LTC6103 データシートの表示（PDF） - Linear Technology

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LTC6103

Dual High Voltage, High Side Current Sense Amplifi er

Linear Technology 

LTC6103

APPLICATIONS INFORMATION

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

bias current and input offset, as well as resistor matching.

Ideally, the circuit output is:

VOUT

=

VSENSE

•

ROUT

RIN

VSENSE = RSENSE • ISENSE

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

which provides an error in gain only. However, offset

voltage, bias current and finite gain in the amplifier cause

additional errors.

Output Error, EOUT, Due to the Amplifier DC Offset

Voltage, VOS

EOUT(VOS)

=

VOS

•

ROUT

RIN

The DC offset voltage of the amplifier adds directly to the

value of the sense voltage, VSENSE. This is the dominant

error of the system and it limits the available dynamic

range. The paragraph, Selection of External Current Sense

Resistor provides details.

supply current can cause an output error if trace resistance

between RSENSE and +IN is significant (Figure 4).

EOUT(RT_+IN) = (IS • RT/RIN) • ROUT

Trace resistance to the –IN pin will increase the effective

RIN value, causing a gain error. In addition, internal device

resistance will add approximately 0.3Ω to RIN.

Minimizing the trace resistance is important and care

should be taken in the PCB layout. Make the trace short

and wide. Kelvin connection to the shunt resistor pad

should be used.

V+

ILOAD RSENSE

LOAD

RIN

RT

RT

+IN –IN

IS

VS

1/2

OUT

LTC6103

V–

ROUT

6103 F04

Output Error, EOUT, Due to Bias Currents

The bias current IB(+) flows into the positive input of the

internal op amp. IB(–) flows into the negative input.

EOUT(IBIAS) = ROUT(IB(+) • (RSENSE/RIN) – IB(–))

Since IB(+) ≈ IB(–) = IBIAS, if RSENSE << RIN then:

EOUT(IBIAS) ≈ –ROUT • IBIAS

For instance, if IBIAS is 100nA and ROUT is 1k, then the

output error is 0.1mV.

Output Error, EOUT, Due to PCB Trace Resistance

The LTC6103 uses the +IN pin for both the positive amplifier

input and the positive supply input for the amplifier. The

Figure 4. Error Due to PCB Trace Resistance

Output Error, EOUT, Due to the Finite DC Open-Loop Gain,

AOL, of the LTC6103 Amplifier

This error is inconsequential as the AOL of the LTC6103

is very large.

Design Example:

If ISENSE range = (1A to 1mA) and:

VOUT = 3V

ISENSE 1A

6103f

10

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