Electronic Design - Steps of step-down converter PCB design configuration

If switched mode power supplies (SMPSs) want to balance performance and stability,PCB configuration is particularly important, but it is often overlooked. If the configuration is wrong, it will cause various problems, such as poor output voltage regulation and switching Abnormal, or even device failure. Since the PCB design is often adjusted during the repair process, such problems should be avoided as much as possible. However, if you can spend time thinking about the configuration process before ordering the PCB, these shortcomings can be easily overcome. This article introduces five simple steps to help you quickly prepare a production prototype when you design the PCB design configuration of the buck converter next time.

When designing servers, tablets, and electronic terminals, if you want to minimize the risk, it is best to directly copy the PCB design configuration examples in the evaluation module or product manual, but many reasons will create resistance. This article details the PCB The five steps of design configuration are applicable to any buck converter with built-in switch of TPS62xxx. The internal MOSFETs and built-in loop compensation circuit can reduce the difficulty and time required for PCB design and configuration, thereby greatly simplifying the PCB design and configuration in the device.

Step 1: Set and connect the input capacitor

For a buck converter to operate stably, the input capacitor is the most important single component. Therefore, the arrangement sequence should be second only to the chip, and the capacitor and chip should be connected immediately to avoid obstacles in the path, because V= L*dI/dt The switching of the power supply and the grounded input capacitor terminal generates additional parasitic inductance, and the excessive voltage surge between the PVIN and PGND terminals of the chip may cause the chip to malfunction.

Step 2: Set up and connect the inductor and SW node buffer

The configuration and connection of the inductor and the SW node buffer (if necessary) are also very important. Sometimes a buffer circuit is needed in the circuit board. By slowing the rise and fall time of the SW node, the electromagnetic interference of the SMPS is reduced, but it also increases the switching loss and reduces efficacy. The rise and fall time of SW node voltage from input voltage to ground is very short, and it is also the main source of electromagnetic interference of SMPS. Modern SMPS usually contains technology to reduce electromagnetic interference. For example, at this time, resistor/capacitor (RC) buffer is set in the PCB design configuration. Establish the shortest connection between SW and PGND pins, and minimize parasitic inductance

Step 3: Set and connect the output capacitor and VOS pin

The output capacitor is the last item in the connection of power components (internal MOSFET, input capacitor, output capacitor, inductor, and optional buffer). This is the last component of the system connected to the power ground terminal to shorten the distance between the inductor and the power ground, and output If the capacitor is not properly configured, it will often cause poor output voltage regulation.

The VOS input pin is the most important small signal connection. If it is not handled properly or there is too much noise, it may lead to poor output voltage regulation, switching jumps, and even chip failure. After the wiring arrangement, the VOS pin is of great importance. For other signal routing, the VOS pin should be short and connected directly to the output capacitor. Because the TPS62130A has pins, two vias and dedicated traces can be used to connect the VOS pin and the output capacitor, so it is better than others in the circuit. Power components.

Step 4: Set up and connect small signal components

Analog and digital components can be called small signal components as long as they are not directly related to power conversion, such as FB pin voltage divider, soft start capacitor, and all small value decoupling capacitors (such as 0.1 µF), compared to power supply Components and their nodes will generate noise. Analog small-signal components are very sensitive to noise. Let each component set point close to the chip and use a direct and short path to reduce noise sensitivity.

The size of the FB node should be reduced as much as possible to reduce noise pickup and provide good output voltage regulation; use common analog or quiet grounding, and place all components on the same side of the PCB to reduce the difficulty of connection. If the small signal components are not properly set up, common problems include poor output voltage regulation, unstable soft start, and device operation problems.

Step 5: Make a single point ground and connect it to other parts of the system

The grounding design must refer to the recommendations in the product manual. This means that the noisy power components are provided with a grounding, and the quieter small signal components are also provided with a grounding. If the above suggestions and steps are followed, the relevant settings have been completed. Next, the two grounds will intersect at the same point, usually located on the heat sink under the chip, and the heat sink should also be grounded.

After the grounding design is completed, this circuit must be connected to other parts of the system, such as through through holes, because the input voltage, output voltage and ground are generally connected to the plane of the internal PCB layer, and then to each circuit, the through hole is from the ground At the beginning, it is best to set it directly under the chip, then the heat sink can transfer heat to the PCB layer to achieve the best thermal performance of the chip.

Through holes are generally located at the ground terminals of input and output capacitors. It is generally not recommended to set through holes in the system ground plane of quietly grounded components, because it may cause ground plane noise in the network. It is best to connect these grounds directly back to AGND The pin serves as a single connection point for the thermal pad.

Special considerations

Please remember to read the device manual to understand the specific recommended configurations and examples. For most device configurations, the descriptions and examples are sufficient. In wafer chip level packaging (WCSP) such as TPS62360, confusing configurations may occasionally appear In many WCSP buck converters, the chip pin puts the SW pin between the VIN and PGND pins. If you follow the first step, unless the SW pin is detoured from below the input capacitor, the input capacitor will block the SW connection. Feet, some designers reject this approach,

Because the trace must be quite thin to connect between the terminals of small components such as input capacitors.

If this trace cannot be established, a through hole can be used to connect the SW pin and the inductor. Although this through hole and a longer connection will cause additional electromagnetic interference, because the parasitic inductance is connected in series with other inductances, these through holes increase Inductance has little effect. Compared to the ideal position of moving the input capacitor, it is recommended to use a through hole in this path.

in conclusion

When designing the PCB design and configuration of SMPS, please refer to the examples and suggestions in the device manual and evaluation module. If it is impossible to copy completely or there is no basis for reference, you can use five simple steps to make a high-quality buck converter:

1. Set and connect the input capacitor;

2. Set up and connect the inductor and the SW node buffer;

3. Set and connect the output capacitor and VOS pin;

4. Set up and connect small signal components;

5. Make a single point ground and connect it to other parts of the system.

If you follow the above steps, it will provide a sound design and superior performance for systems that use step-down converters such as servers, tablets, and electronic terminals.