What Causes Thermal Hysteresis in a Voltage Reference?
Voltage reference circuits need to provide highly stable output over long periods of time.
Thermal hysteresis is one effect that occurs during temperature cycling, where the output from a voltage reference circuit will shift after cycling.
Thermal hysteresis can’t be eliminated, but it can be reduced with some simple steps after mounting to the PCB layout.
The reference voltage circuit in these components will undergo thermal hysteresis during temperature cycling.
Anytime you need to place a voltage reference in your PCB layout, it needs to be ultra-stable against thermal fluctuations and external noise. Drift in a voltage reference creates minor voltage errors that can be unacceptable in some precision measurement systems, as well as in precision regulators and high resolution converters. Reference voltage circuits have a specific quantity that defines how they are affected by temperature cycling, known as thermal hysteresis.
For semiconductor components, thermal hysteresis is unavoidable simply due to the planar construction of semiconductor devices. Although thermal hysteresis can’t be prevented totally, it can be suppressed through proper PCB mounting and electrical testing before deploying the product to its end environment. Here’s what causes thermal hysteresis and how you can eliminate it as you prepare to deploy a new solution.
What Is Thermal Hysteresis?
Technically, any physically measurable quantity can exhibit hysteresis during measurement due to changes in some variable or system parameter, including temperature and quantities that change with temperature. Thermal hysteresis is normally discussed in terms of a separation of freezing and melting points for ice crystals in solutions containing antifreeze proteins/glycoproteins, where the freezing and melting temperatures will shift slightly as the temperature of the solution is cycled between extreme values. Conceptually, thermal hysteresis can be compared to magnetic hysteresis, wherein a cycled magnetic field leaves some remnant magnetization.
Thermal Hysteresis in Circuits
In electronics, thermal hysteresis is used to describe the precision of a voltage reference. These are precision circuits and devices that are used to provide a stable comparison for voltage measurements within some other circuit. Some circuits and components that require stable voltage references are:
Analog-digital convertors (ADCs) and digital-to-analog convertors (DACs): These two circuits use a voltage reference to set quantization values.
Low-dropout (LDO) regulators: A voltage reference is used as one input into an error amplifier to detect when the regulator’s output voltage drops too low. The error amplifier then modulates a MOSFET to correct the output voltage to the required value.
Comparators: A voltage reference provides the basis for a comparator’s high and low thresholds and its own switching hysteresis. This can be supplied by a battery, Zener diode, or silicon bandgap reference.
Thermal hysteresis is formally defined as the change in output voltage at ambient temperature (+25 °C) before and after the device is cycled over its entire operating temperature range. The thermal hysteresis in a voltage reference circuit is usually measured in ppm/°C. This is the amount by which the output reference voltage will change due to temperature cycling over an interval ΔT. In effect, this is a permanent change in the output voltage from the reference voltage circuit when the temperature is cycled throughout ΔT.
If the device is cycled between its low and high temperature rating (e.g., -40 °C to 125 °C for many components), the total change in output can reach ~1 mV for a typical bandgap reference voltage circuit. High precision circuits that are properly mounted to the PCB can have hysteresis values as low as ~105 ppm across the entire operating temperature range. Note that long-term drift also occurs in these circuits even if the temperature of the circuit is maintained at a constant value.
Example thermal hysteresis measurement in the voltage reference used in an LDO regulator.
What Causes Thermal Hysteresis?
Thermal hysteresis is created by mechanical stress that accumulates on the semiconductor die during temperature cycling. The stress distribution and how stress is released from the device depends on whether the die was previously at a higher or lower temperature, and the past history of stress in the device. Stress accumulates and sets in at different locations on the die due to thermal expansion and contraction.
Once a device with a reference voltage circuit comes off the fab line, it is usually briefly tested in standard environmental conditions. What happens next can place stress on the semiconductor die and cause the output from a reference voltage circuit to change in these ways:
Heating and cooling during packaging: When the die is placed in packaging, it is encased in an epoxy package at high temperature. The package then cools and returns to ambient temperature. During this process, stress will accumulate on the die.
Soldering during assembly: Wave soldering requires heating the device up to high temperature and holding that temperature for some time. After cooling, some stress will have accumulated in the die. Hand soldering does not heat up the entire device to the point of causing significant stress accumulation.
Heating during operation: While the device is operating on the PCB, it’s unavoidable for its temperature to change. Heat can flow to the reference voltage circuit from other components on the board or from the external environment.
Placing a cutout slot around a component that is prone to thermal hysteresis is one way to increase the stiffness of the substrate beneath the component. In addition, place the device away from the center of the PCB. Both methods have been experimentally shown to reduce stress accumulation and resulting thermal hysteresis.
The board edge provides a stiffer surface for mounting, which will prevent output voltage changes due to thermal hysteresis.
Finally, to alleviate stress in the die and force a reference voltage circuit to settle to its long-term output, the circuit can be cycled repeatedly while an assembled PCB is running. It may take multiple cycles, but measurements of reference voltages by some component makers have shown that the hysteresis window can decrease over time after repeated cycling.
When you’re ready to plan your next PCB for fabrication and assembly, you can specify test requirements for suppressing thermal hysteresis and you can prepare deliverables with the best PCB design and analysis software. Allegro PCB Editor gives you all the features you need to create advanced circuit boards for a range of applications. You’ll also have access to a complete set of advanced design verification tools and field solver utilities to analyze the behavior of advanced electronics.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.