Decoupling Capacitor in DC/DC Converter

[Nikos A.]

Hi everyone,

I am going to use this DC/DC converter to supply an Oven Controlled Crystal Oscillator (OCCL) and I've got several question regarding the decoupling capacitors

https://eu.mouser.com/datasheet/2/687/tel5-519367.pdf
https://abracon.com/Precisiontiming/AOCJY3.pdf
https://eu.mouser.com/ProductDetail/ABRACON/AOCJY3B-10000MHZ-E-SW?qs=%2Fha2pyFaduheXwhiO7qFNqRHAJBxDf6eSy3uqFqG%252BNw%3D

OCCL output is selected to be 10Mhz sinewave.

I want to calculate the decoupling capacitors after the DC/DC converter and I need your advice.
What I know so far is that in order to calculate a proper decoupling network you need to calculate a target impedance and then select the proper capacitors in order to maintain a low impedance path below the target impedance for the whole frequency range that transient current exist.

The formula for the target impedance is Ztarget = (Rail Voltage x max. Voltage Ripple) / Transient current

From OCCL datasheet,

Rail Voltage = 12V (actually there are three available Vcc options 3.3V, 5V and 12V but at my application I will use 12V)
max. Voltage Ripple = 5%
Transient current while turn ON state (3.6W power consumption) @ 12V = 300mA (this is the maximun transient current isn't it?)

Taking into consideration the information above the target impedance should be Ztarget = (12 x 0.05) / 0.3 = 2 Ohms

So I need a decoupling network with impedance path under 2 Ohms, is this correct?

Using SimSurfing tool provided by Murata and selecting two capacitors 0.1uF and 0.01uF it seems like is maintained an impedance path under 2 Ohms for the frequency range 1Mhz - 1Ghz




Some questions come to the surface...

1. How do I calculate the appropriate frequency range to maintain low impedance path?

I found a formula that estimates the bandwidth, BW = 0.35/RT, where:

BW: Bandwidth
RT: Rise Time

Can I apply this formula in my case?
What is the minimum frequency?

2. The intersection of capacitive (green line) and inductive (blue line) region of the capacitors is going to create an impedance peak (anti-resonance effect).. I don't know the magnitude of this peak and also don't know whether is going to affect the performance.
From your experience how to you manipulate this effect?


3. Converter's datasheet refered Ripple & Noise and Switching Frequency



Should I take into consideration these parameters? Should I include a decoupling capacitor BEFORE the converter due to its switching frequency?

Thanks in advance

[Nikos A.]

Anyone?