Nonisolated Power Converters Simplify Voltage Sequencing

Jun 1, 2003 12:00 PM
By Ashok Bindra, Editor, Power Electronics Technology

Combining techniques such as auto-track, margin up/down control, and pre-bias startup with clever circuit design and double sided surface mount construction, Texas Instruments Inc. (TI) has readied its next-generation nonisolated plug-in power modules that are up to 96% efficient and 70% smaller than previous board-mounted converters. TI's new PTHxx series of point-of-load (POL) plug-in power modules are aimed at networking, server, workstation, and industrial applications using DSPs, FPGAs, ASICs and microprocessors that require multiple voltage rails — low voltage to power a high-speed logic core and a standard voltage to power I/O interface and other supporting system devices like memory, data converters and I/O ports.

Consequently, system designers are developing power solutions that must tackle multiple POL supplies and the ability to sequence the power-up and power-down of multiple supply voltages is critical in such power solutions. TI's unique auto-track sequencing feature in PTHxx series eases this task as it permits nonisolated power modules to track each other, or any external control voltage. In its simplest implementation a designer only needs to connect the track pins of the modules together to seamlessly meet sequencing requirements (figure). The feature makes it possible for the output voltage of these modules to be directly controlled (volt-for-volt) below their respective set-point voltages. This added flexibility allows such modules with different output voltages to be easily configured for simultaneous power-up and power-down voltage sequencing. The auto-track feature is not limited to just two modules or output voltages. You can use it to configure a number of modules with different output voltages for supply voltage sequencing. Its set point tolerance is less than 1%.

In essence, auto-track uses a control pin called Track to control the output voltage of the module over the range, 0V up to the nominal set-point voltage. Within this range the voltage at the module's output will follow the voltage applied to the Track pin on a volt-for-volt basis. However, once the voltage at the Track pin is raised above the module's set-point voltage, the module's output remains at its set-point voltage. As an example, if the Track pin of a 2.5V regulator is at 1V, the regulated output will be 1V. But if the voltage at the Track pin rises to 3V, the regulated output will not go higher than 2.5V.

The PTHxx series also provides advanced pre-bias startup capabilities and margin up/down control. While pre-bias startup prevents modules from sinking current during startup, margin up/down increments output voltage up or down as much as 5%. Other features include on/off inhibit, and output over-current protection. These single output plug-in power modules support step-down dc-dc conversion from a 3.3V, 5V or 12V input with adjustable output voltages from 0.8V to 5.5V at output currents up to 30A. Unlike others, the 30A version is a two-phase solution. For 3.3V input model, the adjustable output range is 0.8V to 2.5V, while the 5V version offers 0.8V to 3.6V, and the 12V input unit provides output of 1.2V to 5.5V. It needs an external resistor for adjusting the output. The size of the 6A member is 18.92 mm × 12.57 mm × 7.62 mm. Thus, it occupies board space that is about half the size of a postage stamp. According to TI, a 40A version is also in the works.

Sampling now, the PTHxx series power modules are slated for production by the end of second quarter. Pricing per module in quantities of 1,000 begins at $9.95.

Texas Instruments, Dallas, www.ti.com CIRCLE 349 on Reader Service Card