Guide to Choosing the Best DC-to-DC Converter for Your Application

By: Faisal Hussien, Ph.D. and Brittany Eckmann
Revised: 3/14/2019 by Stephen M. Nolan, Vidatronic

ABSTRACT

DC-to-DC switching converters are widely used in most portable devices. They efficiently generate various regulated supply voltages for different sub-blocks within the same system board. A typical system board for any portable device can easily contain many switching converters. This huge market leads to the existence of hundreds of different DC-DC converters provided by multiple suppliers. These converters range from tiny packaged parts with no external components to large parts with multiple external components for proper operation, making it difficult to choose the best option.

This white paper introduces a procedure for choosing the proper DC-DC switching converter for a given application. It explains basic, performance, and optional metrics in detail. It also demonstrates other practical aspects that are sometimes overlooked by system designers. Multiple application examples are provided.

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INTRODUCTION & BACKGROUND

All high-performance applications require a specific, regulated, and stable power supply voltage. This cannot be guaranteed by conventional battery technology and it 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. Figure 1 shows the voltage regulation concept and how it helps in the isolation process. Regulating a power supply to meet the minimum requirement of a specific application extends the battery life, which is crucial in all portable applications.

Figure 1. The Use of Voltage Regulators for On- Chip Supply Noise Isolation

Power management technology deals with the optimum design of voltage regulators, voltage monitors, voltage references, and current references. It aims to generate all 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 regulators, and charge pumps.

Linear regulators are used in compact and noisesensitive applications; for example, RF circuits and highprecision 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 linear and switching regulators. The main question for all system designers is “How to choose the best regulator for my application?” To answer this question, one needs to understand the metrics of a converter. These metrics can be divided into basic metrics that most of the 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.

This white paper is intended as a tool to help system designers choose the best DC-DC switching converter for their application. This is done by describing all the above-mentioned metrics and providing a procedure for optimum choice. Application examples from real life are then provided.

A similar white paper provides information on how to choose the best low dropout (LDO) linear regulator for your application, which can be downloaded in the section titled Further Reading below.

DC-DC CONVERTER FEATURES AND PARAMETERS

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. Listed below are the important features and parameters and what they mean.

Input-to-Output Voltage Consideration

Based on the difference between the input and output voltages, different converter types should be used. If VOUT is less than VIN, a step-down (BUCK) converter is used. If VOUT is greater than VIN, a step-up (BOOST) converter is used. If both cases are required, a BUCKBOOST converter is used. If VOUT and VIN have different polarity, an inverting converter is used.

Input-to-Output Isolation Feature

In some applications, complete DC isolation is required between input and output ports of the converter. This usually requires use of a transformer-based converter.

DC Line Regulation Parameter

DC line regulation is defined as the resulting change in the output voltage for a given change in the input voltage. This measurement is made under conditions of low power dissipation.

DC Load Regulation Parameter

The DC load regulation is the change in output voltage for a static change in output (load) current.

Efficiency Parameter

Power efficiency is defined as the percentage of the input power that is delivered to the output and it is expressed as:

An ideal switching converter can approach 100% efficiency. The relationship between the average input current and the average output current is opposite to the relationship between the input voltage and the output voltage. This maintains equality between input power and output power. Real, practical converter circuitry consumes power, both static and dynamic. As a result, POUT is less than PIN. In order to maximize the DCDC efficiency, the converter’s internal losses (static and dynamic) must be minimized.

Input Voltage (VIN) Range Parameter

The input voltage range determines the maximum and minimum allowable input supply for the converter. Input supplies higher than the maximum allowable input can damage the converter.

Maximum Output Current (IOUT) Parameter

This parameter is the maximum output current that the converter can provide while meeting the datasheet parameters.

No Load Current Operation Feature

Several applications need the converter to hold the output voltage stable and provide good performance under a no current load condition (e.g. CMOS RAM keepalive applications). Some converters might suffer from degraded performance or instability under no load conditions.

Output Voltage Noise Parameter

The switching nature of the switching converters results in an output tone with harmonics. This tone corresponds to the power transistors’ switching frequency. Aside from this tone, the output voltage noise parameter is the RMS output noise voltage generated only by the converter (including the voltage reference-noise as it is assumed that the converter has an integrated reference) over a given frequency range (typically 10 Hz to 100 KHz) under a constant output current and a clean input voltage.

Output Voltage (VOUT) Accuracy Parameter

The output voltage accuracy describes the typical and worst-case deviation of the average output voltage with respect to the nominal converter output voltage. The overall output voltage accuracy also includes the effects of line regulation and load regulation.

Output Voltage Range Parameter

The output voltage of the converter can be fixed or adjustable. If the output voltage is adjustable, it is important to know the maximum and minimum allowable output voltage as well as the required programming steps.

Over-Current Protection Feature

This feature limits the maximum amount of current that the converter can source. This limit is established in order to protect the converter and the system under a short-circuit or high current condition.

Over-Temperature Protection Feature

This feature shuts down the converter when the die temperature exceeds the specified high temperature level. The converter is re-enabled when the temperature drops to a safe value.

Power Supply Rejection Ratio (PSRR) Parameter

The PSRR is a measure of how well the converter rejects electrical noise at the power-supply input by measuring the change in the output voltage.

Quiescent Current (IQ) Parameter

Quiescent current, also called ground current, is the current used to operate the converter, and is not delivered to the load. IQ is measured when the converter is enabled and the output/load current is zero (0). A small quiescent current is needed to maximize the converter output efficiency, reduce heat, and extend battery life in battery-operated applications.

Soft-Start Operation Feature

The soft-start feature guarantees that the output voltage will ramp-up slowly from zero to the required output voltage. Soft-start is useful to avoid inrush current during start-up operation that can result in damage to either the converter package or the load. It maintains monotonic ramp-up for safe load startup operation. Also, it avoids false trigger of ESD active power-supply clamps.

Soft-Stop Operation Feature

Soft-stop guarantees that the output voltage will rampdown slowly in a controlled fashion when the converter is disabled.

Shutdown Current (ISD) Parameter

Shutdown current is leakage current through the converter, when the converter is disabled or powered down.

Start-Up Or Turn-On Time Parameter

Start-up time is the time between the rising edge of the enable signal and the output voltage reaching 90% or 95% of its nominal value.

Line Transient Regulation Parameter

The line transient regulation parameter provides a measure of the ability of the converter to maintain a constant output voltage with a transient step at the input.

Load Transient Regulation Parameter

The load transient regulation is defined as a change in the output voltage resulting from a given dynamic (step) change in output current. Load transient regulation includes overshoot (difference between the maximum VOUT and the initial VOUT during a load transient regulation test when the output current changes to a lower value), as well as undershoot (difference between the minimum VOUT and the initial VOUT during a load transient regulation test when the output current changes to a larger value).

Interface Protocol Feature

This specifies the type of digital interface used to control the programmability options and operation behavior of the converter. This Interface can be I2C (Inter- Integrated Circuit), VID (Voltage Identification), or SPI (Serial Peripheral Interface), to name a few.

Temperature Coefficient Parameter

The temperature coefficient describes the output voltage variation with respect to the temperature variation. It is usually measured in “parts per million” (ppm). Output Voltage Ripple Amplitude Parameter The output voltage ripple amplitude identifies the maximum peak-to-peak ripple amplitude of the output voltage.

Output Current Monitor Feature

This feature allows the converter to provide a signal proportional to the average output current.

SELECTING YOUR DC-DC CONVERTER

With so many converter parameters, it is usually difficult for system designers to select an appropriate or best converter for a certain application. Which parameters are critical? Are there any parameters that can be compromised to reduce the overall cost? Is there any hidden information that we should seek for a successful product?

To make the converter selection process easier, we recommend dividing the converter parameters into three groups:

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.

Table 1 shows the features and parameters separated into each of these three groups.

Table 1. Switching Converter Parameters Divided Into Three Groups

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 offchip 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.

Practical Aspects

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.

Operating Temperature

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.

Failure Rate

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.

Converter Selection Procedure

To select the best converter for your application, we recommend the following five-step process:

1. Select a group of converters that meet the basic parameters for your application. For example, what is the required input voltage and output voltage range? Do you require input-output isolation? What is the maximum current requirement?
2. Arrange the parameters of the performance column of Table 1 in priority order based on the importance to your application. Then, from the group of converters chosen in Step #1, choose the converters that meet these arranged parameters in the specified order.
3. In this step, narrow your selection to the one performance parameter that is most important for your application.
4. Study the practical aspects mentioned above in detail to help in your converter selection.
5. Now that you have made your converter selection, find out which optional parameters the converter offers for your application.

CONVERTER APPLICATION EXAMPLES

Listed below are a few examples of switching converter applications and the performance parameters that are important for each. Please contact Vidatronic at sales@vidatronic.com if your particular application is not discussed in this white paper. We will gladly help you with your particular application.

AMOLED (Active Matrix OLED) Displays in Wearable Devices

Because wearable devices require restricted circuit board area and generally long-lasting battery-life, system efficiency is extremely important in designing these products. Powering AMOLED displays in wearable devices requires DC-DC switching converters that offer high power efficiency and tight output voltage accuracy.

DDR-SDRAM Memory

DDR-SDRAM memory requires DC-DC switching converters with tight accuracy requirements and ability to support fast load transients.

Battery Charger/USB Charging Ports in Portable Devices

Everyone wants their smartphone, tablet, or portable battery pack to charge quickly without heating up their portable devices. A synchronous BUCK converter can be used for this application. Typically, a charging port for a mobile device is a micro USB port. It accepts a regulated 5 V. The charging circuits are on the inside of the mobile device, which is often a BUCK converter.

DVFS (Dynamic Voltage Frequency Scaling)

Performance-Power tradeoff in microprocessors and digital signal processing circuits is managed using switching converters. Increasing the digital circuit supply leads to increasing the performance (speed) at the expense of dynamic power consumption and vice versa.

Energy Harvesting Systems

High efficiency switching converters are a vital block in energy harvesting systems. It is the most adequate voltage regulation method in such low power systems. Boost converters with clock-controlled input impedance are used for MPPT (Maximum Power Point Tracking) operation.

VIDATRONIC HIGH PERFORMANCE DC-DC CONVERTERS

Vidatronic switching converters offer unparalleled performance, which makes them ideal for a host of power applications. Our converter technology has several features that add significant value to the overall system solution:

• Smallest overall solution due to on-chip compensation and accurate on-chip voltage and current references (no need for extra off-chip bias components)
• 95% power efficiency at average load, and over
• 85% power efficiency at light loads
• Output voltage ripples less than ±10 mV
• Low-cost wirebond packages
• Output voltage accuracy better than ±1%, without trimming
• Maximum Output current (IOUT) 1 mA up to 5 A
• Overshoot/undershoot less than ±50 mV
• Controlled soft-start and soft-stop operation
• Over-current and over-temperature protection
• Small silicon area and reduced pin count
• Optional I2C interface
• Robust thermal management suitable for small packages
• Different feedback techniques for light load to ensure high efficiency

SUMMARY

Today’s complex systems continue to add more and more functions that require multiple supply voltage levels. DC-DC switching converters are a key component used in multiple applications. With this simple procedure and parameter/application reference, system designers can select the best switching converter for any application.

For more information on Vidatronic’s DC-DC converters and other IP cores, go to: http://www.vidatronic.com/ipsolutions/ or contact Vidatronic to learn how we can provide the best DC-DC converter IP core for your application.

FURTHER READING

LIST OF ABBREVIATIONS