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Right regulator can cool handset

Posted: 16 Aug 2004 ?? ?Print Version ?Bookmark and Share

Keywords:switching regulator? regulator? converter? dc? cellphone?

Today's cellphones allow Web browsing, wireless transfer of e-mails, digital photos and even streaming video. Makers of those phones are under increasing pressure to pack these features into an ever-shrinking form factor while simultaneously gaining longer runtimes.

The increasing number of features is simultaneously driving the need for more low-voltage output rails at varying power levels. One example is the applications processor for image processing, which needs up to 360mW of power during video capture. It is fairly common for more than 4W of peak power to be required by the phone's internal system's load during full-load operation. This type of power consumption can quickly drain a battery's energy. Another important factor adversely affecting a battery's runtime is the power supply efficiency and system power management.

Poor power-conversion efficiency generates heat. This heat is generated from the power lost in the regulator during the energy transfer process. But inside a cellphone, there are no fans or heat sinks for cooling - just a densely-packed PCB. Consequently, there is no path for heat to exit. This heat equates to a reduction of battery life and can degrade product reliability.

Efficiency is calculated as output power divided by input power or, put another way, power to the load divided by power from the input. It should be noted that the input voltage and current must be measured at the node prior to any external components of the DC/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 power conversion, there is a driving need in the industry to rethink the regulator that should be used. Manufacturers are now adopting switching regulators over the simpler but less efficient linear LDO regulators, because of their higher efficiency.

The benefits and drawbacks of different voltage regulators that can be used to meet the power conversion needs inside a cellphone must be considered. There are three choices: linear LDO, inductorless switching and conventional inductor-based switching regulators.

The linear LDO is considered the simplest; it can only step down from the input voltage to a lower voltage. However, its most significant drawback is its thermal management, since its conversion efficiency can be approximated by the output voltage divided by the input voltage. For instance, consider an LDO that provides an output voltage of 1.8V at 200mA of current from the nominal 3.6V of a single-cell Li-ion battery to drive an image processor. The conversion efficiency is only 50 percent, so it generates hot spots inside the phone and reduces battery runtime.

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

A compromise between a linear regulator and a conventional switching regulator is the charge pump. In these devices the external storage elements are capacitors - not an inductor. Having no inductor alleviates any potential EMI concerns that could have an impact on sensitive RF receivers or Bluetooth chipsets. Potential drawbacks are a limited input-to-output voltage ratio and a limited output current capability.

The traditional method for preventing noise interference has been to isolate noise-generating circuits from noise-sensitive circuitry. However, in a modern cellphone, everything is so densely packed that this is no longer possible. Shielding is not practical for cost and size reasons. One trade-off with switching regulators is their potential to generate harmonic noise.

Pulling out noise

A technique that can be used to mitigate this noise is to dither the system clock of the DC/DC converter. This approach, with its resulting spread-spectrum operation, makes it possible to modulate the switching frequency by means of a pseudorandom number sequence to eliminate narrowband harmonics. In essence, the noise is "spread" across the frequency range instead of being concentrated in distinct harmonics.

In a conventional step-down switching regulator, the harmonics are large and could cause potential interference issues in a system. But in an inductorless step-down switching regulator with spread-spectrum implemented on-chip, the input harmonics are virtually eliminated and there is a significant reduction in peak output noise.

In addition to the proliferation of high-efficiency converters due to thermal and noise issues, we will continue to see improvements in I/O noise levels, increased switching frequencies and reduced external-component count in switching converters. Along with this, smaller and lower-profile packages will proliferate.

- Tony Armstrong

Product Marketing Manager

Linear Technology Corp.

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