LMP8601MASLASHNOPB datasheet(16/29 Pages) TI1

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+IN

-IN

Preamplifier

Output Buffer

Gain = K2

Gain = 10

OFFSET

Internal

Resistor

100 k:

OUT

Level

shift

Ri = 100 k: X

Gi - 20

Gi =

Ri - 100 k:

20 Ri

Rr = 100 k: X

20 - Gr

Gr =

Rr + 100 k:

20 Rr

LMP8601, LMP8601-Q1

SNOSAR2E – SEPTEMBER 2008 – REVISED MARCH 2013

www.ti.com

(8)

Given a desired value of the reduced gain Gr, using this equation the required value for Rr can be calculated

with:

(9)

Increase Gain

Figure 36 shows the configuration that can be used to increase the gain of the LMP8601/LMP8601Q.

Ri creates positive feedback from the output pin to the input of the buffer amplifier. The positive feedback

increases the gain. The increased gain Gi becomes:

(10)

From this equation, for a desired value of the gain, the required value of Ri can be calculated with:

(11)

It should be noted from the equation for the gain Gi that for large gains Ri approaches 100 kΩ. In this case, the

denominator in the equation becomes close to zero. In practice, for large gains the denominator will be

determined by tolerances in the value of the external resistor Ri and the internal 100 kΩ resistor. In this case, the

gain becomes very inaccurate. If the denominator becomes equal to zero, the system will even become instable.

It is recommended to limit the application of this technique to gain values of 50 or smaller.

K2 = 2

Figure 36. Increase Gain

BIDIRECTIONAL CURRENT SENSING

The signal on the A1 and OUT pins is ground-referenced when the OFFSET pin is connected to ground. This

means that the output signal can only represent positive values of the current through the shunt resistor, so only

currents flowing in one direction can be measured. When the offset pin is tied to the positive supply rail, the

signal on the A1 and OUT pins is referenced to a mid-rail voltage which allows bidirectional current sensing.

When the offset pin is connected to a voltage source, the output signal will be level shifted to that voltage divided

by two. In principle, the output signal can be shifted to any voltage between 0 and VS/2 by applying twice that

voltage to the OFFSET pin.

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