St.,Cyr-Novak-Biunno-Howard: Using Embedded Resistors to Set Capacitor ESR in Power Distribution
Networks. Manuscript for EPEP2001, October29-31, 2001, Boston, MA
ARIES: Using Annular-Ring Embedded Resistors to Set Capacitor ESR in
Power Distribution Networks
Valerie St. Cyr*, Istvan Novak*, Nick Biunno**, Jim Howard**
*SUN Microsystems, Inc.
One Network Drive, MS UBUR03-205, Burlington, MA 01803
Tel: (781) 442 0982, fax: (781) 442 1575
445 El Camino Real, Santa Clara, CA
Tel: (408) 557 7546, fax: (408) 557 7800
Valerie.St.Cyr@sun.com, Istvan.Novak@sun.com, Nick.Biunno@sanmina.com, Jim.Howard@sanmina.com
Power-distribution networks need to provide flat low impedance over a wide band. Bypass capacitors with different
values, and capacitors and planes create resonance peaks, unless the capacitor parameters are selected properly.
Distributed Matched Bypassing is used to create a smooth impedance profile. The ESR of ceramic capacitors is
increased by adding embedded annular resistors in series to the capacitors.
There has been considerable interest in recent years to improve the power-distribution network of high-end computer and
networking equipment. At the module level, on printed-circuit boards, full-area conductive layers over thin laminates
provide embedded capacitance for high-frequency decoupling , complemented by sometimes several thousands of
capacitors for mid and lower frequency bypassing and decoupling . Conductive plane pairs in printed-circuit boards
(PCB) exhibit multiple resonances , which should be suppressed by proper damping of the structure , .
Bypass capacitors with different values connected to the conductive planes also may exhibit resonances either between
different capacitor banks  or between capacitors and planes. One universal approach to reduce these resonance peaks
is to minimize the inductance connecting the parts. With discrete surface-mount capacitors the loop inductance is several
hundred pH, and usually the dimensions of PCB and capacitor do not allow us to lower it below 70-80 pH, which value
is still too high in some applications to avoid resonance peaks. The ESR of bypass capacitors could also be selected to
provide a flat impedance response, however, the ESR parameter for today’s capacitors is not user defined. ESR of
tantalum and electrolytic capacitors is usually in the order of ohm range, whereas ESR of multi-layer ceramic capacitors
is usually in the milliohm range.
This paper describes a bypass-capacitor selection process with a possible way to set (increase) the ESR of ceramic
bypass capacitors by using embedded printed resistors.
Distributed Matched Bypassing
The cumulative impedance of all bypass capacitors connected to the PCB planes should be a basin-shape impedance
profile. For a lumped-equivalent model, it is shown in Fig. 1. The total capacitance value is determined by the VRM’s
equivalent inductance and the mid-frequency impedance requirement. At very high frequencies, the required cumulative
inductance (which may be the result of detailed simulations of planes and capacitors together) of bypass capacitors will
determine the number of capacitors. For instance, if we find that the capacitors altogether should provide an inductance
of 10pH, and the PCB stackup, layout and capacitor dimensions allow a minimum of 500pH inductance for each bypass
capacitor, we need 500pH/10pH=50 pieces of high-frequency capacitors. With today’s technology, 1uF capacitance is
conveniently available in two-terminal 0805 sizes, and 4x1uF is available in low-inductance eight-terminal 1206 sizes.
Each capacitor may have an ESR of 5-10 milliohms. If we connect 50 such capacitors in parallel, the total capacitance is
50 to 200 uF, and the expected ESR is 0.1 to 0.2 milliohms. For applications, where the mid-frequency impedance
requirement may be 1-100 milliohms, it would be very convenient to have capacitors with user-definable ESR, such as
ESR=50 to 5000 milliohms for the above example.
By matching the mid-frequency impedance requirement to the inherent plane impedance, the Distributed Matched
Bypassing can not only eliminate inter-capacitance resonance peaks, but at the same time can also suppress plane