All high-performance applications require a specific, regulated, and stable power supply voltage. This cannot be guaranteed, however, by conventional battery technology and requires an intermediate voltage regulator circuit to generate the required supply. Voltage regulators not only provide supply regulation, they also maintain noise isolation between different blocks sharing the same supply. Regulating a power supply to meet the minimum requirement of a specific application extends the battery life, which is crucial in all portable applications.
Power management technology deals with the optimum design of voltage regulators, voltage monitors, voltage references, and current references. It aims to generate all of the proper voltage and current references (including the voltage supply) for a certain application efficiently and with minimum power loss.
Voltage regulators can be one of three major types: linear regulators (e.g. low-dropout regulators), switching converters, and charge pumps.
Linear regulators are typically used in compact and noise-sensitive applications; for example, RF circuits and high-precision analog circuits. However, typical linear regulators have relatively low efficiency especially when operated at a large dropout voltage, which directly affects the battery life. Switching converters are typically used in applications that require high efficiency, like processors and memory chips for portable devices. Moreover, all applications that require complete DC input-output isolation have no choice but to use switching converters. Finally, charge pumps can be thought of as a sub-category of switching converters, but due to their low current capability, are separated as they serve different types of low power applications (e.g. LEDs).
The semiconductor market is full of different types of switching converters. The main question for many system designers is “How to choose the best switching converter for my application?” To answer this question, an understanding of the features and parameters of a DC-DC converter is crucial. These metrics can be divided into basic metrics that most designers use as a starting point. These basic metrics can be used to narrow the search space but not to reach the optimum performance. Then comes the performance metrics. These are really what differentiate one converter from another. Only a good understanding of the required application leads to the proper set of performance metrics. Finally, come the optional features. They add some extra features to the converter that increase its robustness and flexibility through different operating scenarios. Last but not least are the practical aspects, mostly overlooked by new system designers. Practical aspects require full knowledge of the application environment as well as what other blocks exist on the same board or chip. These can drastically increase the overall system cost or degrade the performance of neighboring chips.
Basic DC Parameters
The DC-DC converter must operate within the range of required DC parameters to provide the required functionality for a particular application. Typical basic DC parameters are the input voltage range, the output voltage range, and the maximum required output current.
Required Features and AC Performance Parameters
The DC-DC converter must provide the required features and the performance must meet the required AC parameters to provide the proper performance for the particular application. Typical AC performance parameters are efficiency, output voltage ripples, and load transient regulation.
Optional Extra Features
These may or may not be required for a particular application. For example, if the converter is part of a large SoC that already includes a temperature sensor, a temperature sensor inside the converter may not be useful.
When reviewing the parameters and features of DC-DC converters, it is important to understand the different tradeoffs between performance metrics. This helps determine realistic expectations for the DC-DC converter that best fits your application.
- Small output voltage ripples: requires larger off-chip L and C components and thus larger PCB area and cost.
- High efficiency: requires larger switch sizes and thus larger die area.
- Smaller off-chip components: requires larger switching frequencies and thus more dynamic losses and efficiency degradation.
- High output power: requires larger die area for larger switches as well as high performance package technology and thus higher cost.
Besides the above-mentioned parameters, there are multiple practical aspects that need to be taken into consideration. Some of these aspects are mentioned below:
External EMI Reduction Circuits
Some devices have a requirement for off-chip circuit components to handle extreme EMI and maintain device reliability (e.g. off-chip filters).
PCB (Printed Circuit Board) Area
Check all off-chip components’ values, cost, and sizes for minimum PCB area. Also, check the package type and size and PCB design constraints provided by the supplier for optimum performance as well as EMI reduction.
Input Voltage Ripples
Switching converters drain pulsed current from their input voltage source. This pulsed current causes large input voltage ripples. These are always suppressed with an input capacitor to a minimum value. Does this minimum value suit your application? Other blocks sharing the same supply voltage input may experience brownout activity causing multiple resets or system instability.
The system designer needs to know the application ambient temperature, package type and its thermal resistance, system casing and its thermal resistance, and the maximum operating temperature of the switching converter. Using this information, one can decide if a heat dissipation mechanism is required (e.g. heat sinks). This will directly affect the system cost.
What is the MTTF (mean time to failure) of this converter? Does it match your reliability requirements? Or will it be the bottleneck of having a short lifetime product?
Output Ripple Frequency
Large output ripples are not significant if their frequency is out-of-band of your application. It is important to check the switching frequency of the converter and decide what its effect is on your application. Note: detailed analysis of inter-modulation and harmonic distribution needs to be studied for a multi-tone environment.
Other Protection Features
Are there any application-specific risks that should be addressed? For example, is it possible to have input supply overshoot due to supply sharing with other converters? In some cases, over-voltage protection is a must for reliable operation.
Number Of Switching Converters Per Application
High performance switching converters can be bulky and costly. Unless a performance constraint exists, using a single switching converter to power up as many blocks as possible is recommended.
Isolated Versus Non-Isolated Converters
Isolated converters require a transformer with a size proportional to the maximum current requirement. An isolated converter should only be used if needed.
Different applications impose different requirements on DC-DC converters. By clearly understanding the different converter parameters and the requirements from different applications, system designers can choose the proper DC-DC converter for their application. For an depth look at DC-DC converter parameters and applications, download our white paper: