Datasheet

LMP8480, LMP8481
www.ti.com SNVS829A –JUNE 2012–REVISED AUGUST 2012
For best results, the value of the resistor is calculated from the maximum expected load current I
LMAX
and the
expected maximum output swing V
OUTMAX
, plus a few percent of headroom. See the MAXIMUM OUTPUT
VOLTAGE section for details about the maximum output voltage limits.
High values of R
SENSE
provide better accuracy at lower currents by minimizing the effects of amplifier offset. Low
values of R
SENSE
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 V
SENSE
voltage that must be generated across the R
SENSE
resistor will be:
V
SENSE
= V
OUTMAX
/ A
V
. (7)
Note: The maximum V
SENSE
voltage should be no more than 667mV.
From this maximum V
SENSE
voltage, the R
SENSE
value can be calculated from:
R
SENSE
= V
SENSE
/ I
LMAX
(8)
Care must be taken to not exceed the maximum power dissipation of the resistor. The maximum sense resistor
power dissipation will be:
P
R
SENSE
= V
SENSE
* I
LMAX
(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 R
SP
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 R
SP
and R
SN
) 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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