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LMP8480-Q1, LMP8481-Q1

SNVSAL6 – JULY 2016

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Product Folder Links: LMP8480-Q1 LMP8481-Q1

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8 Detailed Description

8.1 Overview

The LMP8480-Q1 and LMP8481-Q1 are single-supply, high-side current sense amplifiers with available fixed

gains of x20, x60 and x100. The power supply range is 4.5 V to 76 V, and the common-mode input voltage range

is capable of 4.0-V to 76-V operation. The supply voltage and common-mode range are completely independent

of each other, which causes the LMP848x supply voltage to be extremely flexible because the LMP848x supply

voltage can be greater than, equal to, or less than the load source voltage, and allows the device to be powered

from the system supply or the load supply voltage.

The LMP8480-Q1 and LMP8481-Q1 supply voltage does not have to be larger than the load source voltage. A

76-V load source voltage with a 5-V supply voltage is perfectly acceptable.

8.1.1 Theory of Operation

The LMP8480-Q1 and LMP8481-Q1 are comprised of two main stages. The first stage is a differential input

current to voltage converter, followed by a differential voltage amplifier and level-shifting output stage. Also

present is an internal 14-V low-dropout regulator (LDO) to power the amplifiers and output stage, as well as a

reference divider resistor string to allow the setting of the reference level.

Internally, the voltage on each input pin is converted to a current by the internal precision thin-film input resistors

the input common-mode voltage for internal use by the differential amplifier.

differential current. The differential amplifier then servos the resistor currents through the MOSFETs to maintain

a zero balance across the differential amplifier inputs.

to correct this current imbalance, and the extra current required to balance the input currents is then reflected

down into the two lower 400-kΩ tail resistors. The difference in the currents into the tail resistors is therefore

same current, then provide voltage gain by changing the current into a proportionally larger voltage. The gain of

The differential amplifier stage then samples the voltage difference across the two 400-kΩ tail resistors and also

The resulting output of the amplifier will be equal to the differential input voltage times the gain of the device, plus

any voltage value applied to the two VREF pins.

The resistor values in the schematic are ideal values for clarity and understanding. Table 1 shows the actual

values used that account for parallel combinations and loading. This table can be used for calculating the effects

of any additional external resistance.

Table 1. Actual Internal Resistor Values

GAIN OPTION

(Each)

(Each)

(Each)

DIFFERENTIAL AMP FB

(Each)

(Each)

20x

98.38 k

491.9 k

393.52 k

1967.6 k

98.38 k

60x

32.793 k

172.165 k

393.52 k

1967.6 k

98.38 k

100x

19.676 k

98.38 k

393.52 k

1967.6 k

98.38 k