SBOS825D September 2019 – July 2021 TMCS1101
The primary design parameter for using the TMCS1101 is selecting the correct sensitivity variant, and because positive and negative current must be measured a bidirectional variant should be selected (A1B-A4B). Further consideration of noise and integration with an ADC can be explored, but is beyond the scope of this application design example. The TMCS1101AxB transfer function is effectively a transimpedance with a variable offset set by VOUT,0A, which is internally set to half of the analog supply as defined by Equation 29.
Design of the sensing solution focuses on maximizing the sensitivity of the device while maintaining linear measurement over the expected current input range. The TMCS1101 has a slightly smaller linear output range to the supply than to ground; therefore, the measurable current range is always constrained by the positive swing to supply, SwingVS. To account for the operating margin, consider the minimum possible supply voltage VS,min. With the previous parameters, the maximum linear output voltage range is the range between VOUT,max and VOUT,0A, as defined by Equation 30.
Design parameters for this example application are shown in Table 10-4 along with the calculated output range.
|DESIGN PARAMETER||EXAMPLE VALUE|
|VOUT,0A at VS,min||2.45 V|
|VOUT,max – VOUT,0A||2.25 V|
These design parameters result in a maximum positive linear output voltage swing of 2.25 V. To determine which sensitivity variant of the TMCS1101 most fully uses this linear range, calculate the maximum current range by Equation 31 for a bidirectional current (IB,MAX).
Table 10-5 shows such calculation for each gain variant of the TMCS1101 with the appropriate sensitivities.
|TMCS1101A1B||50 mV/A||±45 A|
|TMCS1101A2B||100 mV/A||±22.5 A|
|TMCS1101A3B||200 mV/A||±11.25 A|
|TMCS1101A4B||400 mV/A||±5.6 A|
In general, the highest sensitivity variant that provides for the desired full-scale current range is selected. For the design parameters in this example, the TMCS1101A2B with a sensitivity of 0.1 V/A is the proper selection because the maximum calculated ±22.5-A linear measurable range is sufficient for the desired ±20-A full-scale current.