Mixed-Signal ICs Manage Battery-Based Power Supplies
Virtually all battery-based power supplies must be custom-designed with supporting ICs that manage system operation. Some form of battery power management is employed in portable phones, MP-3 players, digital cameras, etc. This has been made possible by the availability of mixed-signal power management ICs that provide the required functions for increasingly complex electronic systems. Fig. 1 shows the basic power management subsystem employed in a battery-based system. Battery-based systems do not necessarily employ all these individual components, because multiple functions can now be integrated onto a single chip.
Generally, these power management subsystems must:
- Minimize battery size and weight while maximizing available run time.
- Provide the appropriate regulated output voltage over the specified input voltage range, load current and ambient temperature.
- Minimize overall space and weight of associated components.
- Minimize heat dissipation to eliminate the need for sophisticated thermal management that adds size, weight and cost.
- Allow a circuit board layout that minimizes EMI.
- Maximize system reliability
Battery Selection
To meet these design objectives, the power management design begins with the battery that may be a non-rechargeable primary battery or a rechargeable secondary battery. Primary batteries examples are alkaline and lithium metal cells. A decade ago, the popular rechargeable batteries were nickel cadmium (NiCd) and nickel-metal hydride (NiMH). Today, the lithium-ion (Li-Ion) and lithium-polymer (Li-pol) are the most widely used.
Lithium-ion batteries have the greatest electrochemical potential and the highest energy density per weight. The Li-Ion battery is safe, provided certain precautions are met when charging and discharging. Li-Ion energy density is about twice that of the standard NiCd. Besides high capacity, Li-Ion load characteristics are reasonably good and behave similarly to the NiCd in terms of discharge characteristics. Its relatively high cell voltage (2.7V to 4.2V) allows one-cell battery packs.
Exercise caution when handling and testing Li-Ion batteries. Do not short circuit, overcharge, crush, drop, multilate, penetrate, apply reverse polarity, expose to high temperature or disassemble. Use the Li-Ion battery with its designated protection circuit.
The Li-pol battery differs from the Li-Ion type in its fabrication, ruggedness, safety and thin-profile geometry. Unlike the Li-Ion, the Li-pol has minimal danger of flammability because it does not use a liquid or gelled electrolyte like the Li-Ion. The Li-pol has simpler packaging and a lower profile than the conventional Li-Ion battery.
Battery Charger ICs
Battery chemistries have unique requirements for their charge technique, which is critical for maximizing capacity, cycle life and safety. Linear topology works well in applications with low-power (e.g., one- or two-cell Li-ion) battery packs that are charged at less than 1A. However, switch-mode topology is better suited for large (e.g., three or four series Li-ion or multiple NiCd/NiMH) battery packs that require charge rates of 1A and above. Switch-mode topology is more efficient and minimizes heat generation during charging, but can produce EMI if not packaged properly.
The charge and discharge capacity of a secondary battery is in terms of “C,” given as ampere-hours (Ah). The actual battery capacity depends on the C-rate and temperature. Most portable batteries are rated at 1C. A discharge of 1C draws a current equal to the rated capacity, that is, a battery rated at 1000mAh provides 1000mA for one hour if discharged at 1C rate.
Li-Ion batteries have a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge when reaching full charge. Charge time for Li-Ion batteries charged at a 1C initial current, is about three hours. Full charge occurs after reaching the upper voltage threshold and the current drops and levels off at about 3% of the nominal charge current. Increasing Li-Ion charge current has little effect on shortening the charge time. Although it reaches the voltage peak faster with higher current, the topping charge will take longer. Li-Ion batteries cannot absorb overcharge, which can cause the cell to overheat. Li-Ion constant-current- constant-voltage (CCCV) chargers are important to get the maximum energy into the battery, without overvoltage.
Lithium-based chargers require tighter charge algorithms and voltages. Avoid a charge rate over 1C for lithium battery packs because high currents can induce lithium plating. With most lithium packs, a charge above 1C is not possible because the integrated protection circuit limits the amount of current the battery can accept.
Analog Device’s ADP5065 charger (Fig. 2) enables charging via the mini USB VBUS pin from a wall charger, car charger, or USB port. It operates from a 4 V to 5.5 V input voltage range, but tolerates voltages of up to 20 V, alleviating concerns of USB bus spiking during disconnect or connect scenarios.
Battery Monitor ICs
Portable systems are sensitive to usable battery life, particularly for computers where a loss of power could mean a loss of stored data. Therefore, it is useful to provide a real time indication of remaining battery life. One approach is a battery monitor that accumulates battery data and transmits it to a host processor. Among the monitored battery parameters are overcharge (overvoltage), overdischarge (undervoltage) and excessive charge and discharge currents (overcurrent, short circuit), important information Li-Ion battery systems.
Another type of battery monitor is the “gas gauge” IC usually found within a battery pack. Specific inputs on the gas gauge IC connect directly to the battery, so those inputs must consume very little power. Otherwise, battery life will be reduced during long storage periods. Initially, the battery must be fully charged and the counters and registers set to states consistent with a fully charged battery. As discharge occurs, the gas gauge IC tracks the amount of charge removed from the battery.
Most battery gas gauges compensate for both temperature and charge/discharge rate. Typically, they display the available charge on LEDs and also can also send charge data to an external processor. The LED presentation usually consists of five or six segments of a “thermometer” display. To conserve battery power, the display is only activated at the user’s command. At full charge all the LED segments are lit. As battery life decreases, the gas gauge IC extinguishes successive segments on the thermometer display.
The gas gauge IC calculates the available charge of the battery while compensating for battery temperature because the actual available charge is reduced at lower temperatures. For example, if the gas gauge IC indicates that the battery is 60% full at 25°C, then the IC indicates 40% full when cooled to 0°C, which is the predicted available charge at that temperature. When the temperature returns to 25°C, the displayed capacity returns to 60%.
Maxim’s MAX17048 and MAX17049 ICs are micropower current “gas gauges” for lithium-ion (Li+) batteries (Fig. 3). The MAX17048 operates with a single lithium cell and the MAX17049 with two lithium cells in series. The ICs automatically detect when the battery enters a low-current state and switches into a low-power 4FA hibernate mode, while still providing accurate fuel gauging. The ICs automatically exit hibernate mode when the system returns to the active state.
Battery Protector ICs
An added requirement for Li-ion battery packs is a protection circuit that limits each cell’s peak voltage during charge and prevents the voltage from dropping too low on discharge. The protection circuit limits the maximum charge and discharge current and monitors the cell temperature. This protects against overvoltage, undervoltage, overcharge current, and overdischarge current in battery packs
Ideally, the protection circuit should consume no current when the battery-powered system is turned off. However, the protector always consumes some small current. A single-cell rechargeable Li+ protection IC provides electronic safety functions required for rechargeable Li+ applications, including protecting the battery during charge, protection of the circuit from damage during periods of excess current flow and maximization of battery life by limiting the level of cell depletion. Protection is facilitated by electronically disconnecting the charge and discharge conduction path with switching devices, such as low-cost N-channel power MOSFETs.
Battery Power Supply ICs
Virtually all battery-based systems are intended for portable operation. As such, their power supplies have requirements that dictate the associated power supply controller IC configurations. This also means that the controller ICs should require very few low-cost external components. To minimize size and weight, the IC should be packaged in some form of small outline package. Plus, the application will determine whether the controller should provide step-up, step-down or some other topology.
One tradeoff in selecting a controller IC is whether it employs external or on-chip power MOSFET switches. On-chip devices minimize external components, but have the potential for increasing the junction temperature and degrading thermal performance. Depending on the package employed, this could also reduce the current carrying capacity of the IC. Some controller ICs have on-chip power MOSFETs, others require external MOSETs. Another design consideration is reducing power dissipated by the power supply, which in turn increases battery run time.
Most controller ICs have a shutdown pin that disables the power supply, cutting battery drain. This can be done in many systems that have a normal “sleep” mode. When the IC comes out of the shutdown mode, it has to do so without upsetting the system. Also available in most battery-based controller ICs is undervoltage lockout (UVLO) that shuts down the power supply if the battery voltage drops below a specific threshold; if the voltage drops too far, the power supply will shut down. Another characteristic of these controller ICs is protection against overcurrent, which protects both the controller IC and associated system components.
Power Management ICs
The ability to integrate many functions on a single chip has led to development of Power Management ICs (PMICs). They can perform multiple functions in a battery-based system, such as battery charging, dc-dc conversion, battery protection, battery monitoring, and power source selection. These mixed-signal devices employ analog and digital circuits to manage power supply functions.
Many PMICs employ the I2C bus, a bidirectional two-wire serial interface. There are two bus lines: a serial data line (SDA) and a serial clock line (SCL). Serial, 8-bit oriented, bidirectional data transfers can be made at up to 100 kbit/s in the standard-mode, and up to 400 kbit/s in the fast-mode. This serial interface adds flexibility to power supply operation by enabling functions to be programmed to new values.
A PMIC is both an I2C slave receiver and slave transmitter that communicates with an associated bus master. Built-in timing delays ensure correct operation when addressed from an I2C compliant master device. There are also input filters that suppress glitches should the bus become corrupted.
A PMIC that uses the I2C bus is Maxim’s MAX8660 and MAX8661 that have four step-down DC-DC outputs, three linear regulators, and an eighth always-on LDO (Fig. 4). Two dynamically controlled DC-DC outputs power the processor core and internal memory. Two other DC-DC converters power I/O, memory, and other peripherals. Additional functions include on/off control for outputs, low-battery detection, and reset output.
The MAX8661 functions the same as the MAX8660, except it lacks the REG1step-down regulator and the REG7 linear regulator. All step-down DC-to-DC outputs use 2MHz PWM switching. In addition, a forced-PWM option provides low noise operation at all loads. Overvoltage lockout protects the device against inputs up to 7.5V.
I2C programmability
Freescale Semiconductor’s MC34704 is a multi-channel PMIC (Fig. 5) that also has I2C programmability. Switching regulators provide either five or eight independent output voltages with a single input power supply (2.7 and 5.5 V) and up to ±2% output voltage accuracy. It is intended for portable devices powered up by Li-Ion/polymer batteries or USB powered devices.
Except REG7, each switching regulator has output under-voltage and over-voltage detection, over-current limit detection and short-circuit protection, dynamic voltage scaling, and thermal limit detection,. REG1, REG3, REG6, and REG8 have integrated compensation, and REG8 has selectable output voltage or current regulation.
ON Semiconductor offers the NCP6914, a member of the company’s mini-PMIC family (Fig. 6). These parts are optimized to supply battery powered portable application subsystems such as camera function and microprocessors. These devices integrate one high efficiency 800 mA step−down DC-to-DC converter with DVS (Dynamic Voltage Scaling) and four LDO voltage regulators. The step-down DC-DC converter (3 MHz, 1 µH/10 µF, 800 mA) exhibits 95% peak efficiency and provides programmable output voltage of 0.6 V to 3.3 V by 12.5 mV steps. The NCP6914 has four LDOs (300 mA) that exhibit 50 µVRMS typical output noise and a programmable output of 1.0 V to 3.3 V by 50 mV steps.
Among the converter’s features are a hardware enable pin, power good and interrupt output pin, external synchronization, and customizable power up sequencer.
The TPS65910 from Texas Instruments is an integrated power management IC available in 48-QFN package and dedicated to applications powered by one Li-Ion or Li-Ion polymer battery cell or 3-series Ni-MH cells, or by a 5-V input; it requires multiple power rails. It includes three step-down converters, and one step-up converter (Fig. 7). Two of the step-down converters provide power for dual processor cores and are controllable by a dedicated class-3 SmartReflex interface for optimum power savings. The third converter powers system I/Os and memory.
EIGHT LDoS
The TPS65910 IC includes eight general-purpose LDOs providing a wide range of voltage and current capabilities; they are fully controllable by the I2C interface. LDO use is flexible; they may be used as follows: Two LDOs are designated to power the PLL and video DAC supply rails on the OMAP based processors, four general-purpose auxiliary LDOs are available to provide power to other devices in the system, and two LDOs are provided to power DDR memory supplies in applications requiring these memories.
Besides its power resources, the IC contains an embedded power controller (EPC) to manage the system’s power sequencing requirements and real time clock (RTC).
Linear Technology offers the LTC3675 (Fig. 8). This part is a digitally programmable high efficiency multi-output power supply plus dual string LED driver IC optimized for high power single cell Li-Ion/Polymer applications. The parts has four synchronous buck converters with outputs of 1A, 1A, 500mA, and 500mA, one synchronous 1A boost DC-DC, and one 1A buck-boost DC-DC, all powered from a 2.7V to 5.5V input. The 40V LED driver can regulate up to 25mA through two LED strings with up to 10 LEDs each. You can also configure the LED driver as a general-purpose high voltage boost converter.
Using the I2C bus, you can independently program the part’s DC-DC converter operation, switch slew rates, and operating modes. Or, the user can program the IC to operate in standalone mode via simple I/O and power-up defaults. The buck DC-DCs may be used independently, or can be paralleled to achieve higher output currents with a shared inductor.
The LTC3675’s functions including LED enable, 60dB brightness control and up/down gradation are programmed using the I2C bus. Alarm levels for low VIN and high die temperature may also be programmed via the I2C bus, with a maskable interrupt output to monitor DC-DC and system faults.
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