Ferrite bead placement on PCB

[abhishek.2138]

I am using power management IC (PMIC) for 32 bit microcontroller (MCU).
The IC has internal synchronous buck converter & two LDOs.

One of the LDO is used for microcontroller supply.
I have following queries.

1) Should I use ferrite bead? does ferrite bead help here to filter noise from (PMIC) to MCU?
2) To use ferrite bead, where should I place it? near to microcontroller Vdd pins or at the LDO output (here LDO is internal of PMIC)?

 

[KlausST]

Hi,

1) An LDO usually is a linear regulator. So how much noise in what frequency range do you expect.
And on the other hand: How much noise your microcontroller can handle?

Klaus

 

[cupoftea]

Always good to use ferrite bead for micro....just place it slightly upstream of the vcc pin capacitor of the micro.

 

[stenzer]

Hi,

place the ferrite bead close to the VDD pin(s) of the MCU followed by a buffer/filter capacitor. By placing the ferrite bead close to the MCU's VDD pin you are not interrupting the power supply towards the MCU or other parts supplied by the LDO i.e. your MCU demands a high current for a short time, than the ferrite bead represents a high impedance. If you use a dedicated "MCU" ferrite bead with a buffer capacitor at its "output", this high current demand will be provided by the capacitor while the ferrite bead avoids (high-frequency) noise traveling toward the MCU's VDD.

Now imagen to use a single ferrite bead at the output of the LDO. If routed in a star-point manner afterwards, a high current demand of one branch might lead to a supply drop also visible to the other star-branches/loads.

Choose a feerite bead which has a rerasonable impedance at the swithing frequency of your PMIC. Further, place the ferrite bead close to the VDD pin of MCU. You can also configure it in a PI-topology, by means placing a capacitors at poth sides of the ferrite bead. Doing so, will also prevent a noise created by the MCU traveling towards the PMIC and other star-branches.

BR