Systems in modern electronics are typically based on Printed Circuit Boards (PCBs). A PCB has multiple active and passive hardware components that are arranged to enable the system to function. Functions that can’t be done in hardware are typically done in software or firmware. Software and firmware need the right type and amount of hardware on the PCB to implement the needed functions.
The costs associated with hardware include board space and size, power consumption, risk, time to market and shipping of a product. Software and firmware have these costs too, but these are indirect because they can be amortized across every unit shipped. This means that companies only have to pay once for software and firmware when they are developing a system. Since companies frequently differentiate their products with software and firmware, they expect to pay these costs rather than incur the expenses associated with a hardware-based solution.
This type of system will typically have multiple active semiconductor devices on the PCB. each of which needs voltage supplied to it via a voltage rail. Many devices need multiple rails, which are called voltage domains, that must each be supplied within certain parameters to enable the device to function properly. The parameters can be as minimal as just a fixed voltage within a certain accuracy and with enough current supply capacity. In complex systems, additional requirements may include:
- Power sequencing—turning on and ramping up the voltage at startup and shutdown at the right time and in the right sequence, compared to other rails, and within the right levels
- Voltage programmability—adjusting the voltage in response to multiple power-saving or performance-enhancing operating modes
- Monitoring and control—monitoring incoming and outgoing voltage and current and thermal limits during operation and sending a signal to trigger the components to respond to any fault conditions
- Multiple operating modes—using aggressive power states to reduce the power consumption of the system
The system in which the PCB operates will not typically supply all the needed voltages to the components. One or possibly two voltages are normally supplied to the PCB, usually at levels that are above what the device needs. Some type of power conversion is done at the Point of Load (PoL) on the board to create the voltages needed by one or more of the active devices. This conversion can take many forms but falls into two general categories: DC-to-DC conversion (DC-DC) and linear voltage regulation, usually in the form of a Low Drop Out (LDO) regulator.
The designer must consider the load’s requirements to decide which conversion option is used. DC-DC conversion is a good option because it is efficient and it minimizes thermal concerns. LDO regulation is a low-noise and low-cost solution that can be implemented with a minimum number of external components.
All the power conditioning and control functionality for a system is called a power tree because it can look like a tree when all the voltages and conditioning stages are traced out. When the PCB is simple, the voltage conditioning stages also tend to be simple. They are usually implemented with discrete DC-DC converters and LDO regulators to reduce costs and provide the most flexibility in selecting a supplier. Many of these devices are commodity products with multiple second sources and available in industry-standard packages and pinouts.
In a complex system, or when the hardware designer needs a more complex power solution, using discrete components can be cost prohibitive. As the number of rails or domains increases, the difficulty of designing a full solution using discrete devices increases significantly. Just adding voltage sequencing and output monitoring to a single rail can require two or more active devices in addition to the DC-DC converter or LDO regulator that is being monitored.
