Power Electronics

Digital Power Controller Is Fast and Flexible

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Mixed-signal IC specialist Silicon Laboratories (Austin, Texas) has entered the power market with its introduction of the Si825x family of digital power supply controllers. These controllers combine the flexibility and programmability of a digital signal processor (DSP) with the fast response of a hardware-based controller. This architecture provides the digital power control and power management functions needed for most isolated and nonisolated switch-mode power supply topologies. These benefits are achieved while using one-tenth of the board space and drawing less than 15% of the supply current of typical DSP-based power supply designs.

The Si825x architecture incorporates high-speed processing hardware for real-time loop control and a flash-based system management controller (see the figure). By splitting up the system-management and PWM control-loop functions, the Si825x is able to maintain a uniform response regardless of system task loading. The control loop maintains a 10-MHz loop update rate, updating all of the output timing signals every 100 ns.

The control loop also features a dedicated A-D converter (ADC), programmable DSP filter engine, six-channel DPWM and programmable overcurrent protection (OCP) hardware detector. To implement OCP, a peak current detector performs cycle-by-cycle current limiting, comparing the peak inductor or transformer current to an internally programmed threshold value.

Meanwhile, the system management controller performs fault detection and recovery, and control-loop optimization, such as dynamic loop compensation and dead time adjustment. The system management controller also handles PMBus communication and external device management. By reprogramming the DSP filter coefficients, the system management processor can adjust the filter's response to handle load transients as they occur.

The system management controller is based on a 50-MIPS 8051 CPU with 32 kB of flash and incorporates a 12-bit self-sequencing ADC, an SMBus interface, a UART, 16-bit timers and additional PWM channels. The 12-bit ADC can assign each of its eight input channels to various power supply parameters, digitize those signals and then store the results separately in its registers. Limit detectors then periodically compare the stored parameters with the programmed limits, and send interrupts to the 8051 processor when those parameters are out-of-bounds.

The controller targets applications that now rely on complex analog PWM controllers and use external circuitry for system-level power management. The Si825x can operate at switching speeds up to 1.4 MHz, a limit set by the DPWM resolution. At 1.4 MHz, output timing resolution is 0.4%. Fabricated in a standard 0.25-µm CMOS process, the Si825x employs automotive-grade flash to accommodate the high substrate temperatures encountered in many dc-dc “bricks.” The operating temperature range for the controller is -40°C to 125°C.

Power supply design is supported with a toolset that leverages users' existing analog design methodology. The toolset consists of a real-time firmware kernel, loop compensation and timing simulator/designer tools, system controller, configuration wizards and an integrated development environment. In addition, the company offers a development kit with a USB debug adapter, a complete half-bridge Si825x-based dc-dc converter, a PMBus interface protocol, and external host interface hardware.

The Si825x is available in a 28-pin QFN or 32-pin LQFP. Pricing for the Si825x family starts at $2.49 in lots of 1000. The chip is now sampling with general availability scheduled for the third quarter. For more information, see www.silabs.com.

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