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Posted on 02 June 2019

Novel fourth generation digital PWM controller eliminates external loop compensation, yet offers inherent stability

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Achieving a stable power supply capable of providing optimum performance over a wide range of operating conditions has always been challenging. The world of power has seen several major shifts in the last two decades as it attempts to deal with this problem.

By Chance Dunlap – Senior Marketing Manager, Intersil

One of the recent innovations in this area is with digital control. To date, several vendors have been successful in applying digital control to power supplies and are beginning to see broad adoption in select markets owing to falling costs and improving performance. This article discusses the latest capabilities of digital control with a new part from Intersil, the ZL8800, a dual step-down PWM controller.

The key reason why digital power is often selected is for the need of telemetry (system monitoring) and control flexibility, whereby one design can support a range of applications. The ZL8800 adds to this capability, through implementation of digital control for stability, giving designers extra degrees of freedom by enabling designs without the considerations of component aging, variation and thermal stress.

The Standard Route to a Stable Power Supply

When designing a supply in a complex system such as might be found in data centers or cellular base stations, the standard power distribution strategy is to provide a dc power bus with local point of load (POL) units supplying the needs of an individual board. This aids system modularity and helps infrastructure operators, who are focused on system up time, and power efficiency.

The traditional design route has been to use an analog fixed frequency switching approach with voltage or current mode feedback. Selecting a fixed switching frequency leads to predictable currents in the passive energy storage components (the inductors and capacitors) easing their selection. These components are then sized based on the needs of the output: load current, output voltage ripple, etc. The challenge for the power designer is stabilizing the loop once the power train components are set. A problem that is compounded when variations and worse case analysis (including environmental) must be taken into account. The end result is often a design that sacrifices performance and bandwidth to satisfy the stability requirements of the system over its entire operating conditions.

Consider the problem of component tolerances with the selection of the inductor. These non-linear components have variances based on current, temperature, switching frequency and time. Non-ferrite inductors are widely used, yet these devices, over their rated current range, may vary by as much as 50%, representing a real optimization challenge. Output capacitors equally exhibit the same variation when you account for temperature, DC bias, and aging. As a result, the only option left for the designer attempting to create a stable loop is to dramatically reduce system bandwidth. Transient performance is only then met through over-sizing the output capacitors to balance the sluggish loop performance, burdening the design with a large footprint and increased bill of materials.

The Digital Way

Digital control can remove this trade off and Intersil, through its acquisition of Zilker Labs, has been at the heart of the digital power revolution since 2003. Now, with the introduction of its latest ZL8800 controller, Intersil has the first product of its type, able to offer a compensation free digital solution. This dual-channel, step-down PWM controller eliminates the need to compensate the loop for stability without compromising system bandwidth. With on-board memory allowing the device to be set up for any application, high power density circuits can be designed with minimal external components (see figure 1).

Typical application of Intersil’s ZL8800 dual PWM digital controller

A key feature of the ZL8800 is the proprietary control loop known as ChargeMode™ control. This high-speed loop allows precise replenishment of any charge loss from the output capacitor during a transient event in a minimal amount of time. Control is performed rapidly on a cycle-by-cycle basis as the digital loop over-samples the output voltage. Ingeniously, the ZL8800 does not need to know the actual output capacitor value, relying on digital control algorithms to make the correct adjustment, even for stability. The result is the reduction in the amount of capacitance needed to support a specific application, while providing a compensation free design. The controller’s response ensures that any transient conditions are met while preserving stability and minimizing any ringing or over-shoot.

The system level benefits derived from this approach are that designers are now no longer tied to a few power component choices. Components can be selected to achieve optimum performance, safe in the knowledge that the controller enforces inherent stability. Furthermore, the controller eliminates the effects of component aging or environmental variations since the digital loop is constantly monitoring and accounting for the change.

The high bandwidth control is enabled through various internal subsystems: a high speed over-sampling ADC gives the control loop its transient responsiveness, while a dual-edge modulator allows the ZL8800 to maintain fixed frequency switching while minimizing any delay through the loop. To provide designers a trade-off in loop bandwidth and gain the ZL8800 provides an optimization setting via a programmable gain control, allowing precise setting of control loop response.

Transient response of ZL8800

Figure 2 illustrates the performance of the ZL8800 in a typical application. Configured as a dual phase, single output running at 550 kHz, the ZL8800 is designed to provide a total of 60A output with less than 2,700uF of output capacitance. In the scope shot below, the application is setup with Vin=12V, Vout=1.2V and a 20A load step applied (20A loading and 20A unloading), with only a total output deviation of 24mV observed (+/-1% of the output).

Additional features of the ZL8800

The ZL8800 operates with input voltages of 4.5 to 14 V and can be programmed to provide an output in the range 0.54 to 5.5 V. It is designed to switch at a frequency between 200 kHz and 1.33 MHz, either through internal selection or accepting an external clock. The device can be configured with dual outputs or be operated in a two-phase mode to support high output current applications. Digital communication with this controller is via PMBus, however, a unique single wire DDC (Digital DC) interface is also included allowing communication between a number of Intersil devices, enabling the construction of complex power architectures. Amongst other features the DDC bus allows complex sequencing and fault management across multiple devices.

In support of telemetry, the ZL8800 provides a parametric snapshot capability, which captures the operational data at the occurrence of a fault event, while on-board non-volatile memory offers local storage of data and user settings. With the release of the ZL8800, Intersil has taken the opportunity to update its PowerNavigator controller interface (GUI), which provides access to all the ZL8800 features via an intuitive programming environment without ever needing to write a single line of code. Two evaluation boards are available: a dual output (30A per output) and a high current 60A 2-phase system.

In conclusion, there seems little doubt that with its fourth generation ZL8800 design, Intersil shows leadership in a strongly growing market.

 

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