LMP8480MMX-SSLASHNOPB datasheet(15/24 Pages) TI

15 / 24 page

LMP8480, LMP8481

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SNVS829A – JUNE 2012 – REVISED AUGUST 2012

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.

(7)

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

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

RSENSE = VSENSE / ILMAX

(8)

Care must be taken to not exceed the maximum power dissipation of the resistor. The maximum sense resistor

power dissipation will be:

SENSE = V

SENSE * ILMAX

(9)

It is recommended that a 2-3x minimum safety margin be used in selecting the power rating of the resistor.

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 tempco of copper is typically 3300-4000ppm/°K, which can vary over PCB process variations and require

measurement correction (possibly requiring ambient temperature measurements).

A typical surface mount sense resistor tempco is in the 50ppm to 500ppm/°C range offering more measurement

consistency and accuracy over the copper trace. Special low tempco resistors are available in the 0.1 to 50ppm

range, but at a higher cost.

INPUT COMMON MODE AND DIFFERENTIAL VOLTAGE RANGE

The input common mode range, where “common mode range” is defined as the voltage from ground to the

voltage on RSP input, should be in the range of +4.0V to +76V. Operation below 4.0V on either input pin will

introduce severe gain error and nonlinearities.

The maximum differential voltage (defined as the voltage difference between RSP and RSN) should be 667mV or

less. The theoretical maximum input is 700mV (14V / 20).

Taking the inputs below 4V will not damage the device, but the output conditions during this time are not

predictable and are not guaranteed.

If the load voltage (Vcm) is expected to fall below 4V as part of normal operation, preparations must be made for

invalid output levels during this time.

LOW SIDE CURRENT SENSING

The LMP8480 and LMP8481 are not recommended for low-side current sensing at ground level. The voltage on

either input pin must be a minimum of 4.0V above the ground pin for proper operation.

INPUT SERIES RESISTANCE

Because the input stage uses precision resistors to convert the voltage on the input pin to a current, any

resistance added in series with the input pins will change the gain. If a resistance is added in series with an

input, the gain of that input will not track that of the other input, causing a constant gain error.

It is not recommended to use external resistances to alter the gain, as external resistors will not have the same

thermal matching as the internal thin film resistors.

If resistors are purposely added for filtering, resistance should be added equally to both inputs and the user

should be aware that the gain will change slightly. See end of the THEORY OF OPERATION section for the

internal resistor values.

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