Recommendation as for DC-DC and LDO for ARM and digital ICs.

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Fahrenheit

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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. 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 chips so that there is minimal impact on all 11 chips 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.
I would be thankful if you would help me with your advice and recommendations.
Best wishes
Georg
 

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Fahrenheit said:
I have the following ICs - 6 sensor with I2C bus, GNSS on a separate pcb, ARM STM32, display control, display backlight, LAN.
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When talking about power supply .. we don´t need to know hwat these devices do .. we need to know their current consumption.
Also it does not matter what bus they ouse or whether they are on one or many PCBs.
Only the current counts.

Fahrenheit said:
The two additinal sensors
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Which additional sensors?

Fahrenheit said:
The device will be powered from general external AC/DC 120-220/5V, backup power from 12 volts.
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What´s your idea how to do the switch over between 5V and 12V sources

Fahrenheit said:
Then 5 volts step down to 3.3 volts using a low-noise LDO
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I´m not sure if the LDO is the right choice. the LDO waste 50% of target power .. to generate heat. Neither heat is a good thing nor wasting energy ... Especially if the 12V is from a battery.
We need the currents .. and we need your goals and requirements.

Fahrenheit said:
I need to power 11 chips so that there is minimal impact on all 11 chips between each other and I would like to pass EMC/EMI Testing and Certification
Click to expand...
Chip count does not matter. EMI is is more a problem of PCB layout and part selection.
PCB layout: one really SOLID GND plane, short current loops, small area of current loops.

Fahrenheit said:
Also, I don't understand if I need to use a Ripple Blocker LDO for each IC?
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What is a ripple blocker? How much ripple do you expect (from which source) and how much ripple do you allow?

****
I´ve done many really noise sensitive nalog/digital designs.. if I´m not mistaken all with DC/DC switchers. Usually the 16 bit ADCs work in my application within the specified performance fo the datasheets. So the DC/DC noise is almost neglegible.

Since you did neither mention sensitive nor analog circuitry .. I see no problem for a purely digital application at all.

Klaus
 

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Thank you for your help and advice Klaus
Best wishes
Georg
 
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Fahrenheit said:
so that there is minimal impact on all 11 chips between each other
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One guideline is to examine whether any of your IC's or circuits:

a) suddenly changes how much Amperes it draws (higher or lower),
or
b) is sensitive and adversely affected by changes to its power supply.

If either is true then it's a good idea to give that circuit its own power supply and/or regulation.
 

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I recently completed a project that was quite similar, involving an ARM microcontroller and various digital components, and I thought I’d pass along what worked well for me. For the main power supply, I opted for a DC-DC buck converter, like the MP1584. It effectively reduces voltage from 12V to 5V while keeping heat production to a minimum. For the sensitive parts, such as the ARM MCU and analog circuits, I added a low-dropout regulator (LDO), like the AMS1117-3.3, to provide a clean 3.3V.

One important thing I learned was the need for proper power decoupling at every IC. By placing 0.1µF and 10µF capacitors close to the power pins, I managed to tackle ripple and transient problems. Also, keeping a solid ground plane and reducing the distance between the DC-DC converter and the LDO helped in maintaining stable performance.

If your project has stricter noise requirements, you might look into ripple-suppressing LDOs such as the LP2985 or TPS7Axx series, as they are great for minimizing output noise. This setup, combining a DC-DC converter for efficiency and LDOs for clear power, worked wonderfully for me. Feel free to ask if you want more information!
 

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Thank you for the help and advice, James.

The MP1584 is an ordinary step-down DC-DC converter and I am considering using the TPS62912.
The AMS1117-3.3 is a classic LDO and doesn't have low noise.

Did you use a single LDO to power all the consumers like ARM and other analog ICs? The LP2985 is very similar to the MIC94325/45/55 family, I can also use the more advanced LDO from the TPS7Axx family as per your advice.

The one thing I can't figure out how important is it to use the 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 several LDO TPS7Axx or MIC94325/45/55, or is it enough to place a "sandwich" of 0.01uF, 0.1uF and 10uF capacitors near the power pins of each sensor?
What about power supply for the digital ICs that are responsible for power/backlighting and display control ICs or is it enough to place a "sandwich" of 0.01uF, 0.1uF and 10uF capacitors near the power pins of each digital ICs?

Best regards
Georg
 
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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.
 

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