How to characterize the external inductor for the STM32H5 SMPS package
Introduction
The STM32H5 device in SMPS package supports SMPS mode only and does not support LDO mode.
For devices in SMPS packages, when operating in bypass mode, the external supply ramp slope must be higher than 3.5 mV/μs. This is mentioned in RM0481 in the “Startup with VCORE provided from an external supply (Bypass)” section.
The STM32H5 devices embed a high-power efficiency SMPS step-down converter that needs external components capacitance and inductance. So, how is this inductance characterized?
Explanation regarding inductor characterization
For correct operation of the STM32H5 device, the inductor used in the SMPS circuit must meet the specification defined in the datasheet (for example, DS14258).
This requirement is based on design simulations and validated product behavior. Reducing the Isat and/or Irms specifications below 1 is not acceptable.
Where:
- Isat is the DC current at which the inductance drops 30% from the value without current.
- Irms is the average current for which the temperature of the inductor is raised 40°C by the DC current.
To ensure stable operation across all conditions, the selected inductor must satisfy the following requirements:
- Isat > 1
- Irms > 1
These values are required to ensure a correct behavior of the STM32H5 devices for the full range of use cases.
The external inductor must still be selected with sufficient margin. This is because of the following:
- The inductor value depends on current, temperature, and process.
- Peak inductor current is higher than average load current.
- Depending on the SMPS topology and switching frequency, the peak current can exceed twice the average current.
- Even though Isat is specified as a DC current, increasing AC ripple current also causes the effective inductance to drop.
The SMPS must maintain a certain range of inductance and capacitance for stable operation. Meeting the datasheet specification ensures that these ranges are preserved under all operating conditions.
Conclusion
The STM32H5 in SMPS package supports only SMPS mode. For the SMPS-based design of the H5 series, the inductor selection is critical. To ensure stable SMPS operation and reliable MCU performance, the inductor must satisfy the requirements.