DC Voltage Management in Your PCB: It's All About the PDN

DC voltage management board in an older iPhone

Power integrity is an important consideration in advanced devices as it is deeply related to many signal integrity problems. The one DC power integrity issue that gets all the attention is transient ringing, where a DC fluctuation is seen between the power rail/power and ground return connection when digital components switch strongly. These fluctuations are minimized when you minimize the impedance of your PDN. However, topology and resistance are also important for power distribution in any power system.

With a DC power distribution system in a PCB, your goal is to ensure the voltage output from your power supply is seen as the correct voltage at the downstream components. Similarly, the current output will be limited, and your components will need a certain input current level to operate properly. If you layout your board correctly and arrange circuit blocks properly, DC voltage management suddenly becomes easy.

DC Voltage Management: A Tale of Many Power Integrity Problems

Power distribution network design is all about ensuring the following requirements are met:

• Minimized PDN impedance. Once the power and ground planes in a PCB are constructed and current is distributed to components, the system will have some impedance. Designing for low PDN impedance is all about preventing ringing when components draw current.

• Power/ground paths have low resistance. The path followed by current in the PDN must be a low resistance path. The goal is to ensure that not power is dissipated as heat in the system (i.e., there is no IR drop).

• Current draw is within supply limits. Power supplies and converters are not perfect components, and current needs to be distributed around your board to reach each component. Your total current draw cannot exceed the maximum supply current from your power block. Likewise, your power supply/converter needs the right input current in order to supply the rated output.

While we’ve discussed stability in DC PDNs many times on this blog, we don’t often talk about topology in DC voltage management and power distribution. With the right topology and choice of supply/converter components, you can ensure the right DC voltage/current levels reach your components and your board operates properly.

You can qualify effective DC voltage management with in-circuit test and measurement

Routing Topology for DC Voltage and Current Management

The basic topology for a PDN is one where components or circuit blocks are laid out in parallel. Components receive input DC power from a power rail, which provides the desired supply voltage to components. In boards that use components running at different voltages (5 V, 3.3 V, 1.2 V, or 1 V), multiple power planes may be used, or voltage dividers can be used to ensure the voltage at a component is the correct value. Voltage dividers are not normally desired as they contribute to PDN impedance, they dissipate power as heat, and they convert ripple in the PDN current at a component to a voltage. A better option is to use different regulators, or a single integrated regulator/converter with multiple outputs

Multiple components in a single circuit block may be connected in a complex mix of series and parallel, depending on the exact components involved. Large ICs are normally arranged in parallel connected directly to their respective power/ground connections. The total current on the PDN is then the sum of currents flowing into each circuit block, according to Kirchoff’s current law.

Because each block is arranged in parallel, they will ideally see the same voltage. However, any designer should be aware of voltage drop between the power supply/converter and the power pin on a component. Every conductor has some resistance, and this resistance causes a slight IR drop as current moves to the downstream device. Any device that is physically located farther from the power supply/converter output will see larger voltage drop. When the current density in the power plane/power rail is greater, the voltage drop across some portion of your PDN will be greater. 

Circuit block topology for DC voltage management

Keeping Voltage and Current Within Specs

If you’re using low power ICs, you will also need to be aware of voltage drops. Supply voltage will drop as you go down long traces with high current densities. If you’re already working with ICs that require low supply voltage (i.e., high-power FPGAs), a small voltage drop can cause the IC to operate intermittently, if it operates at all. A PDN simulator can take your layout and calculate the voltage drop across the PDN to ensure your ICs see the voltage they require. In the event the voltage at a downstream IC is too low, you’ll need to adjust the width of power rails, resize your vias, modify your ground/power planes, and/or rearrange components.

Because the total current is split between different circuit blocks, the current output should not exceed the rated output from your power regulator/converter. Rather than using a drastically oversized power converter, your bypass capacitors will provide some extra charge and current for your components. These capacitors will also help prevent ground bounce during switching.

Highly integrated DC-DC converter modules are available on the market that provide a turnkey solution for many systems. This reduces layout time and space required for the power block in your PCB. These components are typically surface mount components,which provide low inductance contribution to the PDN and small footprint, as well as multiple outputs with different voltage/current levels. Integrating these components into a single package further reduces parasitic inductance in the power block, which provides greater EMI protection and lower impedance at the PDN input.

When you’re designing your power distribution system in a PCB, DC voltage management is easy when you use the right PCB layout and design software to create your PCB layout. Allegro PCB Designer and Cadence’s full suite of design tools include the layout and simulation features you need to ensure your supply power remains stable.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.