Today’s power supplies require flexible power solutions and more digital control than a decade ago, especially in the server cloud computing market. The days of de-soldering resistors and capacitors to tweak a voltage control loop are long gone.
Historically, digital power control started with a single-phase, point-of-load (POL) regulator with PMBus interface. Today, it has migrated to a multiphase control scheme that controls 100s of amps of current for multi-core CPUs, accelerators, artificial intelligence (AI) chips and network processors. Until now, digital power control meant engineers could interface with every switching regulator on their board, with one exception: linear regulators, also known as low drop-out regulators or LDOs. However, with the arrival of the digital power monitor IC, engineers now have complete digital control over every power supply in their system.
This article reviews the power management trends in the high-performance computing market, and describes how to design a digitally controlled LDO regulator circuit that enables voltage and current monitoring, as well as telemetry fault monitoring through PMBus commands.
Server power rail requirements
In general, cloud-computing servers require multiphase power control delivering 30 to 100s of amps for memory (VDIMM) and Vcore, respectively, while power rails under 30 amps are generated with a single-phase POL regulator. The remaining power-supply rails are low power, typically implemented with LDO regulators. Figure 1 shows the number of power supply rails required by servers over successive generations. As you can see, more than 25 rails are projected in future generations.
Figure 1. Power supply rails increase with each successive system
The other computing market trend is the balance of high performance and efficiency. The system must deliver high performance when necessary and deliver the most efficient solution when operating in peak current mode. Standby or low power mode is a typical requirement. Providing digital control to the LDO regulator adds the margining capability needed to margin down the output voltage or even set the voltage to zero volts when low power mode is enabled.
Another benefit of this margining capability is to margin the output voltage +/- 10%. Obviously, another major benefit of having a digitally controlled LDO is that it allows you to use the PMBus to read every rail in your system, making make sure there are no outages or faults.
Designing a digitally controlled LDO regulator
So how exactly does this all work? In the following example, we use a digital power monitor. This device is a bi-directional, current sense amplifier with PMBus interface. It brings digital telemetry to an analog system. With the clever use of certain features, an engineer can now monitor, protect and control every LDO in the system.
Let’s look at the digital power monitor in more detail. As shown in Figure 2, there are many features at the engineer’s disposal, including:
- VBUS/VINP/VINM–These are the primary channel pins used to monitor the LDO’s voltage and current. Note: An external shunt resistor is needed for current measurement.
- DAC_OUT–This is a built-in 8-bit DAC that is used to margin the LDO output up/down.
- SMBALERT2–This is alert pin that is triggered whenever the digital power monitor senses a fault condition. It will enable/disable the LDO.
- GND–The ground pins of the digital power monitor and LDO must be connected together for the voltage and current measurements to function properly.
Voltage and current monitoring
Measuring voltage with the digital power monitor is simple as the engineer connects the VBUS pin to the VOUT rail of the LDO regulator and connects the GND pins together. However, in order to measure the current flowing through the output of the LDO, an external shunt resistor is needed.
In Figure 3, the LDO is used with the digital power monitor to demonstrate the connections needed for voltage and current measurement. The VBUS pin measures the voltage of the LDO VOUT. The VP and VM pins are differential and measure the voltage across Rshunt. This resistance value is set within the digital power monitor and is used to calculate the current:
Eq. 1 Iout = Vshunt/Rshunt
The Vshunt setting within the digital power monitor can be set in the 40mV/80mV range. The key is to maximize the use of this range in the current measurement. For example, the LDO can supply 1A. Therefore, if the 80mV range is used, the Rshunt must be selected to ensure the Vshunt measurement is around 80mV at 1A or Rshunt is equal to 80-mOhm.
Output voltage margining
Using the digital power monitor’s 8-bit DAC output, voltage margining of the LDO can be accomplished. By connecting the DAC_OUT pin to the feedback network of the LDO (ADJ pin), one can margin the output voltage up/down.
Essentially, the FB or ADJ pin voltage is being manually changed through the DAC output. This DAC output voltage, along with R1 and R2, set the output voltage for the LDO regulator, according to Eq. 2.
Eq. 2 Vout = 0.5 + (0.5 – DAC_OUT) * R2/R1
One key point to keep in mind when setting the digital power monitor’s DAC_OUT value of the ISL28023 is that the DAC_OUT voltage must not exceed 0.5V, as this could result in unexpected behavior of the LDO.
Fault triggering & enable/disable
The digital power monitor has two SMBALERT pins that will trigger high/low when a fault condition is met. These alerts can also be force triggered to enable/disable the LDO. The SMBALERT2 is used in this application as shown in Figure 5.
Piecing Figures 3, 4 and 5 together forms the complete circuit. Figure 6 shows how the addition of the ISL28023 digital power monitor and three external resistors can add telemetry to a simple LDO.
Conclusion
In the past, it was typical for only high power rails in a system to have digital control, leaving low power LDOs without a digital interface. Now, these lower current rails are in need of telemetry. The ISL28023 digital power monitor makes this possible. Every rail within a system can be digitally monitored, protected and controlled through PMBus commands. Learn more about digital power monitors.
About the authors
Eric Josefson is a Senior Field Applications Engineer with Renesas Electronics America. He received his BSEE from the Rochester Institute of Technology.
Justin Heisig is a Field Applications Engineer with Renesas Electronics America. He received his BSEE from the Rochester Institute of Technology.
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