7.3.1 Theory of Operation

As seen from Figure 26, the sense current flowing through R_SENSE develops a voltage drop equal to V_SENSE. The high impedance inputs of the amplifier does not conduct this current and the high open-loop gain of the sense amplifier forces its noninverting input to the same voltage as the inverting input. In this way the voltage drop across R_IN matches V_SENSE. The current I_IN flowing through R_IN has the following equation:

I_IN flows entirely across the external gain resistor R_G to develop a voltage drop equal to:

This voltage is buffered and showed at the output with a very low impedance allowing a very easy interface of the LMP8646 with the feedback of many voltage regulators. This output voltage has the following equation:

Figure 26. Current Monitor

7.4.1 Output Accuracy

The output accuracy is the device error contributed by the LMP8646 based on its offset and gain errors. The LMP8646 output accuracy has the following equations:

Equation 7.

Output Accuracy Equations

For example, assume V_SENSE = 100 mV, R_G = 10 kOhm, and it is known that V_OFFSET = 1 mV and Gm_Accuracy = 2% (Electrical Characteristics Table), then the output accuracy can be calculated as:

Equation 8.

Output Accuracy Example

In fact, as V_SENSE decreases, the output accuracy worsens as seen in Figure 27. These equations provide a valuable tool to estimate how the LMP8646 affects the overall system performance. Knowing this information allows the system designer to pick the appropriate external resistances (R_SENSE and R_G) to adjust for the tolerable system error. Examples of this tolerable system error can be seen in the next sections.

Figure 27. Output Accuracy vs. V_SENSE

7.4.2 Selection of the Sense Resistor, R_SENSE

The accuracy of the current measurement also depends on the value of the shunt resistor R_SENSE. Its value depends on the application and is a compromise between small-signal accuracy and maximum permissible voltage loss in the load line.

R_SENSE is directly proportional to V_SENSE through the equation R_SENSE = (V_SENSE) / (I_SENSE). If V_SENSE is small, then there is a smaller voltage loss in the load line, but the output accuracy is worse because the LMP8646 offset error will contribute more. Therefore, high values of R_SENSE provide better output accuracy by minimizing the effects of offset, while low values of R_SENSE minimize the voltage loss in the load line. For most applications, best performance is obtained with an R_SENSE value that provides a V_SENSE of 100 mV to 200 mV.

7.4.2.1 R_SENSE Consideration for System Error

The output accuracy described in the previous section talks about the error contributed just by the LMP8646. The system error, however, consists of the errors contributed by the LMP8646 as well as other external resistors such as R_SENSE and R_G. Let's rewrite the output accuracy equation for the system error assuming that R_SENSE is non-ideal and R_G is ideal. This equation can be seen as:

Equation 9.

System Error Example Assuming R_SENSE is Non-ideal and R_G is Ideal

Continuing from the previous output accuracy example, we can calculate for the system error assuming that R_SENSE = 100 mOhm (with 1% tolerance), I_SENSE = 1A, and R_G = 10 kOhm. From the Electrical Characteristics Table, it is also known that V_OFFSET = 1 mV and Gm_Accuracy = 2%.

Equation 10.

System Error Example Assuming RSENSE is Non-ideal and RG is Ideal

Because an R_SENSE tolerance will increase the system error, we recommend selecting an R_SENSE resistor with low tolerance.