Electronic Design

PMBus Controller Takes A State-Machine Approach

Ease of use, flexibility, and efficiency blend together in this power manager and converter package.

Recent announcements of several PMBus-based ICs for digital control of distributed power makes one fact clear: There are two different approaches to controller architecture. On the one hand, controllers are based on high-resolution, high-speed analog-to-digital converters with DSPs in the feedback loop. On the other, controllers employ comparators and hardwired digital proportional-integral-derivative (PID) state-machine filters. Zilker Labs has introduced the first product to use the latter architecture.

The company's ZL2005 is a complete power manager and converter including drivers for external FETs (Fig. 1). Zilker's positioning for the ZL2005 emphasizes that the device doesn't require software download and initialization or reset from a supervisory microcontroller to begin regulating power. In fact, the board designer needn't write any code at all. That's an option, though, because it also can be configured through an SMBus/PMBus interface to permit real-time modifications and in-field configuration changes.

The ZL2005 is even easier to use than analog converter-only products. For example, loop compensation may be accomplished by looking up pin-strap settings on a matrix using the desired loop cutoff frequency and the natural frequency of the output filter (inductor and capacitor). Also, unlike competing digital controllers, the ZL2005 operates directly from an intermediate bus without the requirement for regulated supplies.

EFFICIENCY
A most interesting point of comparison lies in the inherent power efficiency yielded by a state-machine-based architecture like that of Zilker Labs, compared to a DSP-based design. Some system designers still prefer analog control of distributed power because of its efficiency relative to DSP-based control approaches.

As switching frequency increases, the power consumed by the DSP core in controllers that use such an architecture limits system efficiency (Fig. 2). Meanwhile, the essentially static demands of the state machine provide an efficiency roughly equivalent to purely analog control approaches (Fig. 3).

The state-machine approach also means that the current-limit circuit can respond to an overcurrent condition very rapidly—within one switching cycle. (However, the user can adjust the response time to an overcurrent event.)

CURRENT SENSING
There are several ways to implement current sensing: low-side MOSFET RDS(ON), high-side MOSFETRDS(ON), current-sense resistor, parasitic conductor resistance (RL, also known as DCR) of the averaging inductor, and inductor average current (series R/C) sensing.

For current sensing, the chip has an interesting way of dealing with extreme duty cycles. The ZL2005 can be set to sample either the rising or falling inductor current waveform, depending on which has longer duration. That's a useful capability because of the way the current waveform in the averaging inductor in a synchronous buck converter behaves, being an upslope ramp during the control-FET on-time and a downslope ramp during the synchronous-FET on-time. At extreme duty cycles, related to the difference between input and output voltage, one or the other of those may be too short to measure properly.

Current-sensing accuracy depends on the method used. But excluding external tolerances, it's approximately ±5%. With MOSFET RDS(ON) sensing, the device can use the internal or external temperature sensor to calibrate for the typical temperature coefficient of the MOSFET(s) employed.

For advanced designs, the ZL2005 can bypass the response characteristic of the linear PID regulation loop with a time-based non-linear-response (NLR) loop, overriding the normal pulse-width modulation for up to two switching cycles and retriggering for an additional two switching cycles.

The current-sensing operation during NLR activity is identical to pulse-width-modulator (PWM) operation. However, the switching transitions can occur at times other than those prescribed by the usual PWM generation.

DESIGN TOOLS
Additionally, Zilker Labs offers online design and simulation tools to assist engineers in optimizing their designs and verifying the performance before they order the device (Fig. 4). The software can be found on the company's Web site. It rapidly designs and simulates a variety of different application circuits. Furthermore, it generates a printable schematic with a bill of materials.

Thanks to an evaluation board, designers can gain even more familiarity with the device and the PMBus command set. The evaluation board can be evaluated using pin-strap jumpers and an oscilloscope or by using Windows-based GUI evaluation software and a USB cable.

CHARACTERISTICS
The ZL2005 provides output voltages from 0.6 to 5.5 V from input voltages between 3 and 14 V. Its internal drivers support load currents of up to 25 A. Initial tolerance of the internal voltage reference is less than 0.4%, and it's guaranteed to be 1% over line, load, and temperature conditions. The total output accuracy is guaranteed to be within 3% of configured output voltage, including transient variations, given adequate design of the output filter stage.

The ZL2005 can be configured for switching frequencies between 200 kHz and 2 MHz. The internal oscillator accuracy is specified at ±5%. Output voltage ripple is primarily determined by the switching frequency and the design of the power train, including the output inductor and filter capacitance. A good design will exhibit an output ripple of less than 1% of the output voltage.

Connecting multiple ZL2005s in parallel yields higher total output current capability. Current sharing is primarily passive between paralleled channels thanks to the programmed source V-I characteristic.

Residual current errors can be nulled via the bus with small changes in the assigned set points of the channels. If a phase offset is desired between paralleled channels, multiple ZL2005s can be configured to start their switching cycles at 22.5° increments.

The IC comes in a 6- by 6-mm MLF package. Pricing starts at $4.25 in lots of 1000.

Zilker Labs
www.zilkerlabs.com

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