At the start-up of the monitored device, the LMP8645 works at a common-mode voltage of 0 V, which means that the maximum output limit is 1.3 V (Range 1). To maximize the resolution, the R_SENSE value is calculated as maximum allowed V_SENSE (Refer to Figure 7-3) divided by maximum current (1 A), so R_SENSE=0.5 Ω.

Due to the output limitation at low common-mode voltage, the maximum allowed gain will be 2.6 V/V, which corresponds to R_GAIN = 13 kΩ. With this approach the current is monitored correctly at any working condition, but does not use the full output swing range of the LMP8645.

Alternatively if the monitored device doesn’t sink the full 1 A at any supply voltage, it is possible to design with the full maximum output voltage of the LMP8645 when operating in Range 3 ( V_CM ≥ V_S ).

Also in this case it is possible to maximize the resolution using Rsense = 0.5 Ω, and maximize the output dynamic range with R_GAIN=33 kΩ. With this approach the maximum detectable current, when V_CM is less than 1.8 V, is about 400 mA. While for common-mode voltages of less than 2.5 V the maximum detectable current is 600 mA (Refer to Figure 7-3), and for common-mode voltages at or above the LMP8645 supply voltage, the maximum current is 1 A.

The second approach maximizes the output dynamic but implies some knowledge on the monitored current.