LDO-enabled DC/DC converters to meet next-generation mobile application design requirements (Part 2)

Challenges of High Frequency Switching

As we mentioned earlier, the losses associated with high frequency switching increase. Figure 4 shows the losses of a traditional DC/DC converter at 2MHz using a 2.2uH inductor , and the losses at 8MHz using a 0.47uH inductor.

It can be clearly seen that at a conventional mobile DC/DC frequency of 2MHz, the losses caused by switching only account for about 20% of the total losses. The total loss is about 200mA, which is a typical output current for mobile devices. But at 8MHz, the switching losses rise to more than 40%. Reducing switching losses is the key to being able to switch at high frequencies and is also a prerequisite for small inductors that can be integrated into packages.

Suggested Solutions

Micrel Corporation introduced its first generation of "L Free " DC/DC converters, the first product is MIC3385. Its switching frequency is 8MHz, and the inductor is integrated into the 3mm x 3mm MLF package. The design takes into account the reduction of switching losses, so as to minimize the efficiency loss caused by the increase of switching losses. Figure 6 shows the simplified block diagram of MIC3385, and Figure 7 shows the actual size.

The basic structure of the MIC3385 is a constant frequency PWM converter with a parallel LDO . When the output load is in standby, the LDO acts as a light load mode. This hybrid design provides extremely good noise performance and enables easy transition to high frequencies.

As mentioned earlier, the MIC3385 is optimized to switch at higher frequencies and with lower inductance values. Figure 8 shows the efficiency of a 2MHz converter with the same structure and the MIC3385 at 8MHz. As can be seen, the efficiency drops by only 4% at 200mA. This is an acceptable tradeoff for significantly reducing the size and complexity of the design.

There are other challenges for high-frequency DC/DC conversion besides reducing switching losses. The biggest challenge is to design a control loop with high enough bandwidth to keep the output voltage stable under fast transients while still using small ceramic output capacitors. The MIC3385 does this by using a patented method of paralleling LDOs to achieve the required high bandwidth. Both the DC/DC converter and LDO of the MIC3385 provide full output current to allow a nearly seamless transition from one state to the other; with minimal output voltage deviation. Figure 9 shows the output voltage deviation of the MIC3385 under heavy load transient conditions and compares it with a more traditional DC/DC solution. Heavy load transient conditions are common in mobile devices. It can be seen that the MIC3385 8MHz architecture performs much better than traditional structures, creating a large space for design stability.

Advantages in terms of noise

The inductor of a DC/DC converter generates a magnetic field during operation and switching. The placement of the inductor must be considered during design to avoid causing interference. For example, placing the inductor near sensitive audio components may cause harmful interference. Placing it near a power amplifier may reduce the sensitivity of the device and cause compatibility issues. The larger the inductor, the more difficult it is to control these problems. The MIC3385 has a small inductor and is placed as close to the DC/DC die as possible. This keeps the high-frequency power loop as short as possible, reducing EMI noise compared to low-frequency DC/DCs with external inductors. This is a bit counterintuitive because it is generally believed that higher frequencies will produce more noise.

Conclusion

The results show that the first generation of 8MHz switching frequency DC/DC converters are feasible and provide a beneficial solution that is popular in mobile designs. The design shows low noise, fast transient response and high efficiency, all of which bring DC/DC converters closer to LDO solutions.

As power and size requirements for mobile devices continue to increase, more integrated devices will appear in the market.