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LT6106

6106fa

APPLICATIONS INFORMATION

magnitude. A sense resistor with integrated Kelvin sense

terminals will give the best results. Figure 2 illustrates the

recommended method.

This approach can be helpful in cases where occasional

bursts of high currents can be ignored.

Care should be taken when designing the board layout for

RIN, especially for small RIN values. All trace and inter-

connect resistances will increase the effective RIN value,

causing a gain error.

Selection of External Output Resistor, ROUT

The output resistor, ROUT, determines how the output cur-

rent is converted to voltage. VOUT is simply IOUT • ROUT.

In choosing an output resistor, the maximum output volt-

age must ﬁrst be considered. If the following circuit is a

buffer or ADC with limited input range, then ROUT must be

chosen so that IOUT(MAX) • ROUT is less than the allowed

maximum input range of this circuit.

In addition, the output impedance is determined by ROUT. If

the circuit to be driven has high enough input impedance,

then almost any useful output impedance will be accept-

able. However, if the driven circuit has relatively low input

impedance, or draws spikes of current such as an ADC

might do, then a lower ROUT value may be required in order

to preserve the accuracy of the output. As an example, if

the input impedance of the driven circuit is 100 times ROUT,

then the accuracy of VOUT will be reduced by 1% since:

OUT

IN DRIVEN

OUT

IN DRIVEN

•

()

IIR

OUT

••

•

100

101

099

Error Sources

The current sense system uses an ampliﬁer and resistors

to apply gain and level shift the result. The output is then

dependent on the characteristics of the ampliﬁer, such as

gain and input offset, as well as resistor matching.

Ideally, the circuit output is:

OUT

SENSE

OUT

SENSE

•;

•

In this case, the only error is due to resistor mismatch,

which provides an error in gain only. However, offset volt-

age and bias current cause additional errors.

Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow

Better Low Input Resolution Without Overranging

Figure 2. Kelvin Input Connection Preserves Accuracy with

Large Load Currents

Selection of External Input Resistor, RIN

RIN should be chosen to allow the required resolution

while limiting the output current to 1mA. In addition, the

maximum value for RIN is 500Ω. 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 a very wide dynamic range,

a smaller RIN than the maximum current spec allows may

be used if the maximum current is limited in another way,

such as with a Schottky diode across RSENSE (Figure 3).

This will reduce the high current measurement accuracy

by limiting the result, while increasing the low current

measurement resolution.

LT6106

ROUT

VOUT

6106 F02

RIN

V+

LOAD

RSENSE

V+

V–

OUT

–IN

+IN

V+

LOAD

DSENSE

6106 F03

RSENSE

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