Power Electronics

Wireless Products Signal New Uses for VLDOs

Able to provide compact and efficient step-down conversion at low current levels, very low-dropout regulator can generate many of the low-voltage supply rails required in portable wireless devices.

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The majority of battery-powered handheld devices today feature some form of wireless communication. Even a cellular phone does more than just allow people to talk with each other. Today's state-of-the-art cellular phone allows web browsing, wireless transfer of e-mails, picture taking, streaming video and even gaming. An embryonic trend is to add a micro hard disk drive (less than 1-in.-diameter disk platters) for high-capacity storage capability, allowing these phones to also act as an MP3 player. Clearly, makers of these phones are under increasing pressure to pack these features into an already constrained form factor while simultaneously gaining longer runtimes.

Looking at the cellular phone block diagram in Fig. 1, it is easy to see how the increasing number of features is driving the need for more low-voltage output rails at varying power levels. Whereas the main power rail in a cellular phone used to be 3.3 V, it is has become increasingly common for newer cellular phone designs to use a 1.5-V main power rail. The reason for this is clear; the majority of the digital large-scale integration (LSI) ICs have an operating voltage of 1.5 V or less. Examples of this are the baseband chip sets that run off of 1.375 V and the video processing DSPs that require 1.2 V.

In these applications, the power source is typically a lithium ion (Li-ion) battery with a nominal output voltage of 3.6 V. Because of space, efficiency and cost constraints, it is clearly impractical to use point-of-load dc-dc conversion to generate the lower voltages (below 1.5 V) directly from the battery. Therefore, designers have elected to adopt a two-stage conversion approach instead. Thus, they use a high-efficiency step-down converter to drop the Li-ion battery voltage down to 1.5 V. Then, from this 1.5-V main rail, they simply use very low dropout (VLDO) regulators to supply the low-voltage LSI ICs.

The use of VLDOs is possible in large part because of the low nominal operating currents, which result in conversion efficiencies in the 80% to 90% range. For example, stepping down 1.5 V to 1.375 V to power a baseband chipset core can be accomplished with an efficiency of 91.7%. Another compelling reason to use these VLDOs is that many of the digital ICs being powered are noise sensitive. As such, the output ripple from these regulators must typically be less than 1 mVP-P. Clearly, one can easily use a VLDO as a post-regulator for a step-down switching regulator to ensure low ripple.

One could argue that all of this effort is not necessary, since a higher milliamp-hour (mAH) capacity battery can eliminate the need for this methodology. How-ever, consumers like their cell phones with a compact and lightweight battery. This is why most cellular phone manufacturers supply their phones with a nominal 600-mAH capacity battery and offer a larger-capacity battery as an aftermarket accessory. At the same time, the cellular phones' constrained form factor does not allow for any heatsinking, so high-efficiency dc-dc conversion is a major priority to stay within the tight power budgets necessitated by the phone's high-functional content.

Even when it's offered as an option, the aftermarket battery has several drawbacks. This type of battery is more costly, bulkier and heavier than the lower-capacity battery that is supplied. It would be possible to enlarge the cellular phone form factor to accommodate a larger-capacity battery. However, once again, the consumer does not want the large form factor, so this usually will only apply to Smart phones or hybrid functional phones that are more commonly used by business users.

Design Issues

Poor power-conversion efficiency generates heat. This heat is generated from the power lost in the regulator during the energy-transfer process. Inside a cell phone, there are no fans or heatsinks for cooling, just a densely packed printed-circuit board and battery. And so, there is no path for heat to exit the product. This heat equates to a reduction of battery life and can adversely impact product reliability.

Efficiency is calculated as output power divided by input power. In determining the efficiency of a dc-dc converter stage, the input voltage and current must be measured at the node prior to any external components of the dc-to-dc converter. Likewise, the output voltage and current must be measured after any external components of the dc-dc converter.

As a result of the heat generated during the power-conversion process, there is a new driving force in the industry to rethink the selection of the type of regulator that should be used. Manufacturers had adopted switching regulators over the simpler linear low dropout regulators because of their higher efficiency operation. This made sense when the main power rail was 3.3 V; however, with the newer designs incorporating a 1.5-V rail, this decision does not necessarily follow.

Table 1 highlights some of the benefits and drawbacks of the different types of voltage regulators that can be used to meet the power-conversion needs inside a cellular phone. There are three choices: linear low dropout regulators, inductorless switching regulators (also known as charge pumps) and conventional switching regulators (inductor based).

The linear low-dropout regulator is considered the simplest form of regulator; it can only step down an input voltage to a lower one due to its inherent dc voltage conversion, i.e. no switching. Its most significant drawback is its thermal management aspect, since its conversion efficiency can be approximated by the ratio of the output voltage to the input voltage.

As an example, consider an LDO providing an output voltage of 1.8 V at 200 mA of current from the nominal 3.6 V from a single cell Li-ion battery to drive an image processor (Fig. 1). The conversion efficiency is only 50%, thus generating hot spots inside the phone, as well as a reduction in battery runtime. However, while heat generation may be a problem with large input-to-output voltage differentials, that's not the case when the voltage differential is small. For example, going from 1.5 V down to 1.2 V, the LDO is going to be 80% efficient.

A switching regulator circumvents all of the linear regulator's efficiency shortcomings when the voltage differential between input and output is high. It exhibits efficiencies of up to 96% by using low-resistance switches and a magnetic storage element — thus, drastically reducing the power lost in the conversion process. By operating at switching frequencies greater than 2 MHz, the size of the external inductor and capacitors can be greatly reduced. The disadvantages of a switching regulator are minor and can usually be overcome with good design techniques.

But, what happens when a 1.5-V main power rail is used and there is a need to step down to 1.2 V to power a DSP core? At this level, a switching regulator offers no clear advantage. In fact, a switching regulator cannot be used to go from 1.5 V down to 1.2 V because there is no way to fully enhance the MOSFETs — whether on- or off-chip. Also, a standard LDO will not do the job since its dropout voltage is usually greater than 700 mV. The ideal solution would be to use a VLDO where the input voltage range is close to 1 V, its dropout voltage is less than 300 mV and its internal reference is close to 0.5 V. Such a VLDO could easily step-down from 1.5 V to 1.2 V, and could do so with 80% efficiency. Since the power levels at this voltage are usually around 100 mA or so, the 24 mW of power loss are acceptable.

VLDO Features

A VLDO for use in the application described above would require a low input voltage, output voltage and dropout voltage. Furthermore, since a portable wireless device is usually battery powered, a VLDO should also be able to protect itself against reverse-input and reverse-output voltages. Finally, it should also be able to operate with low equivalent series resistance (ESR) capacitors, have excellent line and load regulation and have fast transient response.

Even if a VLDO's design is stable with a wide range of output capacitors, it should be optimized for low-ESR ceramic capacitors for size and cost considerations. Nevertheless, an output capacitor's ESR affects stability, most notably with low-value capacitors. Therefore, it is important to ensure the right level of capacitance and ESR values. To complicate matters further, in a low-voltage device such as a VLDO, the output load transient response is a function of the output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient response for larger load current changes.

Furthermore, extra care needs to be taken when using ceramic capacitors. Manufacturers make ceramic capacitors with a variety of dielectrics, each with different behavior across temperature and applied voltage. The most common dielectrics are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics provide high capacitance-voltage (C-V) products in a small package at low cost, but they exhibit strong voltage and temperature coefficients. The X5R and X7R dielectrics yield stable characteristics and are more suitable for use as the output capacitor at a slightly increased cost. Both the X5R and X7R dielectrics exhibit excellent voltage characteristics. The X7R type works over a larger temperature range and exhibits better temperature stability, whereas the X5R is less expensive and available in higher values.

Keep in mind that voltage and temperature coefficients are not the only sources of problems. Ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer and microphone work. For a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients. The resulting voltages produced can cause appreciable amounts of output noise (Fig. 2).

VLDOs are available from several analog IC manufacturers. One example device is Linear Technology's LT3021, a VLDO that operates from input supplies down to 0.9 V and delivers 500 mA of output current with a typical dropout voltage of 160 mV (Fig. 3). This regulator optimizes stability and transient response with low-ESR ceramic output capacitors as small as 3.3 µF. Its internal protection circuits, which includes reverse-battery protection, current limiting, thermal limiting with hysteresis and reverse-current protection, guard against the potential fault conditions that may be encountered in wireless product applications.

Table 1. Comparison of linear versus switching regulators.
Features LDO and VLDO Regulators Inductorless Switching Regulator Conventional Switching Regulator
Design complexity Low Moderate Moderate to high
Cost Low Moderate Moderate
Noise Lowest Low Low to moderate
Efficiency Low to moderate Moderate to high High
Thermal management Poor to moderate Good Best
Output current Moderate Low Large
Requires magnetics No No Yes
Limitations Cannot step up VIN/VOUT ratio Layout considerations

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