Flexible and versatile PMBus enabled monolithic buck converter in multi-power rail systems

×

Figure 1

   

Introduction

The integration of buck converters with the Power Management Bus (PMBus) capability into multi-power 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 paper will delve into the technical aspects, benefits, and practical example of implementing PMBus-enabled buck converters in multi-rail 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 not only adopted by major power supply manufactories, but system designers to better 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 LT7171,20A, 16V silent switcher step-down regulator with PMBus, as an example to explore some of the features.

Monitoring Operation

The first benefit of integrating PMBus with power converter IC would be monitoring IC operation.  With system application growing more complex and energy conscious, 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 the power IC greatly simplifies system level design.

Traditionally, if a system needs to monitor the power converter, it would require extra circuitry.  For example, monitoring the output current with analog power IC usually requires having a current sensing shunt resistor and a differential amplifier.  This method not only increases the solution cost and size, but also increases power loss and complexity.  Some power ICs has an analogous IMON signal available, but it still requires additional components and ADC outside of the power IC.  The current monitoring accuracy could be compromised due to the external circuitry.  For power ICs like current mode buck converters, the current information is readily available internally. With integrated ADC and PMBus controller, the system level controller can monitor the output current digitally 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 commends, 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 the 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 occurred in those gaps could be 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 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, Track/SS pin is limited to control the turn-on process. With the growing complexity of devices, especially digital processors and FPGAs, 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 is used to satisfy different power sequence requirement.

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

Case study: multi power rail using same hardware.

As systems growing more sophisticated, different voltage power rails are often required by different segments of a system.  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 rails often requires different set of components which increases complexity, cost, and spaces.  As equally important, it lacks flexibility.   If system requirements change slightly for different application scenario, the converter hardware likely needs to be redesigned.  Nevertheless, if the converter that can be configured digitally, utilizing the same hardware design to achieve similar performance for different voltage power rail become possible.

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

To determine a suitable inductor and output capacitors, we need to consider what a reasonable operating frequency range and desired inductor current ripple is.  The inductor current ripple can be calculated as the following equation.

Because the similar level of output voltage ripple and transient response are desired, it would be easier for output capacitor selection and other adjustment 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 range of 10% to 40% of the maximum output current level.  The exact switching frequency and current ripple are determined by requirements of efficiency, solution size, as well as 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 inductor.  The inductor current ripple peak-to-peak would be about 6A, 30% of the maximum output current 20A.

 
The next step would be 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.  Some may intuitively think the lower frequency configuration would have the higher output voltage ripple.  However, in this scenario, lower frequency configuration also outputs a lower voltage which causes less output capacitor derating.  Other parasitic like output capacitor ESL is not linearly related to 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 of output capacitors, it is recommended to choose the smaller package capacitors that meet capacitance requirement. The figure 3 shows a similar level of output voltage ripples of 4mVpk-pk of LT7171 at 0.8V with 800kHz switching frequency and 1.5V with 1.5MHz switching frequency.

 
Lastly, the compensation network needs to be adjusted to ensure the loop stability and adequate transient responds.  Before adjusting the compensation, we need to understand the basics of control logic.  Figure 4 is a simplified block diagram of a current mode controlled on-time buck converter.

 
Products like LT7171 not only has selectable error amplifier gain transconductance factors, the internal Cth and the internal Rth, but also has four different 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 1.5V output voltage, it requires the output voltage range 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 detail of the current sensing limit and V_out range selection, please refer to the LT7171 datasheet.

Setting the compensation to the highest setting doesn’t always yield the best performance.  In fact, it would often cause loop instability and low efficiency.  It is recommended to start with the default compensation value in the datasheet, then adjust it according to application requirements.  The following are the bode plots and transient responds of 0.8V and 1.5V rails.  Both rails achieve around 150kHz bandwidth with more than 60 degree phase margin.  The undershoot and overshoot in transient condition also meets the typical FPGA requirement of +/-3% nominal voltage.

 
Selecting the compensation network value can be a daunting task for engineers who are not familiar with control theory.  Fortunately, most of current mode products like LT7171 are accurately modeled in LTpowerCAD.  System engineers could use LTpowerCAD design tool to visualize how each compensation value affect the loop stability as well as transient responds.  AN149 also provides a more in-depth guide on loop compensation design.

As the example showed, LT7171 can achieve similar output ripple and transient responds for different output voltage level with the same hardware.  However, an important aspect, the efficiency and power loss, has not been discussed.  The reason is that fundamentally higher output voltage configuration has high efficiency despite more power loss because overall high output power.  It is not reasonable to expect similar efficiency and power loss for different voltage conversion.  In practice, high output voltage rail often requires high efficiency, but less restricted absolute output voltage ripple as well as output over/undershoot during load transient.  Therefore, it is likely the high voltage rail could 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 switching frequency and the size of the inductor.

Conclusion

In conclusion, integrating PMBus enabled buck converters into multi-power 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.

About the Authors