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Analog's Driving Force
Driver ICs take advantage of analog technology
Randy Frank, Contributing Editor -- Design News, June 2, 2008
Utilizing analog building blocks such as comparators, charge pumps, a bootstrap function, under- and over-voltage detection/protection and more, analog and mixed-signal integrated circuits (ICs) make driving lighting and motor loads easier, while providing other advantages including space savings, improved performance and reduced cost. With any IC, the additional advantages are essentially a given. The trick with driver ICs is dealing with the high voltages and currents. Depending on the target applications, the voltage and current capabilities can vary considerably and the newest design approaches integrate even more functionality. The most common driver ICs control LEDs or motors. A few industry firsts result from extending the design technology boundaries.
One of the newest drivers integrates isolated power and isolated gate drivers into a single package. Analog Devices Inc. claims an industry first for its ADuM5230 and ADuM6132 isolated half-bridge gate drivers.
“Customers have been looking for solutions that reduce the number of components in power supplies and motor drives as a way to improve quality and reliability and to reduce size and cost,” says David Krakauer, product line manager, iCoupler Isolation Products, Analog Devices Inc. “We saw an opportunity with our iCoupler technology to integrate the isolated dc-to-dc converter into the same package as the isolated gate drivers.”
Using ADI's iCoupler technology (see sidebar, page A6), ADI engineers were able to integrate an isolated power supply and isolated gate drivers in the same package. In addition to eliminating the separate power isolation required for optoelectronic isolation, ADI's integrated approach using high voltage CMOS technology eliminates components, reducing system cost and size by as much as 50 percent.
Designed for motor drives and plasma displays, as well as power inverters in power supplies and solar power converters, the ADuM5230 and ADuM6132 come with safety approvals normally required for discrete-based isolated power supplies.
Differing in power capability and high-side/low-side isolation configuration, the ADuM5230 supplies 150 mW of power and the high-side and low-side outputs are isolated from each other, as well as from the inputs. In contrast, the ADuM6132 supplies 250 mW of power but only the high-side output is isolated from the input.
Providing true galvanic isolation between the inputs and outputs, the outputs on both drivers may be operated up to peak voltages of ±700V relative to each other. To support low-side switching to negative voltages, the isolated outputs of the ADuM5230 may be operated up to ±700V relative to the inputs.
Driving LEDs
For driving LEDs, Power Analog Microelectronics says it has the industry's first high-voltage LED driver with an integrated 3A power MOSFET. Capable of driving up to 72 white LEDs, the PAM2846 provides six string current regulation for large LCDs that employ an array of LEDs for backlighting.
“We are excited at being able to deliver the industry's first 7W LED driver capable of driving up to 72 LEDs,” says Johnston Chen, president and chief executive officer of PAM. “By integrating the switching device, we truly reduce the external component count and lower the overall system cost for LCD systems.”
Based on high-voltage bipolar-CMOS-DMOS (BCD) technology, the driver has a 4.5 to 26V input voltage range and provides a fixed 20 mA or adjustable 5 to 30 mA full scale LED current. The current mode step-up converter has built-in string-current-control circuits and achieves ±3 percent regulation accuracy between strings to ensure uniform brightness for all LEDs.
Features in the regulator include fault protection, cycle-by-cycle current limiting for consistent operation and soft start capability, programmable dimming control and separate feedback loops that limit the output voltage if one or more LEDs fail in the open or short mode. Housed in a 4 x 4 mm 16-pin QFN package, the driver targets slim LED displays from 7- to 15-inch panel sizes frequently found in notebook PCs, portable DVD players , LCD monitors, avionics and automotive displays.
Other semiconductor companies have also developed improved drivers for LEDs. According to Patrick Durand, worldwide applications manager for Future Lighting Solutions, “The next generation of high brightness LEDs, such as Luxeon® K2, require harder driving in order to maximize lumen output.” For these applications, Zetex Semiconductors designed a family of three flexible LED drivers with higher current capability, as well as increased features.
Chris Jolly, vice-president of marketing, Zetex Semiconductors, says the ZXLD132X drivers combine the dc-to-dc converter, a current monitor for accurately stabilizing the output current, the thermal control circuitry and a 2A NPN switching transistor into a 3 x 4 mm DFN package. Products in the family drive single or multiple LEDs with currents from 700 mA to 1.5A in buck, boost and buck/boost configurations.
Handling an input voltage range of 4 to 18V, the ZXLD1320 buck mode driver delivers up to 1.5A continuous LED current for a maximum power up to 24W. Halogen lamp replacement and battery-driven applications can benefit from the driver's power capabilities, as well as its additional features. Those features include a variable ON and OFF time control scheme, feedback control circuit for load and current regulation and thermal control circuitry.
With similar features, the boost mode ZXLD1321 operates with an input voltage range of 1 to 12V and delivers an LED current of 1A and the buck/boost mode ZXLD1322 produces 700 mA for an input voltage of 2.5 to 15V. These drivers target high-power LED flashlights, LED back-up lighting and general LED applications.
Driving Motors
In motor control applications, engineers want to get more in less space, just like everyone else, and IC suppliers take different approaches to deliver more.
For industrial motor control, International Rectifier integrated an additional gate for a power factor correction (PFC) switch or inverter brake in its protected three-phase 600V driver IC.
“The IRS26302D integrates multiple functions including an extra gate drive and extensive protection features that simplify the gate drive circuit design improving reliability and minimizing system cost,” says Aengus Murray, director for iMOTION Products, Energy-Saving Products Group, International Rectifier.
Operating up to 600V, the IC has three high-side and three low-side referenced output channels that provide 200 mA/350 mA drive current at up to 20V MOS gate drive capability, as well as the low-side driver for a PFC switch or inverter brake. With its integrated bootstrap functionality, the driver can reduce the bootstrap power supply components to only three instead of six.
Using an advanced high-voltage IC process with high-voltage level-shifting and termination technology, the design includes over-current and over-temperature detection input, as well as under-voltage lock-out protection, integrated deadtime protection and compatibility with 3.3V input logic. “IR's next generation level shifting technology has improved robustness, noise immunity and new circuit features that improve immunity to negative voltage spikes generated in typical high current industrial drive systems,” says Murray.
In some cases, especially space-conscious motor control in portable electronic products, a single motor driver is not enough. For example, Linear Technology Corp.'s LT3572 dual piezo motor driver targets auto focus and zoom motor control for digital still and video cameras.
“The reason for two motor drivers in this IC was to create image stabilization — one driver for each axis,” says Steve Pietkiewicz, vice-president and general manger, Linear Power Products, Linear Technology.
The integrated dual full-bridge driver can handle two piezo motors operating at up to 40V with independent ON or OFF capability. With 2.7 to 10V input voltage, the driver works in both single- and dual-cell Li-Ion powered digital cameras. To keep external components small, the switching frequency of the integrated 900 mA boost converter can be programmed between 500 kHz and 2.25 MHz. The 4 x 4 mm QFN packaged driver includes soft-start and external synchronization features.
All of these drivers deliver on the semiconductor promise of space savings, improved performance and reduced cost while providing application-specific features for the targeted applications. With voltage capabilities up to 700V and the ability to handle several amperes of current, they demonstrate one of the domains that rely heavily on analog technology.
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