LPC55S DC/DC converter inductor

[jpowning]

I would like to specify an inductor for the LPC55S DC/DC converter, but I do not see any requirements other than the inductance. Can anyone tell me what other characteristics are important?

[jpowning]

Also, what is the min/max duty cycle of the DC-DC controller MOSFET switch?

thank you,

John

[ZhangJennie]

Hi @jpowning 

inductance L1 of 4μ7H and the capacitance C1 of 22μF are calculated by LPC55xx internal circuit. When designing, we must strictly follow the parameters recommended in the manual, otherwise DCDC circuit can’t work normally.

For more about DCDC power supply circuit analysis and detection method, see

https://community.nxp.com/t5/LPC-Microcontrollers-Knowledge/LPC55xx-Power-Supply-Circuit-Design-and-...

Hope this helps,

Jun Zhang

[jpowning]

Thank you for your help. I have some more questions:

1, What is the control scheme of the converter? (constant frequency, burst mode, pulse skipping, current controlled….).

2. Are there any features in the converter to minimize emissions? (spread spectrum, dithering….)

3. What is the operating frequency of the internal DC-DC Converter?

4. What is the max/peak cycle current?

5. What is the max average current? What is the spec for the supply current into VDD_PMU?

6. AN12325 recommends minimum saturation current for the inductor >350mA, but tables 16, 17, 18 in the data sheet would imply the max average current <6.1mA.

John

[ZhangJennie]

John

LPC5500 DCDC uses PFM control scheme. See AN13532 chapter 4 DC-DC operating mechanism and control regulation

https://www.nxp.com.cn/docs/en/application-note/AN13532.pdf

The DCDC converter spec that we can share is listed in above AN as well, chapter 5 DC-DC converter specs. Besides we don’t have other spec record for external.

Inductor >350mA. This is requirement for selecting inductor. see below

While Datasheet table 16-18 is for Idd. They are different.

Hope this helps,

Jun Zhang