LTC1921 datasheet(10/22 Pages) LINER

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LTC6101/LTC6101HV

6101fh

APPLICATIONS INFORMATION

Figure 2. Kelvin Input Connection Preserves

Accuracy Despite Large Load Current

LTC6101

ROUT

VOUT

6101 F02

RIN

V+

LOAD

RSENSE

V+

V–

OUT

–IN

+IN

Useful Gain Conﬁgurations

Gain

RIN

ROUT

VSENSE at VOUT = 5V IOUT at VOUT = 5V

499

10k

250mV

500μA

200

10k

100mV

500μA

100

10k

50mV

500μA

Selection of External Current Sense Resistor

The external sense resistor, RSENSE, has a signiﬁcant effect

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

reproduced signal, and is limited primarily by input DC

offset of the internal ampliﬁer of the LTC6101. In addition,

RSENSE must be small enough that VSENSE does not exceed

the maximum input voltage speciﬁed by the LTC6101, 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 amp is limited by the

input offset. As an example, the LTC6101B 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 resistor

will reduce the error due to offset by increasing the sense

voltage for a given load current.

Choosing a 50mΩ RSENSE willmaximizethedynamicrange

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 5mΩ sense resistor is

employed, then the effective current error is 30mA, while

the peak sense voltage is reduced to 10mV at 2A, dis-

sipating only 20mW.

The low offset and corresponding large dynamic range of

the LTC6101 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

max, and over 70dB of dynamic range if the rated input

maximum of 500mV 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 x 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

by orders of magnitude. A sense resistor with integrated

Kelvin sense terminals will give the best results. Figure 2

illustrates the recommended method.

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