LPC553x: Using internal LDO instead of internal DCDC converter

[danielholala]

Hello,

I see that the LPC553x series include an internal LDO which can be used instead of the internal DCDC converter. At least this is my understanding from reading AN13528.

Is it correct that to use the internal LDO, I just have to drop the inductor (which is connected to LX in order to use the internal DCDC converter) so that I end up with this configuration:

danielholala_0-1695396449458.png

Now I have these question:

Do I just have to connect VDD_MAIN to 3.3V to get the MCU powered? I'm asking because the application note states "The internal LDO allows the MCU to use an external regulation element". What does that mean? Which "external regulation element" do I have to add and where? Is that a requirement to use the internal LDO?

Further, Do I have to "enable" the internal LDO?

Thanks.

[xiangjun_rong · Accepted solution]

Hi,

Yes, the LPC5536/LPC55S36 has both internal DC/DC converter and LDO for VDD_CORE power supply, if you use internal LDO, it is okay to just connect a 4.7uF cap to the VDD_CORE pin.

You can connect 3.3V power supply to the VDD_MIAN and VDD_MAIN_PWR pins.

The remark "The internal LDO allows the MCU to use an external regulation element" just means that the 3.3V power supply which is connected to VDD_MIAN and VDD_MAIN_PWR can be from external DC/DC converter or external LDO.

For example, you have 5V power supply, there is an external Buck DC/DC converter which convert 5V to 3.3V, the 3.3V can power the VDD_MIAN and VDD_MAIN_PWR pins.

Hope it is helpful

BR

XiangJun Rong

[danielholala]

Hello @xiangjun_rong ,

Thank you for your reply. That clears thing up. I understand that I do not have to enable the internal LDO. It is automatically active when the DCDC converter output (LX pin) is not connected to VDD_CORE via an inductor.

Regarding the remark "The internal LDO allows the MCU to use an external regulation element" you wrote that this means that the 3.3V power supply which is connected to VDD_MIAN and VDD_MAIN_PWR can be from external DC/DC converter or external LDO. However, I think, this also holds true for devices without internal LDO, e.g., LPC5526. They can also be powered from LDO or DC/DC converter. Don't you agree?

[xiangjun_rong]

Hi,

I agree with you that MCU can be powered from LDO or DC/DC even if there is not internal LDO or DC/Dc converter for MCU.

BR

XiangJun Rong