Linear Technology has put the whole of a supercapacitor power back-up system on one chip.
Rated at 2A, it will back-up a voltage rail (Vsys) between 1.71 and 5.25V.
Key to the chip, called LTC3110, is a 1.2MHz four-switch bi-directional dc-dc converter that both charges the capacitor and can also extract almost all of the energy remaining in it – as it keeps on switching until the capacitor has only 100mV left. Two voltage supervisors automate direction control and end of charge.
One or two (at up to 5.5V, set by a resistor) supercaps can be used, with an active (non-switching) balancing circuit dealing with capacitance imbalance when two are used in series – avoiding the constant drain of passive balance resistors.
With a buck-boost converter on-board, designers have freedom to design a nominal Vcap above or below the backed-up system voltage.
Direction control is through the DIR pin – via a logic level from a microcontroller, or by sensing a voltage using the comparator internally connected to the pin – which has a small amount of hysteresis.
To prevent over-loading the system rail when the capacitor is charging, maximum input current in charge mode can be set by a resistor (125mA to 2A average, +/-2% accuracy).
Optional burst-mode operation improves light-load efficiency, and reduces standby to 40µA. Shutdown is <1µA.
Thermal overload protection is included, and there is a spare general-purpose comparator with an open-collector output for interfacing with a microcontroller.
Although the LTC3110 will run from 100mV on the capacitor bank, at least 1.8V is needed for a cold start-up.
Operation is not limited to using supercapacitors, and the data sheet includes examples of Pb-acid and NiMH energy stores.
For more detail: LT puts supercapacitor power back-up on a chip