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Improving Power Management
Common and unique requirements determine different approaches
Randy Frank, Contributing Editor -- Design News, June 2, 2008
Managing the power in different products frequently dictates a somewhat unique solution for each application. Among the key differences are operating voltage range, temperature and unique interface requirements. However, common circuitry in some applications also allows an integrated power management chip to cover a broad range of end products. Before looking at the unique requirements, an example of a power management circuit that can handle the common requirements of several products provides a good starting point for understanding the difference between the two approaches.
Addressing Common Power Requirements
Some of the common requirements for power management include shrinking the size and increasing the efficiency of the power supply. A switching regulator that targets these goals and meets other common requirements, including the temperature and voltage range, can find a home in many products. ROHM Electronics' recently introduced BD9870, a single-channel step-down switching regulator, falls into this category.
In addition to integrated protection and compensation circuitry, by operating at 900 kHz, the BD9870 uses smaller external components and is compatible with a ceramic input/output capacitor resulting in space savings of up to 60 percent. The regulator integrates a switching voltage regulator, a 1.5A P-channel power MOSFET and compensation circuitry into a 6.5 x 9-mm footprint with a TO-252S-5 package. Using the IC, a complete power supply requires only an external coil, diode and output capacitor.
The unit can achieve 88 percent efficiency in systems where the input voltage can range from 8 to 35V. Additional performance capability includes output accuracy of ±1.0 percent, as well as 3 mA bias current and zero standby current. With its efficiency and space savings, the switching regulator meets the requirements of a variety of applications, including DVD players, TVs, gaming consoles, PCs, car audio and navigation systems, and industrial and office equipment.
Unique Power Management Solutions
Automobiles provide one of the more difficult power management environments. In addition to temperature extremes for electronic components that can range from -40 to 85C in the passenger compartment and up to 125C under the hood, the 12V car battery is rarely 12V. The vehicle's charging system maintains a temperature compensated voltage of about 13V on hot days and around 15V on very cold days. A jump start could double the voltage to over 24V and a fault condition called load dump, where the alternator is momentarily disconnected from the battery, causes even higher voltage transients.
For this more challenging power management environment, Microchip Technology's MCP1790 and MCP1791 high-voltage linear regulators (LDOs) provide operation at continuous high-input voltages. With the ability to operate at a continuous input voltage of up to 30V and a handle load dump up to 43.5V, the regulators deliver a continuous 70 mA output. Additional capability for automotive applications includes low quiescent and shutdown current.
The LDOs target automotive applications that use power directly from the car's battery, such as instrumentation lighting, power windows and locks, stereo systems and electronic accessory power adapters. In addition, the LDOs also meet the voltage requirements of industrial applications, including security systems, fire alarms, thermostat controls and sensors.
A much more focused automotive IC, STMicroelectronics' L5961 is an integrated power-management solution for MOST (Media Oriented Systems Transport) networks. According to Domenico Rossi, general manager of ST's Car Multimedia Div., “The L5961 is the industry's first integrated MOST power-management solution, implementing both power supply and power management for the network processor.”
In addition to both the power supply and MOST-compliant power-management control logic, the IC has diagnostic and full system monitoring for the power supply, network status, wake-up events and temperature. Offering multiple power modes, including an Ultra-Zero Power Mode with a typical quiescent current of only 5 µA, the L5961 has an integrated 5 or 3.3V, 650 mA dc-dc converter, two voltage regulators for powering the MOST fiber-optic transmitter and fiber-optic receiver, a diagnostic transceiver for the network physical layer and three battery detectors for voltage monitoring.
In vehicles, the MOST technology the power management IC addresses primarily connects multimedia devices, such as CD or DVD players, car radios and personal navigation devices.
While quite different from automotive, portable consumer electronic products have numerous power management challenges. Temperatures are a mere fraction of the automotive range but the power frequently comes from a single lithium-ion battery and different voltages are required depending on the subsystem. Focusing on the requirements for the LCD or OLED displays used in these portable products, austriamicrosystems designed a dc-dc boost converter to deliver 50V from a single Li-Ion cell. Offering micro-power consumption with an efficiency of up to 90 percent, the AS1340 delivers 50 mA at 24V or 30 mA at 36V with adjustable output voltages in the range of 2.7 to 50V. Since the unit operates from a single 2.7 to 5.5V supply at a 1 MHz fixed switching frequency, the predictable output noise can be easily filtered. In addition, the high frequency allows the use of small inductors and capacitors minimizing the PCB footprint.
Ultimately, any power management system is embedded in an end product — unless the end product is simply an application-specific power supply. This is the case with Pepperl+Fuchs' ODZ-MAH-CHARGER. Designed specifically for use with the company's GRIP2 and GRIP3 model bar code readers, the battery charger has a footprint of 9.125 x 3.4 x 2 inch and weighs only 4.8 oz. In addition to the 5V dc power supply, the unit has an LED indication of the battery charge status and an optional Bluetooth modem provides greater functionality. The battery charger charges a GRIP2 handle bar code reader in approximately three hours and a GRIP3 handle in six hours.
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