1 Introduction

Placing current shunt resistors in parallel is a common design strategy, particularly in high-current applications. It distributes current among multiple resistors, allowing for increased power and better heat dissipation. Moreover, it enables the use of small shunts which are relatively cheaper and more easily available.

Shunts in parallel also have a lower effective resistance, minimizing the voltage drop across the shunt, which reduces power loss ( $P = I^{2}R$ ) and increases system efficiency. In addition, a single shunt resistor designed for high currents can have a larger physical size, and therefore, a higher parasitic inductance. Using multiple smaller resistors in parallel can help to reduce the overall inductance of the shunt . This is particularly important in fast-switching circuits, like those in motor drives or power supplies, where inductance can cause voltage spikes and measurement errors. ​

On the other hand, parallel shunt designs may introduce uneven current sharing since it is difficult to ensure that the current is split evenly among parallel resistors. Variations in resistance due to manufacturing tolerances and differences in the resistance of the PCB traces leading to each resistor causes an unequal current distribution. This can lead to one resistor carrying more current than intended, causing it to overheat, which can alter the resistance and further worsen the imbalance. Figure 1-1 details the various sources of resistances that may become significant as shunt resistances in the µΩ range are used.

Aside from the shunt resistors, auxiliary resistances such as the solder between the shunt and resistor pad, solder between shunt and trace, and copper around the shunts contribute to the effective resistance, approximately 10 to 100µΩ, 1 to 10µΩ, and 500µΩ/square respectively, as shown in Figure 1-1.

Figure 1-1 Sources of Resistance in Parallel Shunt Layout

An accurate parallel shunt design requires a very careful and symmetrical PCB layout. The layout must make sure that copper, solder, and trace resistances are compensated for, so that the shunt resistors predominantly contribute to the overall shunt voltage drop. To mitigate the issue of uneven current sharing, the traces connecting to each resistor should be as identical as possible in length and width. Moreover, it must make sure that the current prefers the path through the shunt, as opposed to through the sense traces.

One method to minimize the effect of trace resistance is to use kelvin connections. However, when multiple shunts in parallel are involved, it is important to ensure the voltage is measured from a central, symmetrical point or that all shunts have independent kelvin connections, otherwise, the measurement accuracy will be compromised.

This paper examines the effectiveness of various parallel resistor layouts on total shunt voltage drop in shunt-based current sensing. The analysis is supported by TINA-TI simulations and experimental PCB measurements. Recommendations are provided for achieving layout practices when designing with the INA190, one of TI's ultra-precise current sense amplifiers with a maximum of 3nA of input bias current. This focus helps ensure proper current sharing among the shunts. This layout can be applied to other devices in our portfolio by taking into account any additional input bias current.