System clock frequencies are increasing while logic operating voltages are dropping, resulting in designs that experience high switching-current demands. To meet those demands, designs depend on noise-reduction capacitors to store charge between clock cycles and deliver that energy during switching. A capacitor's effectiveness in this application, however, depends on its equivalent-series resistance (ESR) value. Because this resistance is in series with the energy source, the ripple voltage on a board's supply power will be directly proportionate to the ESR of the board's capacitors.
Major sources of ESR in a capacitor at today's switching frequencies are dielectric loss and cathode connection resistance. To control those losses, capacitor manufacturers have developed a number of formulations for the dielectric, as well as differing construction approaches. The types of low-ESR capacitors today fall into six major categories:
• Liquid electrolyte aluminum electrolytic: This choice offers high capacitance values and voltage ranges at low cost, but tends to be large and capable of drying out under high temperatures.
• Solid polymer electrolyte aluminum electrolytic: These capacitors offer very low ESR but at relatively higher cost, with moderate capacitance values and lower voltage ratings.
• Hybrid electrolyte aluminum electrolytic: They offer very low ESR at lower cost, but come with lower voltage ratings and dry-out concerns.
• Solid electrolyte tantalum electrolytic: These are small but offer limited transient tolerance.
• Polymer cathode instead of the typical MnO2: These capacitors offer very low ESR but at low voltage ratings and high cost.
• Ceramic chip: It offers ultralow ESR and nonpolarized operation but can suffer large effective capacitance loss at low operating voltages.
According to Patrick Wastal, senior vice president for IP&E marketing at distributor Avnet Inc., another approach to lowering ESR in capacitors has recently begun entering the market. This scheme uses multiple anodes to lower ESR by paralleling the internal connections in the capacitor. Several vendors now offer this multiple-anode construction.
The table shows representative low-ESR capacitor product families available in surface-mount packaging, along with their key characteristics.
Richard Quinnell is an engineer-turned-technology-journalist. He can be reached at RichQuinnell@att.net.
See related chart
