Conflict between SMPS and LDO regulator during startup on STM32H7 microcontrollers.

[SBett.1]

I am working on a PCB design with a STM32H735IG uC, and I would like to use the integrated SMPS to provide VCORE directly, without the LDO. This corresponds to figure 19.2 of RM0468 (the reference manual). In this configuration the VFBSMPS is connected directly to VCAP and to VDDLDO (I also add 100nF on each of the three VCAP pins and on each of the three VDDLDO pins).

 

Section 6.4.1 ("System supply startup") stipulates that at startup:

• When the system is powered on, the POR monitors V DD supply. Once V DD is above the POR threshold level, the SMPS step-down converter and voltage regulator are enabled in the default supply configuration:

1. The SMPS step-down converter output level is set at 1.36 V.
2. The voltage regulator output level is set at 1.0 V in accordance with the VOS3

level

So, at startup, before the LDO is disabled by software, there is a conflict on the VCAP pins: the SMPS is driving them at 1.36V, while the LDO provides 1.0V internally. Is this out of spec? Is the LDO going to be damaged? What happens exactly, and which voltage prevails?

I am interested to know what happens to VCAP and to the LDO also in case the LDO is erroneously not disabled by software during startup (that is, if the conflict is permanent instead of temporary).

Note: In the AN5419 (the "Getting started" guide) I have found the following remark:

>> 2.2.6 Bypass mode

>> [...] To avoid conflict with the LDO, the external voltage must be kept above 1.15 V until the LDO is disabled by software. [...]

So, it seems that delivering 1.36V from the SMPS to the VCAP pins is fine with the LDO. Nonetheless, I would like to know more about the situation when the VCAP pins are potentially driven both externally and by the LDO.

[JBURB · Accepted solution]

Hello,

the device is defined to deal with the situation where LDO and SMPS are both enabled and connected to VCAP, it can keep this not optimal situation without damage, but the power performance might be impacted.

Note that you are in that situation when the bootloader is used, because the bootloader does not know about PCB configuration.

As you noted, if the power supply voltage applied to VCAP is below the enabled LDO output, the LDO will try to reach a voltage and this must be avoided.

Br,

Jacky