Whenever you need to place a reference voltage in a PCB layout, it must have strong stability against temperature fluctuations and external noise. The drift of the reference voltage source can generate small voltage errors, which are unacceptable in some precision measurement systems, precision regulators, and high-resolution converters. The reference voltage circuit has a specific quantity that defines how temperature cycling affects the reference voltage, known as thermal hysteresis.
For semiconductor components, thermal hysteresis is inevitable, simply due to the planar structure of the semiconductor device. Although thermal hysteresis cannot be completely avoided, it can be suppressed by performing appropriate PCB installation and electrical testing before deploying the product to the final environment. This is the reason for the thermal lag and how to eliminate it when preparing to deploy a new solution.
What is thermal hysteresis?
Technically speaking, due to changes in certain variables or system parameters (including temperature and the amount that varies with temperature), any quantity that can be physically measured will exhibit hysteresis during the measurement process. Usually, the thermal hysteresis phenomenon is discussed by separating the freezing and melting points of ice crystals in solutions containing antifreeze proteins/glycoproteins. As the solution temperature cycles between the limit values, the solidification temperature and melting temperature will slightly change. Conceptually, thermal hysteresis can be compared to magnetic hysteresis, where the cyclic magnetic field leaves some residual magnetization.
Thermal hysteresis in circuits
In electronic products, thermal hysteresis is used to describe the accuracy of the reference voltage. These are precision circuits and equipment used to provide stable comparisons for voltage measurements in certain other circuits. Some circuits and components that require stable reference voltage include:
Analog to Digital Converter (ADC) and Digital to Analog Converter (DAC): These two circuits use a reference voltage to set the quantization value.
Low dropout (LDO) regulator: The reference voltage is used as the input of the error amplifier to detect when the output voltage of the regulator has dropped too low. Then, the error amplifier modulates the MOSFET to correct the output voltage to the desired value.
Comparator: The reference voltage source provides the basis for the high and low thresholds and switching hysteresis of the comparator. This can be provided by batteries, Zener diodes, or silicon bandgap reference sources.
Formal definition
The form of thermal hysteresis is officially defined as the change in output voltage at ambient temperature (+25 ° C) before and after equipment cycling throughout the entire operating temperature range. The thermal hysteresis in voltage reference circuits is usually measured in ppm/° C. This is the output reference voltage due to Δ The amount of change caused by temperature cycling in T. Actually, when the temperature Δ During the T cycle, this is a permanent change in the output voltage of a reference voltage circuit.
If the device cycles between its low temperature rating and high temperature rating (for example, many components have a temperature range of -40 ° C to 125 ° C), for a typical bandgap reference voltage, the total output variation can reach~1 mV of the circuit. The hysteresis value of high-precision circuits correctly installed on PCBs can be as low as~105 ppm throughout the entire operating temperature range. Please note that even if the temperature of the circuit remains constant