LMP8481 datasheet(13/29 Pages) TI

13 / 29 page

VCC = +4.5V to +76V

CBYPASS

0.12F

To Load

+4.0V

+76V

RSENSE

ISENSE

RSN

RSP

REFA

REFB

GND

LMP8481

VSENSE

VOUT

VCC

OUTPUT

LMP8480, LMP8481

www.ti.com

SNVS829C – MARCH 1999 – REVISED SEPTEMBER 2015

Feature Description (continued)

Figure 21 shows the basic connections for the LMP8481 for Bidirectional applications centering the output at

one-half the applied VREF or VCC voltage. If VREFA is connected to VCC, then the output "zero" point will be VCC/2.

If VREFA is connected to the ADC VREF line, then the "zero" output will be at mid-scale for the ADC.

Figure 22. LMP8481 Connections for Unidirectional Configuration (Equivalent to LMP8480 Unidirectional)

Figure 22 shows the how to connect the LMP8481 for Bidirectional applications, thus making it equivalent to the

LMP8480 in Figure 19.

8.3.2 Selection of the Sense Resistor

The accuracy of the current measurement depends heavily on the accuracy of the shunt resistor RSENSE. Its

value depends on the application and is a compromise between small-signal accuracy, maximum permissible

voltage drop and allowable power dissipation in the current measurement circuit.

The use of a “4-terminal” or “Kelvin” sense resistor is highly recommended. See the Layout Guidelines.

For best results, the value of the resistor is calculated from the maximum expected load current ILMAX and the

expected maximum output swing VOUTMAX, plus a few percent of headroom. See the Maximum Output Voltage

section for details about the maximum output voltage limits.

High values of RSENSE provide better accuracy at lower currents by minimizing the effects of amplifier offset. Low

values of RSENSE minimize load voltage loss, but at the expense of accuracy at low currents. A compromise

between low current accuracy and load circuit losses must generally be made.

The maximum VSENSE voltage that must be generated across the RSENSE resistor will be:

VSENSE = VOUTMAX / AV

(1)

NOTE

The maximum VSENSE voltage should be no more than 667 mV.

From this maximum VSENSE voltage, the RSENSE value can be calculated from:

RSENSE = VSENSE / ILMAX

(2)

Take care not exceed the maximum power dissipation of the resistor. The maximum sense resistor power

dissipation will be:

PRSENSE = VSENSE × ILMAX

(3)

TI recommends using a 2-3x minimum safety margin in selecting the power rating of the resistor.

8.3.3 Using PCB Traces as Sense Resistors

While it may be tempting to use a known length of PCB trace resistance as a sense resistor, it is not

recommended.

The temperature coefficient of copper is typically 3300-4000 ppm/°K, which can vary over PCB process

variations and require measurement correction (possibly requiring ambient temperature measurements).

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