Supercapacitors (or ultracapacitors) are increasingly being used for short-term energy storage and applications that require intermittent high energy pulses. This article introduces a power ride-through circuit application, in which a backup energy source cuts in and powers the load if the main power supply fails for a short time. In the past, this type of application was dominated by batteries, but electric double layer capacitors (EDLCs) are quickly gaining ground due to falling price-per-farad, size, and effective series resistance per capacitance (ESR/C). In a power ride-through application, series-stacked capacitors must be charged and cell-voltage balanced. Supercaps are switched into the power path when needed and the power to the load is controlled by a DC/DC converter. The LTC3225 supercapacitor charger has several features that make it a good choice for power ride-through applications. It comes in a small, 10-lead 3mm × 3mm DFN package and features programmable charging current, automatic cell voltage balancing, low drain current on the supercapacitors, and a patent pending, low noise, constant current charger.
This document is an application note from Linear Technology Magazine (September 2008) focusing on replacing batteries with robust supercapacitors in power ride-through circuits. It introduces the LTC3225 as a compact solution for charging series-stacked supercapacitors and maintaining cell voltage balance.
The text details the advantages of supercapacitors over batteries, such as long cycle life (greater than 500,000 cycles) and low maintenance requirements. It provides design equations for estimating effective capacitance and circuit resistance to determine ride-through time, along with example application circuits using the LTC3225 in conjunction with DC/DC converters.
| Package Type | 10-lead 3mm x 3mm DFN package |
|---|---|
| Charging Current | up to 150mA |
| Charge Voltage Options | 4.8V or 5.3V |
A capacitor's cycle life is quoted as greater than 500,000 cycles; batteries are specified for only a few hundred cycles.
The charger features a programmable charging current of up to 150mA.
The device charges two series supercapacitors to either 4.8V or 5.3V.