Whenever you need to place a reference voltage in a PCB layout, it must have strong stability against temperature fluctuations and external noise. Drift of the reference voltage source can result in 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 new solutions.

What is thermal hysteresis?

Technically speaking, due to changes in certain variables or system parameters (including temperature and temperature dependent quantities), any physically measurable quantity will exhibit hysteresis during the measurement process. Usually, the phenomenon of thermal hysteresis 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 a 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 Voltage Difference (LDO) Voltage Stabilizer: The reference voltage is used as input to the error amplifier to detect when the output voltage of the voltage 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 switch 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 formally 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 units of ppm/° C. This is the change in output reference voltage caused by temperature cycling in Δ T. In fact, when the temperature Δ T cycles, 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), the total output variation for a typical bandgap reference voltage can reach~1 mV circuit. The hysteresis value of a high-precision circuit correctly installed on a PCB 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, long-term drift may occur in these circuits.

What causes thermal hysteresis?

Thermal hysteresis is caused by the mechanical stress accumulated on the semiconductor chip during temperature cycling. The stress distribution and how to release stress from the device depend on whether the chip was previously at a high or low temperature and the past stress history in the device. Due to thermal expansion and contraction, stress will accumulate and solidify at different positions in the tube core.

After the equipment with a reference voltage circuit is taken offline, it is usually tested for a short period of time under standard environmental conditions. The following events may exert pressure on the semiconductor chip and cause changes in the output of the reference voltage circuit in the following ways:

Heating and cooling during packaging: When placing the mold into the packaging, place it in a high-temperature epoxy resin packaging. Then cool the packaging and restore it to ambient temperature. During this process, stress will accumulate on the mold.

Welding during assembly: Wave soldering requires heating the equipment to high temperature and maintaining it for a period of time. After cooling, some stress will accumulate in the mold. Manual welding will not heat the entire equipment to the extent of significant stress accumulation.

Heating during operation: When the device is working on a PCB, the temperature will inevitably change. Heat may flow from other components on the circuit board or the external environment to the reference voltage circuit.

Placing notches around parts that are prone to thermal hysteresis is a way to increase the stiffness of the substrate beneath the parts. In addition, place the device away from the center of the PCB board. Both methods have been experimentally proven to reduce stress accumulation and thermal hysteresis.

The edge of the board provides a hard mounting surface to prevent output voltage changes caused by thermal hysteresis.

Finally, in order to reduce stress in the chip and force the reference voltage circuit to stabilize to its long-term output, the circuit can cycle repeatedly during the operation of the assembled PCB. This may require multiple cycles, but measurements of the reference voltage by some component manufacturers indicate that the hysteresis window will decrease over time after repeated cycles. The PCB processing factory explained the thermal hysteresis of the voltage reference caused by PCB design, and what is thermal hysteresis.