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Electronic Design

Processor's Precise Pulse Targets Power PWM

This novel approach for generating high-speed timing avoids going gigahertz.

Timing is everything in dc-dc conversion. A high-resolution, 16-channel pulse-width-modulation (HRPWM) module from Texas Instruments can control the edge position to a 150-ps accuracy. This is a neat trick for the 100-MHz TMS320F28044, since it doesn't generate a faster clock internally to handle the 24-bit PWM support (Fig. 1).

Instead, the chip uses the upper 16 bits in a conventional fashion with counter/compare registers. The lower 8 bits delay the pulse edge based on a timing system that's accurate to 150 ps. This permits increased edge timing without resorting to a clock that would have to run 256 times faster than its current speed.

A pair of registers controls the low and high times within a cycle. The PWM module can operate in the 24-bit HRPWM mode or by using conventional 16-bit PWM timing. This makes it possible to attain the accuracy when necessary.

The 16-channel PWM unit can be used for a variety of purposes. Its primary target is power control, like that for dc-dc buck converters. Precise timing can significantly improve the analog response, generating a more stable output (Fig. 2). With a large number of channels, the DSP can control redundant, parallel converters as well as support multiple converters generating different voltages.

The TMS320F28044 is a 32-bit DSP, so it's quite capable of handling an array of chores while the HRPWM operates in parallel. The processor operates at 100 MHz and has up to 128 kbytes of flash and 20 kbytes of RAM. The digital peripherals are varied but not numerous, since the HRPWM will use most of the multiplexed pins. The chip does feature a 12-bit analog-to-digital controller with 16 channels to match the HRPWM.

TI makes a large amount of C source code available. Therefore, developers needn't start from scratch. It's designed to work with the TMS320F280xx and TI's Code Compuser Studio DSP integrated development environment.

Pricing starts under $5. It comes in a 64-pin thin quad flat pack. The starter development kit costs $495 and includes a copy of C2000 Code Composer Studio. PFC and a dc-dc adapter board go for $295.

Texas Instruments

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