LTC6103CMS8 datasheet(8/16 Pages) LINER

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LTC6103

6103f

APPLICATIONS INFORMATION

Sense Resistor Connection

Kelvin connections should be used between the inputs (+IN

and –IN) and the sense resistor in all but the lowest power

applications. Solder connections and PC board intercon-

nections 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 0.5m

Ω. 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.

Selection of External Input Resistor, RIN

The external input resistor, RIN, controls the trans-

conductance of the current sense circuit. Since:

OUT

SENSE

, transconductance g

For example, if RIN = 100Ω, then:

OUT

SENSE

100

Ω

or IOUT = 1mA for VSENSE = 100mV.

RIN should be chosen to allow the required resolution

while limiting the output current. At low supply voltage,

IOUT may be as much as 1mA. By setting RIN such that

the largest expected sense voltage gives IOUT = 1mA, then

the maximum output dynamic range is available. Output

dynamic range is limited by both the maximum allowed

output current and the maximum allowed output voltage,

as well as the minimum practical output signal. If less

dynamic range is required, then RIN can be increased

accordingly, reducing the maximum output current and

power dissipation. If low sense currents must be resolved

accurately in a system that has very wide dynamic range,

a smaller RIN than the maximum current speciﬁcation

allows may be used if the maximum current is limited in

another way, such as with a Schottky diode across RSENSE

(Figure 3a). This will reduce the high current measurement

accuracy by limiting the result, while increasing the low

current measurement resolution.

This approach can be helpful in cases where occasional

large burst currents may be ignored. It can also be used

in a multi-range conﬁguration where a low current circuit

is added to a high current circuit (Figure 3b). Note that a

comparator (LTC1540) is used to select the range, and

transistor M1 limits the voltage across RSENSE(LO).

Figure 2. Kelvin Input Connection Preserves

Accuracy Despite Large Load Current

Figure 3a. Shunt Diode Limits Maximum Input Voltage to Allow

Better Low Input Resolution Without Overranging

–

+IN

OUT

1/2

LTC6103

–IN

RIN

RSENSE

V+

V–

ROUT

6103 F02

LOAD

ILOAD

V+

LOAD

DSENSE

6103 F03a

RSENSE

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