LT6106

7

6106fa

APPLICATIONS INFORMATION

Introduction

The LT6106 high side current sense amplifier (Figure 1) 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,

The high impedance inputs of the sense amplifier will not

conduct this current, so it will flow through an internal

The output current can be transformed into a voltage by

Table 1. Useful Gain Configurations

GAIN

20

499Ω

10k

250mV

500μA

50

200Ω

10k

100mV

500μA

100

100Ω

10k

50mV

500μA

GAIN

20

249Ω

5k

125mV

500μA

50

100Ω

5k

50mV

500μA

100

50Ω

5k

25mV

500μA

Selection of External Current Sense Resistor

fect on the function of a current sensing system and must

be chosen with care.

First, the power dissipation in the resistor should be con-

sidered. The system load current will cause both heat and

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 mea-

sured signal, and is limited primarily by input DC offset of

maximum input voltage specified by the LT6106, even un-

der 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,

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

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

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

5mΩ sense resistor is employed, then the effective current

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 appli-

cations. 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 cur-

rent 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 by orders of