Leveraging DC/DC Converters with LDO Features to Meet Next-Generation Mobile Application Design Requirements (Part 1)

Today's mobile phones continue to get smaller and thinner. Not surprisingly, most advances in technology size are a key issue that can determine the fate of product development. As mobile devices continue to shrink in size, component size and number must do the same. As the space around each component shrinks, component placement becomes more important. Interference and "low noise placement" become part of the engineer's job. Components smaller than 0.6mm are now a standard requirement. However, there are some limiting factors that are limiting this trend of shrinking size. The first factor is that for every new feature added to a mobile phone, the power consumption also increases accordingly. The most obvious example is that 10 years ago, the power consumption of the display screen was less than 50mW, today it has risen to 150mW-200mW, and it is expected to rise to 3-500mW in a few years. Add to that the multimedia processor, camera module, TV tuner, etc., and it is easy to see why the power consumption of mobile phones continues to increase.

Unfortunately, battery technology has not kept pace with this demand. Lithium-ion energy density has only doubled, from around 100Whr/Kg to around 200Whr/kg, while mobile phone power consumption has tripled. Even accounting for density improvements, the size of the average battery today is the same or larger than it was a few years ago, while talk and standby times have decreased. Considering all of these factors, it's easy to see that power management plays an increasingly important role in today's mobile products.

Types of Power Management Devices

As an example, let's look at the heart of a cell phone - the baseband processor. The baseband processor in a cell phone is traditionally powered by a low-dropout linear regulator (LDO) . The advantages of an LDO are low output noise deviation under all conditions, small size, ease of use, no reflected noise on the battery that could affect other components, and fewer external components. The disadvantage of an LDO is that its efficiency is generally lower than that of a DC/DC converter, and that efficiency decreases as the chipset voltage requirement decreases, but the battery voltage remains the same. As the power requirements of mobile phone functions continue to increase, many designers are using DC/DC converters instead of LDOs to improve efficiency and maintain battery life.

DC/DC converters offer designers a viable alternative; they are efficient over a wide range of loads, which LDOs cannot match. However, DC/DC converters are inferior to LDOs in almost every other respect: they are larger, more difficult to use, require more external components, and generate more noise. The largest external component is the inductor. In order for a DC/DC converter to operate effectively, the device must switch a storage element, usually an inductor, on and off at a high frequency. This function necessarily generates noise and increases the size of the regulator. This "noise" can be transferred to the device it is powering, the baseband processor, causing system problems. It can also contaminate the battery, causing the noise to spread to every part of the phone. To reduce this phenomenon, phone designers must add additional filtering components such as capacitors and inductors to isolate and suppress the noise. This increases product size and complexity. It also requires careful planning of board space to keep sensitive areas away from the DC/DC converter and as isolated as possible. Noise is not always predictable, and predictability and a conservative approach to risk are extremely important when designing high-volume consumer products.

Figure 1 shows the difference between an LDO and a traditional DC/DC converter for mobile phones.

Ideal power management components

From an efficiency perspective, it is clear that DC/DC converters are the future of power management. The challenge is to reduce the size of the DC/DC converter to a small, simple, low-noise, and inexpensive device like an LDO. Market forces and demand for miniaturization of mobile products will force this to happen.

To understand how this is accomplished, let’s first look at the components that make up a DC/DC converter. The largest component is the inductor. The inductor is a switching storage element, so it is not only large in size, but it also generates magnetic fields that can cause noise issues in board designs. Obviously, the area and height of the inductor must be reduced to approach an ideal LDO type product. Let’s look at the function of the DC/DC converter and why the size of the inductor needs to be so large. Figure 2 shows the basic operation of a non-synchronous buck DC/DC converter. Mobile DC/DC converters are usually synchronous in nature, using MOSFETs instead of diodes to improve efficiency. For ease of understanding, a non-synchronous buck converter is used to illustrate the operation.

The switch has two operating modes, on and off, and the number of times it switches per second is the switching frequency. When the switch is off, energy is delivered to the output load and stored in the inductor. When the switch is on, the energy stored in the inductor is transferred to the output. The ratio between the on and off of the switch is called the duty cycle, and controlling this ratio can control the output voltage. As can be seen from Figure 2, the inductor current consists of two parts. The first is the DC output current, and the second is the current delta IL caused by the switch inductance. Delta IL is mainly determined by E=Ldi/dt. Here E is the voltage on the inductor when the switch is off (input voltage minus output voltage), di is delta IL, and dt is inversely proportional to the switching frequency. Delta IL is actually a redundant part that flows through the output capacitor, diode and generates noise, and causes additional losses for the switch when the switch is on. Choosing an inductor for a given design is entirely a balance between delta IL, noise and loss. But one thing is clear: for a given input and output voltage, the switching frequency is the main factor in determining the inductor value. The higher the switching frequency, that is, the lower the dt, the smaller the inductance.

Unfortunately, increasing the switching frequency has a significant negative effect. The main one is that the efficiency of the DC/DC converter will decrease. The reason for this is simple. The switch uses a certain amount of energy to turn on and off. This energy is actually a loss, so the more times the switch is turned on and off per second, the greater the energy loss and the lower the overall efficiency. Controlling this "loss" is the key to increasing the switching frequency.

Today's popular DC/DC converters are targeted at mobile products operating at 1-3MHz. At a switching frequency of 1Mhz, a 4.7uH inductor is usually required, and at a frequency of 3-4MHz the inductor can be reduced to around 1-1.5uH. Figure 3 shows the relationship between inductor size and switching frequency for mobile products. As can be seen, in order to approach the size of an LDO, the inductor needs to be less than 1uH. This allows the switching frequency to be set above 6MHz. The most advanced 500mA 0.47uH inductor uses a 0805 case size with a height of 0.55mm.

As can be seen in Figure 3, at a switching frequency of 8MHz, it is tempting to place the inductor inside the IC package. The inductor height is currently less than 0.6mm. There are some challenges in this regard.