SMPS with/without LDO comparison

Andrew Sterian · ‎2021-07-22

I'm trying to understand the benefits/tradeoffs of the "Direct SMPS supply" and "SMPS supplies LDO" options for the STM327H (red and blue options in the attached figure). Since the LDO adds an extra source of power loss, and the SMPS can supply all desired VOS0-VOS3 voltage levels, why would someone choose the "SMPS supplies LDO" option? What is the benefit?

Andrew Sterian · ‎2022-01-07

Here is a reply I received from another ST FAE:

"Due to the switching nature of SMPS, they can be noisier than LDO's. Using an LDO after the SMPS can isolate the loads from both switcher noise and EMI noise while providing highly accurate voltage. The tradeoff to having the LDO included is, of course, higher power consumption.

We also recommend using the LDO if your application will be running at higher frequencies as it is less noisy than the SMPS."

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Bouraoui Chemli · ‎2021-07-23

Hi,

The case "SMPS supplies LDO" allow to have the 480MHz via VOS0 available only with LDO.

Also, you can refer to RM0399, section 7.4 power supply.

Ciao

Andrew Sterian · ‎2021-07-23

Thank you Bouraoui, but you are referencing a different part. RM0399 applies to the STM32H74x series and I am using the STM32H725/735, as described by RM0468 (sorry, should have been more clear).

The data sheet says VOS0 requires 1.36V, and Section 6.4.1 System Startup of RM0468 says: "When the system is powered on....The SMPS step-down converter output level is set at 1.36 V"

Thus it appears the SMPS can drive the voltage necessary for VOS0.

Furthermore, AN5419 says in Section 2.1.7: "The SMPS is directly connected to the VCAP pin and provides the regulated supply to the Vcore...Run mode: The converter can be dynamically scaled by programming the voltage scaling (VOS0 to VOS3) to the required performance"

Are there any other explanations for why "SMPS supplies LDO" would be a better option than "Direct SMPS supply"?

Manuel Del Basso · ‎2022-01-07

Hi Andrew,

are you able to answer the question "why would someone choose the "SMPS supplies LDO" option? What is the benefit?" right now?

I'm not, can someone explain the benefit of "SMPS supplies LDO" option?!

Best regards, Manuel

Andrew Sterian · ‎2022-01-07

Here is a reply I received from another ST FAE:

"Due to the switching nature of SMPS, they can be noisier than LDO's. Using an LDO after the SMPS can isolate the loads from both switcher noise and EMI noise while providing highly accurate voltage. The tradeoff to having the LDO included is, of course, higher power consumption.

We also recommend using the LDO if your application will be running at higher frequencies as it is less noisy than the SMPS."

Bouraoui Chemli · ‎2022-01-10

One additional comment regarding the difference between SMPS and LDO is the BOM. Where SMPS requires additional inductor and capacitator.