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SMPS (Switched-Mode Power Supply) Theory
In packages with an integrated SMPS, the STM32 embeds two regulators that can be selected when the application runs, depending on the application requirements: one LDO (Linear Voltage Regulator) and one SMPS step-down converter in parallel to provide the VCORE supply to the digital peripherals.
The SMPS reduces the overall power consumption, however the drawback is that some applications may be perturbed by the noise generated by the SMPS. The SMPS can operate in two modes: main regulator mode (used when performance is needed) and low-power regulator mode. SMPS can be used in all voltage scaling ranges and in all stop modes.
How do I know if the specific STM32 part I am using includes SMPS signals available on the pins?
Note that the SMPS power supply pins (VLXSMPS, VDD11, VDDSMPS and VSSSMPS) are available only on packages with a SMPS. In devices without a SMPS, the I/Os and system peripherals (e.g., PLLs and reset blocks) are fed by the VDD supply source. The VCORE power supply for digital peripherals and memories is generated from the LDO.
The best way to verify if these SMPS pins are available on your MCU package is by doing a cross check with the package pinout available in the datasheet. The following steps will show you how to decipher your STM32 part number to retrieve the package type.
Step 1. Have your STM32 full part number handy. In our example, we’ll be using the STM32 part: STM32U585CIU6.
Step 2. Navigate to your STM32’s landing page, to locate the MCU’s appropriate datasheet. Note that you will need to launch the MCU's datasheet in preparation for the next step.
Step 3. Locate the Ordering Information section within the MCU’s datasheet. Note that each MCU is unique, therefore the ordering information section will vary between datasheets.
Step 4. Navigate to the ordering information section. Here, you will want to decipher your device’s part number and make note of your specific MCU’s pin count and package type.
In our example, for the STM32U585CIU6 we can see that C refers to a pin count of 48 and U refers to package type UFQFPN.
Step 5. Search for your MCU package type pinout within the device’s datasheet. In our example here, we’re doing a search for UFQFPN48. In the datasheet section under list of Figures, we see the UFQFPN48_SMPS denotation however let’s look at the pinout diagram to further confirm that the SMPS signals are available on this package.
Looking at our selected MCU’s package pinout, we see the SMPS signals (VLXSMPS, VDDSMPS and VSSSMPS, VDD11) and now can confirm that our MCU has SMPS available.
The SMPS signals are pulled out to pins on my package, but I don’t plan to use the SMPS. How should I connect these signals?
Each STM32 device, has an associated getting started hardware development application note. This application note provides guidelines for the minimum hardware implementation and recommendations for power supply, clock, reset control, boot mode settings, and debug management for specific MCU product families.
You can find your specific MCU’s getting started hardware development application note under the “Documentation” tab > “Application Notes” section of the MCU’s page.
Similarly to the datasheet, each MCU's guidelines and contents within the getting started hardware application note will differ slightly. Therefore, we always recommend to review your specific MCU's hardware getting started application note to get the most accurate guidelines and recommendations for hardware implementation.
Every hardware getting started application note has a section denoted as power supply management. Typically, in this section users will be given a guideline on how to connect various power pins depending on the user's application needs.
In our example case, the Power Supply Management section doesn't detail what to do with SMPS pins when we don't plan to use them, so we must look elsewhere in the application note.
The key is to look for sections or tables within the application note that detail configuration recommendations. Often, we recommend doing a search within the document and look for key words such as SMPS in this case.
In AN5373, the guideline shows two power supply overviews. One with SMPS, and one without. Since our MCU has the SMPS, we will focus on figure showing the power supply overview with the SMPS.
As indicated in the diagram below, there's a note that discusses the connections to make should you not choose to utilize the integrated SMPS in your design.
Following these guidelines, we can now confirm that if we're not using SMPS in our design, but our MCU includes the signals pulled out to pins we should connect them as follows:
- VDDSMPS to VSS
- VLXSMPS to VSS
- VSSSMPS to VSS
- Connect VDD11 pins to VSS through a pair of decoupling capacitors (two 2.2uF and two 100nF capacitors)
Relevant Documentation and Links:
