OPA454 High-Voltage (100-V), High-Current (50-mA) Operational Amplifiers, G = 1 Stable (Rev. B)
SBOS391B – December2007 – revisedMarch 2016
12.1 Layout Guidelines
12.1.1 Thermally-Enhanced PowerPAD Package
The OPA454 comes in an 8-pin SO with PowerPAD version that provides an extremely low thermal resistance (θJC) path between the die and the exterior of the package. This package features an exposed thermal pad. This thermal pad has direct thermal contact with the die; thus, excellent thermal performance is achieved by providing a good thermal path away from the thermal pad.
The OPA454 SO-8 PowerPAD is a standard-size SO-8 package constructed using a downset leadframe upon which the die is mounted, as Figure 84 shows. This arrangement results in the lead frame being exposed as a thermal pad on the underside of the package. The thermal pad on the bottom of the IC can then be soldered directly to the PCB, using the PCB as a heatsink. In addition, plated-through holes (vias) provide a low thermal resistance heat flow path to the back side of the PCB. This architecture enhances the OPA454 power dissipation capability significantly, eliminates the use of bulky heatsinks and slugs traditionally used in thermal packages, and allows the OPA454 to be easily mounted using standard PCB assembly techniques.
Because the SO-8 PowerPAD is pin-compatible with standard SO-8 packages, the OPA454 is a drop-in replacement for operational amplifiers in existing sockets. Soldering the PowerPAD to the PCB is always required, even with applications that have low power dissipation. Soldering the device to the PCB provides the necessary thermal and mechanical connection between the leadframe die pad and the PCB.
12.1.2 PowerPAD Layout Guidelines
The PowerPAD package allows for both assembly and thermal management in one manufacturing operation. During the surface-mount solder operation (when the leads are being soldered), the thermal pad must be soldered to a copper area underneath the package. Through the use of thermal paths within this copper area, heat can be conducted away from the package into either a ground plane or other heat-dissipating device. Soldering the PowerPAD to the PCB is always required, even with applications that have low power dissipation. Follow these steps to attach the device to the PCB:
- The PowerPAD must be connected to the most negative supply voltage on the device, V–.
- Prepare the PCB with a top-side etch pattern. There must be etching for the leads as well as etch for the thermal pad.
- Use of thermal vias improves heat dissipation, but are not required. The thermal pad can connect to the PCB using an area equal to the pad size with no vias, but externally connected to V–.
- Place recommended holes in the area of the thermal pad. Recommended thermal land size and thermal via patterns for the SO-8 DDA package are shown in the thermal land pattern mechanical drawing appended at the end of this document. These holes must be 13 mils (.013 in, or 0.3302 mm) in diameter. Keep them small, so that solder wicking through the holes is not a problem during reflow. The minimum recommended number of holes for the SO-8 PowerPAD package is five.
- Additional vias may be placed anywhere along the thermal plane outside of the thermal pad area. These vias help dissipate the heat generated by the OPA454 IC. These additional vias may be larger than the 13-mil diameter vias directly under the thermal pad. They can be larger because they are not in the thermal pad area to be soldered; thus, wicking is not a problem.
- Connect all holes to the internal power plane of the correct voltage potential (V–).
- When connecting these holes to the plane, do not use the typical web or spoke via connection methodology. Web connections have a high thermal resistance connection that is useful for slowing the heat transfer during soldering operations, making the soldering of vias that have plane connections easier. In this application, however, low thermal resistance is desired for the most efficient heat transfer. Therefore, the holes under the OPA454 PowerPAD package must make the connections to the internal plane with a complete connection around the entire circumference of the plated-through hole.
- The top-side solder mask must leave the terminals of the package and the thermal pad area exposed. The bottom-side solder mask must cover the holes of the thermal pad area. This masking prevents solder from being pulled away from the thermal pad area during the reflow process.
- Apply solder paste to the exposed thermal pad area and all of the IC terminals.
- With these preparatory steps in place, the PowerPAD IC is simply placed in position and run through the solder reflow operation as any standard surface-mount component. This preparation results in a properly installed part.
For detailed information on the PowerPAD package, including thermal modeling considerations and repair procedures, see technical brief SLMA002 PowerPAD Thermally-Enhanced Package, available for download at www.ti.com.
12.2 Layout Example
12.3 Thermal Protection
Figure 86 shows the thermal shutdown behavior of a socketed OPA454 that internally dissipates 1 W. Unsoldered and in a socket, θJA of the DDA package is typically 128°C/W. With the socket at 25°C, the output stage temperature rises to the shutdown temperature of 150°C, which triggers automatic thermal shutdown of the device. The device remains in thermal shutdown (output is in a high-impedance state) until it cools to 130°C where it again is powered. This thermal protection hysteresis feature typically prevents the amplifier from leaving the safe operating area, even with a direct short from the output to ground or either supply. The rail-to-rail supply voltage at which catastrophic breakdown occurs is typically 135 V at 25°C. However, the absolute maximum specification is 120 V, and the OPA454 must not be allowed to exceed 120 V under any condition. Failure as a result of breakdown, caused by spiking currents into inductive loads (particularly with elevated supply voltage), is not prevented by the thermal protection architecture.
12.4 Power Dissipation
Power dissipation depends on power supply, signal, and load conditions. For DC signals, power dissipation is equal to the product of the output current times the voltage across the conducting output transistor, PD = IL (VS – VO). Power dissipation can be minimized by using the lowest possible power-supply voltage necessary to assure the required output voltage swing.
For resistive loads, the maximum power dissipation occurs at a DC output voltage of one-half the power-supply voltage. Dissipation with AC signals is lower because the root-mean square (RMS) value determines heating. Application bulletin SBOA022 explains how to calculate or measure dissipation with unusual loads or signals. For constant current source circuits, maximum power dissipation occurs at the minimum output voltage, as Figure 87 shows.
The OPA454 can supply output currents of 25 mA and larger. Supplying this amount of current presents no problem for some op amps operating from ±15-V supplies. However, with high supply voltages, internal power dissipation of the op amp can be quite high. Operation from a single power supply (or unbalanced power supplies) can produce even greater power dissipation because a large voltage is impressed across the conducting output transistor. Applications with high power dissipation may require a heatsink or a heat spreader.
NOINDENT:NOTE: R1 = R3 and R2 = R4 + R5.
Power dissipated in the OPA454 causes the junction temperature to rise. For reliable operation, junction temperature must be limited to 125°C, maximum. Maintaining a lower junction temperature always results in higher reliability. Some applications require a heatsink to assure that the maximum operating junction temperature is not exceeded. Junction temperature can be determined according to Equation 3:
Package thermal resistance, θJA, is affected by mounting techniques and environments. Poor air circulation and use of sockets can significantly increase thermal resistance to the ambient environment. Many op amps placed closely together also increase the surrounding temperature. Best thermal performance is achieved by soldering the op amp onto a circuit board with wide printed circuit traces to allow greater conduction through the op amp leads. Increasing circuit board copper area to approximately 0.5 in2 decreases thermal resistance; however, minimal improvement occurs beyond 0.5 in2, as shown in Figure 88.
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