Dedicated multiphase controller ICs address processor power specs

In addition to voltage and current control, the new multiphase controllers were required to satisfy many other specification requirements: VID programming, load line regulation, power sequencing, phase current balance, monitoring, and protection. The PWM function in the multiphase controller is becoming a small part of the controller functionality. This demand for more functionality from the controller and the continuing advancement of digital technology has pushed many companies to look at digital multiphase control. The proposed resolutions vary from the use of digital processors (DSP), microprocessors, and microcontrollers to the latest software-programmable mixedsignal IC. Though analog systems continue to be the price/performance method of choice in the vast majority of applications, digital power management has carved out a niche in high-end systems. This is because digital control allows designers to create flexible power solutions that address the growing complexity of today's electrical systems.BENEFITS OF DIGITAL CONTROL

Digital controllers offer many advantages over their analog counterparts: improved system reliability, flexibility, and ease of integration, optimization and the ability to implement advance control techniques. Overall, they offer an elegant way to address many requirements in the Vcore power regulation specifications.



A system based on a digital controller requires fewer components, which decreases the mean time before failure (MTBF) of the system. For example, all the components for the feedback loop are eliminated; the select on test and select according to design specification components are also replaced by software programming. A change to the design to meet a new requirement may not require new board layout and more engineering time; the changes could be implemented in software.



The added capability of monitoring protection and prevention will also increase the system reliability. For instance, an engineer can choose to monitor the system temperature to decrease the current limit level, or to turn on a fan. This scenario will decrease the stress on the power components and fans that, in turn, will improve the system reliability and would eliminate over specification of components.



The use of software to change the controller functionality makes a system based on a digital controller very flexible. The digital controller offers the ability to add, eliminate or change any system parameter in order to meet new requirements or to optimize and calibrate the system. For example, the same voltage regulator model (VRM) can be programmed to meet different processor specifications such as load line (LL), voltage identification (VID), and current or voltage requirements without any hardware changes. Due to the ease of integrating communication capability, the digital controller also facilitates the ability to integrate and cascade multiple systems together. For example, in multi- VRM boards, current sharing could be implemented through a standard communication bus without the need for any hardware additions. Digital controllers also offer the ability to implement advanced control techniques elegantly and without much added cost or complexity.



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DIGITAL CONTROL IC IMPLEMENTATION

In order to choose the ideal digital controller IC for the application, the power supply engineer will have to take into consideration the performance and the capabilities of many digital IC blocks that normally do not exist in an analog controller IC.



a) Anti–aliasing filter

The importance of the anti-aliasing filter arises from its effect on the total system bandwidth. For example, assume that we only can have an RC type of low pass filter, with transfer function of the form:

To have –20dB gain at 1MHz, half the sampling frequency of 2MHz, the pole of H_Alias has to be at 100kHz, which will introduce a phase lag of 45° at 100kHz. If a power stage is switching at 1MHz, an analog control loop with a crossover frequency of 200kHz can easily be obtained. With this digital implementation, the crossover frequency will be limited to below 100kHz if we do not allow any phase lag to be introduced in the feedback loop by the antialiasing filter.



b) A/D converter

The ADC block consists of two main sub-circuits: the sample and hold, and the ADC itself. The sample and hold block adds a delay in the loop according to e-sTADC that can be approximated as:

In order to decrease the delay, i.e. decrease the phase lag, the sampling period TADC of the ADC needs to be increased. A wide choice of ADC architectures exists that differs in resolution, bandwidth, accuracy, and power requirements.



The flash architecture: Sets of 2n-1 comparators are used to directly measure an analog signal to a resolution of n bits. The flash architecture has the advantage of being very fast because the conversion occurs in a single cycle. The disadvantage is that it requires a large number of comparators; the number of comparators needed for an n-bit ADC is equal to 2n-1.



The successive approximations architecture: This approach uses a single comparator over many cycles to make its conversion. The successive approximation (SAR) converter only needs a single comparator, but it requires n comparison cycles to achieve n-bit resolution. For example, a 10bit conversion will introduce 10gTADC delay.



The pipelined architecture with multiple flash: A pipelined converter divides the conversion task into p consecutive stages. Each of these stages consists of a sample and hold circuit, an n-bit flash converter, and an n-bit DAC. A pipelined converter with p-pipelined stages, each with an n-bit flash converter, can produce a high-speed ADC with a resolution of k=pgnbits using pg(2n-1) comparators.



From the discussion regarding the anti–aliasing filter and the delay introduced by the ADC, we see clearly the need for high sampling rate.



The number of bits needed can be based on the resolution of the measurement required. If the maximum output regulation is äVout, the maximum voltage of the ADC is VADCmax and the output voltage Vout is scaled by a gain G to meet the ADC voltage levels, then the least significant bit of the ADC has to be less than the product of the maximum ripple and the gain.



c) Digital pulse width modulator An analog controller has no inherent limit on the possible pulse width generated, but the DPWM produces a discrete and finite set of PWM widths. From the output point of view, only a set of discrete output voltages is possible. Because DPWM is part of the feedback loop, it is necessary that the resolution of the DPWM be high enough so that the output will not display what is known as a limit cycle.



The minimum number of bits needed depends on the topology, the output voltage, and the ADC resolution. For example, in the buck converter:



PAY ATTENTION

Over the years, power supply designers have gained a strong knowledge of how to choose an analog power supply controller in order to meet the required specifications. With the introduction of digital controllers, the choices are not as clear. The digital controller requires the designer to pay attention to more complex IC blocks, in order to achieve the digital controller advantages.



The anti-aliasing filter, ADC, and the DPWM are the heart of the digital controller IC. Special design considerations of these blocks are key to achieving superior overall system performance.