beyond design
pdn planning and capacitor selection, part 2 continues
Figure 7: Target frequency and optimized value approaches including VRM, loop inductance and plane
capacitance of 3M ECM.
• A capacitor's equivalent circuit is basically
a series capacitor, resistor and inductor. These
are referred to as the capacitance value, ESR and
ESL respectively.
• The downward slope of the capacitor is
capacitive, then as the capacitor approaches its
SRF it becomes resistive, then as the frequency
increases the inductance takes over raising the
impedance again.
• To meet the target (low) impedance at a
particular frequency, a capacitance value is chosen so that when mounted on the PCB, it will
resonate at the desired frequency, and have an
impedance that is equal to its ESR.
• The combination of capacitors causes antiresonant peaks, where one goes capacitive while
the other is inductive.
• When the SRFs are spread, the parallel
resonant impedance sets the limits to the PDN
performance.
• The mounting inductance is comprised
of three components: Capacitor footprint and
fanout, capacitor height above or below the
plane and power plane spreading inductance.
• Embedded Capacitance technology allows
for a very thin dielectric layer (0.24 – 2.0mil) that
provides distributive decoupling capacitance
and takes the place of conventional discrete decoupling capacitors over 1GHz. PCBDESIGN
40
The PCB Design Magazine • January 2014
References
1. Barry Olney, Beyond Design: Power Distribution Network Planning and Material Selection for SERDES Design
2. Henry Ott, Electromagnetic Compatibility Engineering
3. Istvan Novak, Quiet Power: Resonances in
Power Planes
4. Eric Bogatin, Signal and Power Integrity
Simplified
5. Johanson Dielectrics, Basics of Ceramic
Chip Capacitors
6. Kemet, SMT Capacitor Comparison
7. X2Y, Understanding capacitor inductance
8. The ICD PDN Planner is available at www.
icd.com.au
Barry Olney is managing director of In-Circuit Design Pty Ltd
(ICD), Australia. ICD is a PCB
design service bureau that specializing in board-level simulation. The company developed
the ICD Stackup Planner and
the ICD PDN Planner software. To read past
columns or contact Olney, click here.
