To ensure consistent and accurate signals from the analog-to-digital converter (ADC) or digital-to-analog converter (DAC) of the field sensor during startup, a precise reference voltage is crucial. The system also requires an adequate startup cycle in case it encounters a large current draw. This current, known as inrush current, is the maximum current drawn from the power supply during startup. Since capacitive loads in the system typically occur around the power rails, inrush current can be significantly larger than the typical current designed for the system. This can be problematic because inrush current during startup can cause low-dropout regulators (LDOs) and series voltage references to exceed their regulation range, negatively impacting system accuracy and voltage regulation.
Inrush current is not only a problem for factory automation systems such as PLCs and field transmitters, but also for industrial systems requiring isolation and other 4-20mA current loop applications. This is because these systems are typically turned on and off periodically for measurement and automation. Consider a system without inrush current protection during startup; the impedance of the inrush current is extremely low. This is because power supplies typically have low series impedance, and the RDSON of any LDO is low, resulting in insufficient impedance to limit the current flowing to the capacitors during startup. This current can damage circuit board traces and devices not built to handle it.
Figure 1 shows a typical connection example from a voltage source to a voltage reference. The inrush current from VCC is caused by capacitors CS and CLoad, which can both be several microfarads for stability. The typical total value of the series impedance limiting the current is a few ohms or less; as a result, the inrush current can reach hundreds of milliamps, causing the voltage reference to go out of regulation or damaging equipment or the power supply.
Figure 1: Precision voltage reference lacking surge current protection
A common solution is to use Rs to limit the peak current, as shown in Figure 1. In some applications, this resistor can be up to several hundred ohms to limit the peak current. The current-limiting resistor sets an upper limit but does not control the slope of the voltage and current.
One potential problem is that if Cs after Rs is small, the capacitor may not provide enough current to the reference during startup. Insufficient current will cause a very slow ramp-up during startup, which can affect the device's internal power-on initialization. A second problem is that the presence of Rs, due to the continuous current across the resistor, results in wasted power and a voltage drop from the original supply voltage. In energy-efficient applications where voltage reference power dissipation is a critical parameter, the power dissipation of Rs can be significant. You can see the large drop caused by the large series resistance Rs in Figure 2.
Figure 2: Precision voltage reference with Rs for limiting inrush current.
Precise and clean start
While external discrete solutions exist to protect voltage references from inrush current, they are often bulky or inefficient. The REF2125 voltage reference introduces a new feature called "Clean Start," similar to soft start on other devices, to prevent inrush current. Both Clean Start and Soft Start prevent the device from entering undesirable states that could affect the regulated voltage by regulating the current flowing into the device.
The difference between a typical device soft-start and a REF2125 clean start lies in the mechanism behind the soft-start. Soft-start works by connecting an external capacitor C<sub>SS</sub> to the soft-start pin. The device's output charging rate is then proportional to the current ramp rate on C<sub>SS</sub>. While soft-start does provide inrush current protection, this feature is not commonly found in voltage references.
Clean startup provides the REF2125 with programmable inrush current protection and additional programmability lacking in soft startup. Clean startup limits the output current to a voltage proportional to the reference voltage at the CS pin. This allows for flexibility in ramp-up rate adjustment using resistor RCS and capacitor CCS, making it as gradual or rapid as necessary. When only RCS is present at the REF2125's CS pin, the device's input current is limited to Iin,peak. In this state, device startup appears similar to soft startup because the current increases linearly with the resistance. Equation 1 calculates Iin,peak based on RCS.
I in,peak ≈ 466uA + 13.54µA*Rcs (1)
One benefit of clean start is the added programmability, which comes in the form of adding a capacitor C<sub>CS</sub>. In addition to the resistor on the CS pin, this capacitor allows the output current ramp to increase according to the RC time constants of C<sub>CS</sub> and R<sub>CS</sub>, as shown in Figure 3.
Figure 3: REF2125 clean boot example with R CS and C CS
If programmability of the CS pin is not required but inrush current protection is needed, the REF3425 can handle this problem. In the REF3425, the ramp-up current during startup is linearly related to the fixed turn-on time to ensure that the device always turns on as expected.
Factory automation systems using PLCs and field transmitters require inrush current protection. These are essential for automation systems, especially since every measurement and analog input must be accurate and reliable. While discrete solutions exist to help limit inrush current, built-in solutions are irreplaceable.
