Micropower dc-dc converters promote battery life in handhelds

Dave Bell, Vice President, General Manager, Power Business Unit, Linear Technology Corp., Milpitas, Calif.

Consumers expect more functionality to be packed into ever-smaller products, and they also expect longer battery life. These expectations are forcing changes in the way power supplies are constructed. Linear regulators have been popular because of their small size and cost-effectiveness; however, their low efficiencies severely impact run-time, especially as supply voltages continue to decline.

The battery of choice for advanced handheld products is the single Lithium Ion cell — voltage ranges from approximately 4.2V to 3.0V during discharge. An increasing array of micropower dc/dc converters is becoming available to convert a Li Ion battery's voltage to the various supply rails needed within a handheld product. Many of these new dc/dc converters deliver excellent power conversion efficiency over a broad range of load current, maximizing battery life.

As the functionality of handheld products increases, so does the number of supply rails. Processors and logic generally need low voltages (1.8V is common), so one or more step-down dc/dc converters are needed. It is often necessary to generate voltages in the range of 4V to 5V for analog and backlighting functions, so a step-up (boost) converter may be needed. Most difficult is generating a well-regulated supply voltage that is in the middle of the battery's voltage range (3.3V), since a buck-boost architecture is required. Fortunately, very small and efficient micropower dc/dc converters are available to fill all these needs.

A variety of synchronous buck (step-down) dc/dc converters are a designed to operate from a single Li Ion cell. They are capable of delivering up to 300mA load current, which make them ideal for powering microprocessors with supply voltages as low as 0.8V. The regulator size is very small because few external components are required, and the 1.5MHz switching frequency keeps the output inductor and capacitor very small and low profile. A ThinSOT package allows the entire dc/dc converter can be constructed with chip passives in less than 1mm height.

Most handheld products have two operating modes: active and standby. During the active mode, current consumption is typically high, so excellent conversion efficiency is essential for maximum battery life. However, many handheld products spend the vast majority of their time in standby, drawing little power from the battery. It's equally important that the power supply have high efficiency under these conditions. This implies that the power supply's quiescent current (the current drawn with little or no load) must be much less than the load current to maintain high efficiency.

It is usually and highly desirable for a synchronous regulator to maintain high efficiency over a three-decade range of load current (from 0.1 to 100 mA). Synchronous switching results in a peak efficiency of approximately 92 percent for a 1.8V output, while Burst Mode operation (a "keep alive" function for a switching regulator) reduces quiescent current to only 20µA under light load. Also desirable is an efficiency remaining up 87 percent even with only 1mA load current.

Although there is a clear trend toward lower supply voltages, many handheld products still require some 5V for analog circuitry or even higher voltages for LCD bias. Synchronous switching is practical for boost converters up to 5V output. For example, Linear Technology's LTC3400 boost converter family delivers conversion efficiencies up to 95%, while Burst Mode operation keeps quiescent current very low for excellent light-load efficiency. Members of the family have different current capabilities.

Synchronous switching is less important at higher voltages because the output rectifier drop is a smaller percentage of the output voltage. For applications such as LCD bias generation, there are regulators that can generate output voltages up to +/-34V with very good efficiency. Specialized ICs are also available for white LED backlighting — the high forward drop of white LEDs requires boosting a single Li Ion cell's voltage.

The 3.3V challenge

It may be surprising, but generating 3.3V from a single Li Ion cell can be difficult. Because the cell voltage can be either higher or lower than the output, a unique dc/dc architecture is needed. Traditionally, two methods have been used: a boost converter followed by a linear regulator, or a Single-Ended Primary Inductance Converter (SEPIC). Both solutions are complex, inefficient and consume considerable pcb area. Power-supply experts would recommend a monolithic buck-boost converter.

Buck-boost is hard to implement on a single chip. LTC's implementation ( such as the LTC3440) is based on a full-bridge (4-switch) architecture using a unique control scheme. The dc /dc converter switches from step-down to step-up operation as the battery voltage changes, and delivers 94 percent efficiency across the entire battery voltage range. Because all four MOSFETs are included in the MSOP package, and switching frequency may be user-adjusted up to 2MHz. A complete 3.3V, 600mA power supply consumes only 150mm2 of pcb area. The LTC3440 can be pin-selected for continuous switching or Burst Mode operation where quiescent current drops to 25µA.

Handheld product designers may need to be concerned with two types of noise generation: RF noise and audio noise. When wireless communication circuitry is embedded within the handheld product, the fundamental and harmonics of the power supply's switching frequency may cause radio interference. Good layout techniques are essential to minimize noise coupling, but synchronizing the switching frequencies can also ensure that switching noise is located away from sensitive frequencies. Most of the dc/dc converters support synchronization; however, this requires that the converter operate at a fixed frequency independent of load current (Burst Mode operation defeated). Noise is predictable when synchronized, but quiescent current is high, typically several milliamps, because the power MOSFETs are continuously switching.

Preventing the dc/dc converter from entering Burst Mode operation also solves another problem — audible noise generation. When a converter operates with Burst Mode switching it turns on and off at a load-dependant rate, and may generate output ripple with audio frequency components. This is obviously a problem for products such as cellular phones and MP3 players that process audio signals. Selecting continuous switching or synchronization inhibits Burst Mode operation, and eliminates audio noise generation, but again, at the expense of light load efficiency.

The solution to the noise versus quiescent current tradeoff is actually quiet simple. Most of the dc/dc converters discussed include a MODE/SYNC pin that allows continuous switching, synchronized switching (also continuous), or Burst Mode switching. The handheld product's microcontroller can select either continuous switching or synchronized mode when the product is operational to minimized both RF and audio noise. However, when the product is in standby the microcontroller can select Burst Mode operation for minimum quiescent current and maximum battery life. With modern micropower dc/dc converters, it's now practical to simultaneously achieve the goals of long battery life, small size and noise reduction.