Low Inductance Capacitors
Introduction
The signal integrity characteristics of a Power Delivery Network (PDN)
are becoming critical aspects of board level and semiconductor
package designs due to higher operating frequencies, larger power
demands, and the ever shrinking lower and upper voltage limits around
low operating voltages. These power system challenges are coming
from mainstream designs with operating frequencies of 300MHz
or greater, modest ICs with power demand of 15 watts or more, and
operating voltages below 3 volts.
The ESL of a capacitor determines the speed of energy transfer to a
load. The lower the ESL of a capacitor, the faster that energy can be
transferred to a load. Historically, there has been a tradeoff between
energy storage (capacitance) and inductance (speed of energy
delivery). Low ESL devices typically have low capacitance. Likewise,
higher capacitance devices typically have higher ESLs. This tradeoff
between ESL (speed of energy delivery) and capacitance (energy
storage) drives the PDN design topology that places the fastest low ESL
capacitors as close to the load as possible. Low Inductance MLCCs are
found on semiconductor packages and on boards as close as possible
to the load.
The classic PDN topology is comprised of a series of capacitor stages.
Figure 1 is an example of this architecture with multiple capacitor
stages.
An ideal capacitor can transfer all its stored energy to a load instantly.
A real capacitor has parasitics that prevent instantaneous transfer of a
capacitor’s stored energy. The true nature of a capacitor can be modeled
as an RLC equivalent circuit. For most simulation purposes, it is possible
to model the characteristics of a real capacitor with one capacitor, one
resistor, and one inductor. The RLC values in this model are commonly
referred to as equivalent series capacitance (ESC), equivalent series
resistance (ESR), and equivalent series inductance (ESL).
Slowest Capacitors
Fastest Capacitors
Semiconductor Product
VR
Bulk
Board-Level
Package-Level
Die-Level
Low Inductance Decoupling Capacitors
Figure 1 Classic Power Delivery Network (PDN) Architecture
LOW INDUCTANCE CHIP CAPACITORS
INTERDIGITATED CAPACITORS
Thekeyphysicalcharacteristicdeterminingequivalentseriesinductance
(ESL) of a capacitor is the size of the current loop it creates. The smaller
the current loop, the lower the ESL. A standard surface mount MLCC is
rectangular in shape with electrical terminations on its shorter sides.
A Low Inductance Chip Capacitor (LICC) sometimes referred to as
Reverse Geometry Capacitor (RGC) has its terminations on the longer
side of its rectangular shape.
The size of a current loop has the greatest impact on the ESL
characteristics of a surface mount capacitor. There is a secondary
method for decreasing the ESL of a capacitor. This secondary method
uses adjacent opposing current loops to reduce ESL. The InterDigitated
Capacitor (IDC) utilizes both primary and secondary methods of
reducing inductance. The IDC architecture shrinks the distance between
terminations to minimize the current loop size, then further reduces
inductance by creating adjacent opposing current loops.
When the distance between terminations is reduced, the size of
the current loop is reduced. Since the size of the current loop is the
primary driver of inductance, an 0306 with a smaller current loop has
significantly lower ESL then an 0603. The reduction in ESL varies by EIA
size, however, ESL is typically reduced 60% or more with an LICC versus
a standard MLCC.
An IDC is one single capacitor with an internal structure that has been
optimized for low ESL. Similar to standard MLCC versus LICCs, the
reduction in ESL varies by EIA case size. Typically, for the same EIA
size, an IDC delivers an ESL that is at least 80% lower than an MLCC.
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