

PNP to the output pin as IOUT.

The output current can be transformed into a voltage by

adding a resistor from OUT to V–. The output voltage is

then VO = V– + IOUT • ROUT.

Table 1. Useful Gain Configurations

GAIN

RIN

ROUT

VSENSE at VOUT = 5V

IOUT at VOUT = 5V

20

499Ω

10k

250mV

500µA

50

200Ω

10k

100mV

500µA

100

100Ω

10k

50mV

500µA

GAIN

RIN

ROUT VSENSE at VOUT = 2.5V IOUT at VOUT = 2.5V

20

249Ω

5k

125mV

500µA

50

100Ω

5k

50mV

500µA

100

50Ω

5k

25mV

500µA

Selection of External Current Sense Resistor

The external sense resistor, RSENSE,hasasignificanteffect

on the function of a current sensing system and must be

chosen with care.

First, the power dissipation in the resistor should be

considered. The system load current will cause both heat

and voltage loss in RSENSE. As a result, the sense resis-

tor should be as small as possible while still providing

the input dynamic range required by the measurement.

Note that input dynamic range is the difference between

the maximum input signal and the minimum accurately

measuredsignal,andislimitedprimarilybyinputDCoffset

of the internal amplifier of the LT6106. In addition, RSENSE

must be small enough that VSENSE does not exceed the

maximum input voltage specified by the LT6106, even

under peak load conditions. As an example, an application

may require that the maximum sense voltage be 100mV.

If this application is expected to draw 2A at peak load,

RSENSE should be no more than 50mΩ.

Once the maximum RSENSE value is determined, the mini-

mum sense resistor value will be set by the resolution or

dynamic range required. The minimum signal that can be

accurately represented by this sense amplifier is limited by

the input offset. As an example, the LT6106 has a typical

input offset of 150µV. If the minimum current is 20mA, a

sense resistor of 7.5mΩ will set VSENSE to 150µV. This is

the same value as the input offset. A larger sense resis-

tor will reduce the error due to offset by increasing the

sense voltage for a given load current. Choosing a 50mΩ

RSENSE will maximize the dynamic range and provide a

system that has 100mV across the sense resistor at peak

load (2A), while input offset causes an error equivalent to

only 3mA of load current. Peak dissipation is 200mW. If a

5mΩsenseresistorisemployed,thentheeffectivecurrent

error is 30mA, while the peak sense voltage is reduced to

10mV at 2A, dissipating only 20mW.

The low offset and corresponding large dynamic range of

the LT6106 make it more flexible than other solutions in

this respect. The 150µV typical offset gives 60dB of dy-

namic range for a sense voltage that is limited to 150mV

maximum, and over 70dB of dynamic range if the rated

input maximum of 0.5V is allowed.

Sense Resistor Connection

Kelvin connection of the –IN and +IN inputs to the sense

resistor should be used in all but the lowest power ap-

plications. Solder connections and PC board interconnec-

tions that carry high current can cause significant error

in measurement due to their relatively large resistances.

One 10mm × 10mm square trace of one-ounce copper

is approximately 0.5mΩ. A 1mV error can be caused by

as little as 2A flowing through this small interconnect.

This will cause a 1% error in a 100mV signal. A 10A load

current in the same interconnect will cause a 5% error

for the same 100mV signal. By isolating the sense traces

from the high current paths, this error can be reduced