LPC55xx DC to DC converter

[ianbenton]

Fig. 4 of the datasheet give a circuit for a DC to DC converter. Why? What does it do?

There's no mention of it anywhere else in the datasheet.

[ianbenton]

I note that use of the DC-DC converter is optional and the core can be supplied by a linear regulator, but it does not state whether the linear regulator is built-in or the voltage of 1.0V has to be supplied externally.

The datasheet specifies the current consumption at approximately 3.4mA. For an application that is not battery powered the efficiency advantages are marginal, so I am not sure if it is worth the extra complexity. 

How do I connect the various pins if I do not wish to use the DC-DC converter?

I asked about the self-resonant frequency as it does not specify in the datasheet the frequency at which the DC-DC converter operates, and therefore it would be necessary to find an inductor suitable for operation at that frequency.

I also wonder about the need for 500mA saturation current when it would appear that the core only need 3.4mA (Section 10.3 of the datasheet)

[EdwinHz]

Hello @ianbenton 

The use of the DC-DC converter is strongly recommended. If you do not wish to utilize it at all, you would have to use an external regulator like you mentioned. Please refer to this community post for more information on how to properly bypass the DC-DC converter:

https://community.nxp.com/t5/LPC-Microcontrollers/Properly-bypassing-LPC5500-s-internal-DCDC-convert...

There should be no need to worry about the self-resonating frequency in the case you use an external regulator, as long as it is contained within the 1.0 – 1.2V range. Please refer to note [4] of the datasheet (p. 74) for more information about the recommended voltages for different frequency ranges: https://www.nxp.com/docs/en/data-sheet/LPC55S6x.pdf

As for the saturation current, it is recommended at 500mA to allow ample leeway as not to interfere with the DC-DC converter circuitry.

I hope this helps,

Edwin.

[EdwinHz]

Hello Ian!

While the datasheets don’t go into much detail about the function and usage of the DC-DC converters, you may find more information about this topic in the LPC55xx User Manuals. I would advice for you to inquire on the matter with the User Manual of your MCU of choice. Here are the links:

LPC55S0x/LPC550x: https://www.nxp.com/docs/en/user-guide/UM11424.pdf

LPC55S1x/LPC551x: https://www.nxp.com/webapp/Download?colCode=UM11295

LPC55S6x/LPC55S2x/LPC552x: https://www.nxp.com/webapp/Download?colCode=UM11126

Regards,

Edwin.

[ianbenton]

I still didn't find anything useful, apart from a register which I could use to change the core voltage.

I still don't know whether I have to include LX in figure 4 in the datasheet, nor are there any specs for LX such as the saturation current or the self-resonant frequency. Section 6.2.2. in the datasheet does not contain any text whatsoever, just an isolated circuit diagram.

[danielholala]

For the 4.7 uH inductor (LX in figure 4) I'm using WE-TPC inductor, order no. 744025004  : Ir=1.35 A, Isat = 1.7 A, fres = 84 MHz. It's quite tiny and works for me.

That being said, the documentation on this inductor is scarce:

• In Application Note AN12325, the recommended saturation current is 500mA (or higher).
• In LPC55S69 eval board, inductor part number is  MLZ2012M4R7HT000 (rated current is 400mA).

[EdwinHz]

Hello again!

There is more information in the application note: “Using the DCDC feature”. While there is no mention about the self-resonant frequency that you asked about, there is a small section that talks about the saturation current (3.3.1) which could prove useful for your inquiry. Here’s the link for the application note:

https://www.nxp.com/docs/en/application-note/AN12325.pdf

The recommendation in this case would be to follow this document as closely as you can, in order to ensure the proper function of the LPC55xx.

I hope this helps,

Edwin.

[ianbenton]

OK Thanks.

There's a big blank space on page 36 of the Datasheet underneath figure 4. Perhaps you could put "This is how you power up the core" in it!

Followed by " you can use a linear regulator if you really insist, but we think this is the best way".