Recommendation as for DC-DC and LDO for ARM and digital ICs. - Page 1
Forum section: electronics / projects

author: Fahrenheit
author_rank: Newbie
timestamp: December 23, 2024, 03:43:08 pm
content: Dear colleagues,At the moment I work on designing a device and I can't understand which principle/rules should be used to select a DC/DC and LDO for power supplying digital ICs. I have the following ICs - 6 sensor with I2C bus, GNSS on a separate pcb, ARM STM32, display control, display backlight, LAN (Ethernet with LAN IC 10/100 Base-T).All ICs are powered from 3.3 volts. The two additinal sensors are powered by 5 volts. The device will be powered from general external AC/DC 120-220/5V, backup power from 12 volts.My vision is the following: 12 volts step down to 5 volts using a low-noise DC-DC. Then 5 volts step down to 3.3 volts using a low-noise LDO. I need to power 11 ICs so that there is minimal impact on all 11 ICs between each other and I would like to pass EMC/EMI Testing and Certification. Also, I don't understand if I need to use a Ripple Blocker LDO for each IC? When laying out a PCB, how often should ground layers be used? Device will not be produced in the millions and I would not like to save on the components.This is the voltage and current consumption of some ICs: 5 volts-0.5A, 3.3 volts-0.05A, 3.3 volts-0.07A, 3.3 volts-0.08A, 3.3 volts-0.1A.I am considering using the DC-DC TPS62912, LDO TPS7Axx family or MIC94325/45/55 family.Do I need to use one LDO or several to power all the consumers like ARM and other analog/digital ICs? Does it make sense to use a more advanced LDO from the TPS7Axx family?How important is it to use an advanced LDO from the TPS7Axx family or can I use, for example, the MIC94325/45/55 family?Does it make sense to power the sensors from multiple TPS7Axx or MIC94325/45/55 LDOs, or is it enough to "sandwich" 0.01uF, 0.1uF, and 10uF capacitors near the power pins of each sensor?What about powering the digital ICs that are responsible for power/backlight and display control, or is it enough to "sandwich" 0.01uF, 0.1uF, and 10uF capacitors near the power pins of each digital ICs?Detailed information about Ripple Blocker LDO - digikey.com/es/product-highlight/m/micrel/ripple-blockerI would be thankful if you would help me with your advice and recommendations.Best wishesGeorg

author: ajb
author_rank: Super Contributor
timestamp: December 23, 2024, 08:12:27 pm
content: Welcome to the forum!It's hard to give specific advice without knowing what your requirements are, but unless you have some very sensitive analog circuitry, you would probably be fine to run everything off of two buck converters -- one for 5V and one for 3.3V. It might be helpful to have a separate regulator for the analog supply, but that is more complexity for uncertain benefit without knowing more about the purpose of this device. You definitely do NOT need an LDO for every IC! Whether you go with a DC-DC or an LDO depends primarily on how far you need to reduce the voltage and how efficient you need to be. Going from 12V to 5V via an LDO gets you only 42% efficiency, versus 80%+ with a DC-DC, with the remaining 58% of the power consumed by the LDO being dissipated as heat.Quote from: Fahrenheit on December 23, 2024, 03:43:08 pmThe 'sandwich' capacitors you're thinking of are "decoupling" capacitors, and they're there to stabilize the local supply against the brief pulses of high current consumption from the IC. You generally need at least one cap on every supply pin of every IC, as close to the IC as possible, regardless of what is supplying the power. There are tons of articles and app notes and forum threads on the subject of decoupling, but many datasheets will give some guidance as a place to start.QuoteYou should have at least one solid ground plane on the PCB. Since you've got Ethernet and an ARM processor, you probably want a 4-layer board where one internal layer is ground and the other is power, and all of your routing should be on the outer two layers. Both of the internal planes should have as few breaks in them as possible.If you want more specific advice, it would help to know more about what your device is meant to do