The Tech Blog

I was doing some layouts for high current measurements and thought we should address it here. Current Shunt resistors are normally connected in series with a load to measure the current flowing across it. So you tap the voltage across the two terminals across a shunt and feed to an amplifier(if needed) and then to an ADC to measure the voltage across it.

Since these resistors are usually for accurate measurements you really need to keep the following layout considerations in mind.

1. The length of the connections from the shunt to the amp should be symmetrical or be length-matched so that the PCB trace lengths are the same on both arms. It needs to be as short as possible to prevent any trace losses.

2. Connect them preferably with thick traces to reduce impedance issues(But not a big factor as the input impedance of the amplifier is very large anyway).

3. Make sure to tap it at two distinct points such that it’s away from the normal current path. This is known as a 4-wire Kelvin connection. This is a method used to measure very low resistances with high accuracy. It involves the use of two pairs of wires: one pair delivers the supply current to the component being measured, and the other pair is used to measure the voltage drop across the component (We can discuss this in a future post on why it’s needed). This method eliminates the impact of the resistance of the lead wires and contact resistance in the circuit, which can introduce significant errors in low resistance measurement. For this reason, you will find current sense resistors with unique 4 terminals. Check images.

4. If you are having multiple current sense resistors in parallel(for power dissipation reasons), make sure to connect it in a way that current flows through each of the resistors, Tapping at let’s say a wrong pad can cause a current mismatch in the arms.

Hoping these help next you do a physical layout for current sensing. Happy Designing!

When designing circuits for reasonably high-power DC applications you will run across the case of handling thermals. It’s not just about picking the right IC for your DC power conversion; thermal management plays a key role. Here’s a primer on designing with thermals in mind.

For linear power regulators, the math is straightforward. Subtract the output voltage from the input voltage, multiply by the output current, and voilà, you have the power dissipation. This lost power needs to be dissipated in the IC package. So get the datasheet of the IC, find the thermals section, and hunt for Junction-to-ambient thermal resistance. Say it’s 45°C/W; this means for every watt lost, the IC temperature spikes by 45°C above ambient. So if it’s 25°C ambient, the IC hits 70°C.

For a DC-DC converter, it’s a bit more trickier. Here Efficiency = Pout/Pin and Power Loss = Pout – Pin. Now in a DC-DC converter datasheet, you need to find your efficiency value. You will get this mostly from a graph in the datasheet that shows the efficiency of the converter for a particular load and particular output current. Put that value in and you get the power lost.

As a designer, it’s your task to manage this dissipated heat somehow. It can be via a large heatsink(which has a smaller thermal resistance) or can even be via PCB ground planes. There is a TI app note (AN2020 worth reading) that tells you what size of PCBs you want to have to dissipate heat properly without heatsinks. Check images for the rule-of-thumb calculations. Don’t overlook thermal vias and multi-layer PCB stacks as other ways to remove this heat.

In summary, always keep thermals in mind else the moment you power on the circuit the first time, you will see the magic smoke. Speaking from experience.