Battery-based systems cover a multitude of applications ranging from cell phones to Personal Data Assistants (PDAs), games, and medical instruments. These systems require effective power management to optimize equipment size and battery lifetime.
A battery-based power management subsystem includes the battery and the voltage regulation circuit that supplies power to the system. Its important design objectives include:
- Achieving performance and time-between charge goals, while minimizing battery size and weight through an efficient system design.
- Providing the appropriate output voltage regulation over a wide input voltage range, letting the battery-based system operate properly when battery voltage drops.
- Decreasing the pc-board space required by the power-management subsystem because it affects overall system size.
- Minimizing the heat dissipation of the power-management subsystem to eliminate the need for sophisticated thermal management that adds size, weight, and cost.
- Optimizing circuit layout of the power-management subsystem to prevent electromagnetic interference (EMI).
- Maximizing the reliability of the power-management subsystem.
Begin With Battery Selection
To meet these design objectives, the power-management subsystem design starts with the battery. Categories include the primary type, or non-rechargeable batteries, and rechargeable batteries. Popular primary batteries are the alkaline and conventional lithium metal cells, considered to be throwaway items. Exercise care with lithium metal cells because they are flammable.
Some popular rechargeable batteries are:
- Nickel Cadmium (NiCd) has long life, high discharge rate, and an economical price.
- Nickel-Metal Hydride (NiMH) has a higher energy density than NiCd at the expense of reduced cycle life.
- Lithium ion (Li-ion) provides high energy density and light weight.
- Lithium Polymer (Li-Pol) has chemistry similar to the Li-ion in terms of energy density, but it is safer to use and has better package flexibility.
Every Application Is Unique
Different loads call for different batteries and different approaches to managing the power for the battery and its load.
Microprocessors:Core power handles the processor, memory, and so forth, which requires the designer to know what the loads are before the design can begin. Synchronous rectifier outputs are the most efficient for loads in the 200- to 500-mA range. Step-down regulators power processors that operate in the 1- to 1.8-V range.
If the processor stays on during standby mode, you must consider the efficiency of the regulator at very light loads. Modern processors have leakage currents in the 100- to 200-mA range, where it is difficult to make an efficient regulator. If efficiency is less important, you can use regulated charge pumps, whose typical efficiency is about 70%. Consider employing an LDO if the loads require 2.5 V or more (but under the minimum battery voltage) at 250 mA, or as low as 1.5 V at 100 mA or less, or when the usage of the device is low.
Display Lighting: Power management of light-emitting diodes (LEDs) involves the use of voltage regulators specifically intended for these applications. Power-converter technologies for these applications include either charge pumps, step-up or step-down switch-mode regulators, or LDOs. Some lighting ICs may have more than one type of topology.
LED voltage regulators supply current to either individual or series-connected devices. When driving series-connected LEDs, the regulator IC must supply a high enough voltage to accommodate the number of LEDs in the series string. Thus, these regulator ICs usually step up their applied voltage. Individual LED drivers provide only the voltage required by a single LED, but each output must be capable of providing the appropriate current.
Cell phones now employ color liquid-crystal displays (LCDs) that require a clean white backlight. Although LEDs can provide good backlighting for these LCDs, there are some design challenges:
- Their forward voltage drop is in the 3- to 4-V range, which requires a buck-boost voltage-regulator IC with Li-ion batteries.
- LEDs can be connected in series or parallel.
- The parallel connection requires matched LEDs, or their forward voltages will vary.
Nowadays, charge-pump and boost-regulator ICs are the most widely used topologies.
RF Power: LDOs are used to dominate this power-management application because they are optimized for noise and power-supply rejection ratio (PSRR). The LDO is important because RF loads tend to require higher voltages. PSRR is important because it affects the LDO’s ability to prevent output voltage fluctuations caused by input voltage variations. Output capacitors having low equivalent series resistance (ESR) and added bypass capacitors improve PSRR performance. Today, switch-mode buck regulators can power 1 to 3 cores plus memory, as well as the RF power amplifier, because these devices tend to consume the most power and their operating voltages are approaching 1 V.
Typical cell-phone handsets can have up to 10 separate power supplies, usually implemented as independent LDO regulators. In addition, the phone’s power amplifier obtains its power from its own independent power source, usually a separate voltage regulator.
The power amplifier’s power source must exhibit high efficiency, so it includes an on-chip synchronous rectifier. In addition, it must consume very little power in shut-down mode. Furthermore, it must employ small surface-mount packages that occupy minimal space.
To maximize efficiency and extend battery life, unused cell-phone subsections must be powered down. For example, the phone can’t ring while the power amplifier transmits, so ring circuits can be shut down. Similarly, when the phone is in standby mode, the power amplifier is not needed, so it can be shut down.