The next generation of handheld devices with wireless Internet connectivity is beginning to appear, fueling still further the growth of the market for handhelds. The challenge is to make them smaller and to extend their run-time and standby time on a set of batteries.
To do this the dc/dc voltage regulators used must offer extremely high efficiencies over currents ranging from a few milliamperes (standby mode) to a few hundred. The regulators also must occupy as little space as possible in those very space-conscious applications.
The choice of voltage regulator IC will directly influence the battery life of the handheld device. The IC typically is in one of three modes when used in a typical handheld device such as a personal digital assistant (PDA) or a wireless phone. For example, a dc/dc regulator IC that is supplying the microprocessor and digital circuitry of a PDA will be in:
- Run mode, when the PDA is on and is being used. The IC is supplying the microprocessor and other circuitry, which are on and drawing current. In a typical PDA this is a few hundred milliamperes of supply current.
- Standby or sleep mode, when the user hasn't interacted with the PDA for about an hour. Here the PDA has only its keep-alive and its wakeup circuitry on. In this mode the regulator IC is supplying a few hundred milliamperes or less.
- Shutdown mode, when the user turns off the PDA using its on/off button. The regulator ICs that are not supplying the wakeup circuitry will be shut down. In this mode the IC is not supplying any load current.
The key parameters of the voltage regulator IC that affect battery life are:
- The no-load quiescent current (IQ) is the quiescent supply current needed to keep the internal circuitry of the IC on. This significantly affects the battery life in sleep mode. ICs with a lower IQ permit longer battery life.
- The shutdown quiescent current (ISD) is the leakage current the IC draws from the battery when it is in shutdown. ICs with a lower ISD permit longer battery life.
- The operating efficiency under load (h) is the operating efficiency of the regulator IC under loaded conditions; it can be very important, depending on the PDA's usage. In a typical PDA, the regulator IC that supplies the microprocessor and digital circuitry has a full load of a few hundred milliamperes. Dc/dc ICs with a higher h mean a longer battery life.
The amount of time that a handheld device spends in each of these modes depends on the usage pattern. As a result, a voltage regulator IC that has a good combination of IQ, ISD and h is needed to maximize the battery life of handheld devices. Makers need to evaluate different regulator ICs under a typical usage pattern of load to determine which IC will give the longest battery life.
A prudent choice of the voltage regulator IC will result in space savings for today's highly space-constrained handheld devices. Sometimes the handheld designer faces a trade-off between the solution size of the regulator and the battery life provided by that regulator.
There are several types of voltage regulator ICs that provide different trade-offs between solution size and battery life. They include switching voltage regulators, linear voltage regulators, low-dropout (LDO) regulators and charge pump-switched capacitor regulators.
Certain factors affect the dc/dc regulator size .For example, size of the regulator IC is determined by the feature size of the silicon process used and the maximum load current the IC is able to supply. With the latest developments in semiconductor process technology, processes with features sizes 1 micron or smaller are used to make regulator ICs that are available in small packages, such as SOT-23 and MSOP.
Another factor is the number of external components needed. Of the regulator types, switching regulators need the greatest number of external components-typically diode, inductor, and input and output capacitors. LDOs need the fewest components, typically one input and one output capacitor. Charge pump regulators need slightly more, typically an input capacitor, an output capacitor and one or two small charge pump capacitors. Unlike the switching regulators, the LDO and the charge pump regulators do not need an inductor, which can save a lot of board space and minimize height.
Meanwhile, the size of the external components is also an important factor. In switching regulators and charge pump regulators, the switching frequency of the regulator is the most important parameter that determines the size of the external components needed for given voltage and current requirements. Higher switching frequencies lead to smaller component sizes (smaller inductor and capacitor sizes) and thus smaller regulators. The switching frequency does not apply to LDO regulators as these devices operate in a linear mode.
In designing handheld devices, prudent selection of dc/dc voltage regulator ICs will result in very good battery life while minimizing the board size. Understanding the different types of regulator ICs and the efficiency and solution-size trade-offs associated with them is important. When it comes to handhelds, the mantra is: "Maximize battery life, minimize solution size."