SBOA569 may 2023 OPA2828
1 Introduction
The 36 V single, ±18 V dual-supply capability, 30 mA maximum output drive current OPA2828 op amp is finding design-in opportunities in different data acquisition, testing, and other precision applications. Such applications often require high precision over ranges of temperature and power demands, to lessen the effects of thermal drift on the system's accuracy.
The DAC8811EVM is an evaluation module to showcase the performance of both the DAC8811 and the OPA2828. The board features a configurable circuit for the OPA2828, allowing for the circuit design, seen in Figure 1-1, to be easily created and tested.
The report demonstrates how the addition of a thermal pad to a package greatly improves the thermal metrics of the OPA2828 and other op amps. The most commonly reported thermal metric is the junction-to-ambient thermal resistance (Θja). The Θja is the difference in temperature between the internal device junction temperature and the ambient temperature, divided by the dissipated power of the device. This thermal metric is important as it provides a way to compare the thermal performance of devices between companies. The thermal metric can easily be used to determine what the maximum junction temperature will be, if the ambient temperature, Θja, and power dissipation is known.
The Θja is impacted by several other thermal resistances, including the board-to-ambient thermal resistance. The addition of a thermal pad greatly decreases the case-to-board thermal resistance, as more heat can be dissipated to the PCB and to the environment, which lowers the maximum junction temperature. If the internal temperature of the device is lower, many electrical characteristics, including accuracy parameters, can also be affected. This includes offset voltage (Vos) and input bias current (Ib), which typically become larger as temperature increases. The addition of a thermal pad can reduce these error sources and improve the accuracy of the device. For more information about thermal metrics, please refer to the Semiconductor and IC Package Thermal Metrics application note.
As the OPA2828IDGN is a closed package, the internal junction temperature cannot be measured by hand. The OPA2828 was designed to have a quiescent current (Iq) that directly relates to the junction temperature. The OPAx828 Low-Offset, Low-Drift, Low-Noise, 45-MHz, 36-V, JFET-Input Operational Amplifiers data sheet has a plot of quiescent current over temperature, but it is only for an unloaded setting. When the circuit in Figure 1-1 was simulated, it was verified that the non-driving supply current similarly has a direct relationship with junction temperature when the output is loaded with 1 V over 24.9 Ω. This current will be referred to as supply current for the remainder of the document.
By measuring the supply current over an ambient temperature range and calculating the respective junction temperature, the Θja of the HVSSOP was measured and compared to the verified value. To emulate a typical SOIC package without thermal enhancement, the same HVSSOP op amp was elevated slightly above the PCB and its thermal pad connection was severed. This process is visualized in Figure 1-2. The same characterization of the Θja was repeated for the emulated SOIC scenario. The observed difference was used to compare accuracy between the two package types.
Figure 1-2 Detachment of
Thermal Pad from PCBThe emulated SOIC package will have similar thermal characteristics to typical packages which are not thermally enhanced. The emulated package will be referred to as typical package for the remainder of this document.