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
The key physical characteristic determining equivalent series inductance
(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.
The Important Information/Disclaimer is incorporated in the catalog where these specifications came from or
available online at www.avx.com/disclaimer/ by reference and should be reviewed in full before placing any order.
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