AN-2605: Employing Flexible and Versatile PMBus-Enabled Monolithic Buck Converters in Multipower Rail Systems

Introduction

The integration of buck converters with the power management bus (PMBus) capability into multipower rail applications represents a significant advancement in power management technology. By leveraging the PMBus interface, designers can achieve precise control and real-time monitoring, which is critical in applications ranging from cloud infrastructure to integrated circuit testers. This application note delves into the technical aspects, benefits, and practical examples of implementing PMBus-enabled buck converters in multirail systems, providing a comprehensive design guideline.

Benefit of PMBus Enabled Converters

In the past couple of decades, PMBus protocol has gained its popularity because its flexibility, efficiency, and ease of use. It was adopted by both major power-supply manufacturers and system designers to improve adjusting and monitoring power during operation. The PMBus protocol specification provides standard power management commands and allows manufacturers to define specific commends for enhanced functionality. This section uses the newly released LT7171, a 20A, 16V silent switcher step-down regulator with PMBus, as an example to explore some of these features.

Monitoring Operation

The first benefit of integrating PMBus with a power converter IC is monitoring IC operation. With system applications growing more complex and energy conscious, a system level controller often requires telemetry features, such as monitoring temperature, output voltage, and current, for analyzing system health and calculating energy consumption. Although similar features can be implemented within analog power ICs, converting those signals into digital PMBus commands within power ICs greatly simplifies system level design.

Traditionally, if a system needs to monitor the power converter, it requires extra circuitry. For example, monitoring the output current with analog power IC usually necessitates having a current-sensing shunt resistor and a differential amplifier. This method not only increases the solution cost and size, but it also increases power loss and complexity. Some power ICs have an analogous IMON signal available, but they still need additional components and external ADCs. Additionally, the current monitoring accuracy can be compromised due to the external circuitry. For power ICs, such as current-mode buck converters, the current information is readily available internally. With integrated ADC and PMBus controllers, the system level controller can digitally monitor the output current with just two communication wires. Products like the newly released LT7171 family can guarantee an accuracy of ±300mA below 10A output current and ±3% above 10A output current in most applications.

Beyond the standard PMBus commands, LT7171 also offers ADI-specified enhanced telemetry commands. The MFR_VOUT_PEAK, MFR_VIN_PEAK, and MFR_IOUT_PEAK detection feature can be particularly useful for analyzing system health. Although the input voltage, output voltage, and output current are monitored constantly within the power IC, due to delay of other tasks, there could be gaps between each PMBus inquires by the system controller. Thus, abnormalities occurring in those gaps could go undetected. This issue could be further amplified if the system controller sends PMBus commands in a fixed frequency. To combat this issue, LT7171 records the highest voltage and current measured. The system controller can access this report anytime during operation to ensure the system operates as expected. The peak records can be reset by using MFR_PEAK_CLEAR command.

Configuring the Converter Setting

An advanced PMBus feature is that using digital commands to configure the power IC settings such as setting output voltage, PWM frequency, and turn on/off time delay, etc. With the help of the internal logic controller, many of the configuration components become optional or eliminated completely. At the same time, it also unlocks new features and offers greater design flexibility

Most of buck converters have a tracking and soft-start (TRACK/SS) pin that can be driven externally by other voltage source for power sequence or connected to a capacitor for a soft-start function. Although it works for many applications, specific applications still require careful design considerations. First, when using the tracking functions, the output voltage follows the slew rate of the TRACK/SS pin. If a system requires the output turns on faster, slower, or be delayed, external circuitry would be needed. Second, when using the soft-start function, the external soft-start capacitor may not be discharged completely during a frequent on/off and cause inconsistent behavior. Again, external discharging circuit is needed to resolve such issue. Lastly, the TRACK/SS pin is limited to control the turn-on process. With the growing complexity of devices, especially digital processors and FPGAs, the power-down sequence is also critical and restricted.

LT7171 offers PMBus configurable commands to simplify the design. The TON_RISE command can be used to control soft-start precisely. Additionally, there is a TOFF_FALL that sets the output falling slew rate. TON_DELAY and TOFF_DELAY are used to satisfy different power sequence requirements.

These commands suffice the same functions of the TRACK/SS pin without external circuit and provide features such as controlling turn-on and turn-off slew rate independently, which was difficult to achieve before. The LT7171 also offers other PMBus configurable features. For a full list of commands, refer to LT7171/LT7171-1 PMBus/I²C Reference Manual.

Case Study: Multipower Rail Using the Same Hardware

As systems grow more sophisticated, different voltage power rails are often required by different system segments. Using a typical FPGA as an example, it requires voltages ranges from 0.8V to 5V for its core, transceivers, high-speed I/Os, etc. Each power rail often requires a different set of components that increases complexity, cost, and spaces. Equally important, it lacks flexibility. If system requirements change slightly for different application scenarios, the converter hardware likely needs to be redesigned. Nevertheless, if the converter can be configured digitally, utilizing the same hardware design to achieve similar performance for different voltage power rail becomes possible.

Take this example: using the LT7171 for two voltage power rails from a 12V input. The V_OUT of one of the rails is 1.5V, and V_OUT of the other is 0.8V. The maximum output current can be up to 20A for each rail. These requirements can commonly be found in FPGA, ASIC, or similar applications. Since most configurations such as output voltage, switching frequency, and compensation values are stored within the internal NVM of the LT7171, the only critical components selections are output inductors and output capacitors.

To determine a suitable inductor and output capacitors, consider a reasonable operating frequency range and desired inductor current ripple. Calculate the inductor current ripple with the following equation:

Because similar levels of output voltage ripple and transient response are desired, output capacitor selection and other adjustments are easier if the inductor current ripple is the same across all configurations. Thus, the switching frequency needs to be adjusted close to proportionally to output voltages. The typical inductor current ripple is usually in the 10% to 40% range of the maximum output current level. The exact switching frequency and current ripple are determined by efficiency, solution size, and the availability of inductors. In this example, the switching frequency range is 800kHz (0.8V output) to 1.5MHz (1.5V output), and XGL6060-221MEC (220nH) is selected for the inductor. The peak-to-peak inductor current ripple is approximately 6A, 30% of the 20A maximum output current.

The next step is selecting suitable output capacitors. Output capacitors determine not only the output voltage ripple, but also the load transient performance. Because the compensation network needs to be adjusted for each output voltage configuration later, only the output voltage ripple is considered at this step.

The output voltage ripple is affected by switching frequency, actual output capacitance, capacitor ESR, and other parasitic factors. The lower frequency configuration intuitively may suggest having a higher output voltage ripple. However, in this scenario, the lower frequency configuration also outputs a lower voltage that reduces output capacitor derating. Other parasitic factors, such as output capacitor ESL, is not linearly related to the switching frequency. Therefore, it is important to start adjusting the output capacitors with the most restricted rail, then check every configuration. To minimize the parasitic draw of output capacitors, it is recommended to choose the smaller package capacitors that meet the capacitance requirement. Figure 3 shows a similar level of output voltage ripples of 4mV_PK-PK of the LT7171 at 0.8V with a 800kHz switching frequency and 1.5V with a 1.5MHz switching frequency.

Lastly, adjust the compensation network to ensure the loop stability and adequate transient response. Before adjusting the compensation, the basics of control logic need to be understood. Figure 4 illustrates a simplified block diagram of a current-mode controlled on-time buck converter.

Products like the LT7171 not only have selectable error amplifier gain transconductance factors, the internal C_TH and the internal R_TH, but also include four output current-limit settings and three output voltage ranges. Changing the current limit and output voltage range affect the current-sensing gain KI and feedback gain, respectively. In the case of the 1.5V output voltage, the output voltage range needs to be in the middle output voltage range (0.8V < V_OUT < 2.75V), so it is important to select a higher amplifier gain transconductance factor for gm to compensate. For details of the current-sensing limit and V_OUT range selection, refer to the LT7171 data sheet.

Setting the compensation to the highest level doesn’t always yield the best performance. In fact, it can often cause loop instability and low efficiency. It is recommended to start with the default compensation value as described in the data sheet, then accordingly adjust it to the application requirements. Figure 5 shows the bode plots. Figure 6 and Figure 7 illustrate the transient response of 0.8V and 1.5V rails, respectively. Both rails achieve approximately 150kHz bandwidth with more than a 60° phase margin. The undershoot and overshoot in the transient conditions also meet the typical ±3% nominal voltage FPGA requirement.

Selecting a compensation network value can be a daunting task for engineers who are not familiar with control theory. Fortunately, most current-mode products, like the LT7171, are accurately modeled in LTpowerCAD. System engineers can use the LTpowerCAD design tool to visualize how each compensation value affects the loop stability as well as transient response. Application Note 149 (AN-149): Modeling and Loop Compensation Design of Switching Mode Power Supplies also provides a more in-depth guide on loop compensation design.

As Figure 6 and Figure 7 demonstrate, the LT7171 can achieve a similar output ripple and transient response for different output voltage levels using the same hardware. However, the importance of efficiency and power loss has not been discussed; a fundamentally higher output voltage configuration has high efficiency despite more power loss because of overall high output power. It is not reasonable to expect similar efficiency and power loss for a different voltage conversion. In practice, a high-output voltage rail often requires high efficiency, but also a less restricted absolute output voltage ripple, plus an output overshoot/undershoot during a load transient. Therefore, it is likely the high-voltage rail can accept a relatively lower switching frequency than the 1.5MHz given in this example as a design tradeoff for low power loss. These design tradeoffs need to be considered before selecting a switching frequency and inductor size.

Conclusion

In conclusion, integrating PMBus-enabled buck converters into multipower rail systems offers significant advantages in terms of flexibility and control. These converters provide real-time monitoring capabilities, which are crucial for optimizing performance and ensuring system reliability. The ability to program and adjust parameters remotely enhances adaptability to varying load conditions, reducing downtime and maintenance costs. Overall, PMBus-enabled buck converters represent a robust solution for modern power management challenges, paving the way for more efficient and resilient electronic systems.