Today's integrated circuits (ICs) run faster than ever. Running speed acceleration can result in very dynamic power demand from the power supply, which is challenging during testing if power is supplied using programmable sources. High speed current waveforms can cause voltage drops across the IC. If the voltage drop is too steep, it can cause the microprocessor to reset or cause abnormal test results. This article explains why this voltage drop occurs and how to mitigate it.
Optimization of wiring and step capacitor
In many cases, physical limitations force us to place the power supply very far from the IC breadboard, which means using meters of wires. The impedance of the wires can very quickly degrade the impedance of the source reaching the IC. Almost all programmable power supplies incorporate sense wire inputs that allow us to select the voltage regulation point by connecting the sense wires at that location. In this application, the detection point would be as close as possible to the CI. However, the voltage regulation loop can only remove voltage transients at this detection point that are within its own control bandwidth. A local bypass capacitor in the IC can help provide low impedance at frequencies where the combination of the power supply output and the leads is too high impedance.
Let's look at a 25 A application with 5 A surges where the power supply is set to 2,5 V and is connected to the IC breadboard with a 14 meter 1,52 AWG wire.
Since this is a low voltage application, voltage undershoots greater than 100 mV are generally not acceptable. 14 AWG wire has a resistance of 8,2 mΩ per meter, so the round trip path between the power supply output and the IC breadboard has a total resistance of 25 mΩ. As we can see, the resistance of the wires alone is enough to cause an unacceptable voltage drop across the IC, while the inductance of the wires of approximately 557,75 nH/m only makes things worse.
However, the incorporation of a step capacitor, as seen in Figure 1, can provide a considerable improvement.
The interaction between the power supply voltage control loop, the wire network, and the step capacitance can be somewhat complex. However, some useful approximations can help you with the selection of the initial value of the capacitor.
The process will be:
1. Calculate the peak impedance of the network
Determine the desired wire network peak impedance and step capacitance using the following equation:
2. Calculate the value of the step capacitance
Establish the desired peak impedance that satisfies the equation for the characteristic impedance of the inductance-capacitance (LC) tank formed by the inductance of the wires and the step capacitance. In this example, we assume that 4 runs of twisted pair cable are used to reduce the inductance by a factor of 4. Solve the equation to get the value of the capacitance.
3. Calculate the resonance frequency of the tank
If the output impedance of the power supply at the tank resonance frequency is higher than Zpeak, the tank resonance frequency should be lowered by increasing the capacitor until the above condition is met.
4. Select the desired Equivalent Series Resistance (ESR) of the capacitor to ensure proper damping of the LC tank
It is essential to obtain good damping of the resonant tank, as a poorly damped tank will tend to cause transients and can also have a destabilizing effect on the power supply control loop. The combination of the resistance of the wires and the ESR of the capacitor will take care of damping the resonant tank.
We will try to get a damping ratio of 0,5 to get faster response and lower peak voltage by matching the tank resistance to the characteristic impedance of the LC tank.
If necessary, different combinations of capacitors can be used in parallel to achieve the desired ESR.
Result
Figure 2 shows the voltage transient response observed across the load when using the Agilent Technologies N7950A Dynamic DC Power Supply. This device is optimized to operate at low voltage and high current, and offers a very low output impedance, ideal for this application.
Two of the situations described in the example can be seen: both use four 14 meter runs of 1,52 AWB twisted pair cable, but one is aided by the local bypass capacitor located in the device under test (DUT). . A third situation is also shown. It uses four times the local step capacitance to reduce the tank impedance by a factor of about two.
Summary
In this article we have analyzed the difficulty of providing a very dynamic load with a stable voltage using a power supply several meters from the DUT.
Although the impedance of the wires can greatly degrade the transient response performance of a high performance power supply, with the above mitigation methods we can obtain the necessary performance in the DUT. Correctly calculating the DUT step capacitor network allows improving the stability of the voltage level subjected to fast current transients coming from the DUT.
