Milpitas, CA—Supercapacitors are a viable and often attractive solution for supporting surge-power needs, providing system backup, and even allowing "last gasp" system operation and orderly shutdown. However, they need careful charging management, as well as balancing when stacked in a series-pair configuration (as they often are). The LTC4425 charger from Linear Technology Corp. greatly simplifies the task by providing constant-current/constant-voltage (CC-CV) charging as well as overall charge management, in a 12-lead package. The IC operates from, and charges, the supercaps from a Li-ion/polymer battery, a USB port, or a 2.7 to 6.5 V DC supply.
Figure 1: LTC4425 charger IC manages a series pair of supercapacitors, charges them from Li-ion/polymer cells, USB port, or DC source.
(Click on image to enlarge)
The LTC4425 normally charges the top of the stack to Vin
with charge current which varies inversely with the input/output differential, which prevents overheating. In LDO mode, it charges the stack using a user-set fixed charge current, reaching a resistor-programmed final voltage. Maximum charge current is 2 A.
In addition, the IC includes a near-ideal (low forward drop), 50-mΩ diode so the battery stack will not be able to "back drive" the input voltage. Such cell-balancing in the charger IC ensures each cell in the series pair has equal voltages, thus eliminating the need for external balancing resistors.
Six passive external component are needed for full operation of the IC, which operates with 20 μA quiescent current when the output voltage is in regulation. In full shutdown, quiescent current is 2 μA. It also offers Vin power-fail indication output, and current and thermal limiting which reduces the charge current when temperature become excessive.
Packaging, Pricing, and Availability: The LTC4435 is available now from stock, and is offered in a 3×3 mm×0.75 mm high DFN package, and a MSOP package (both with 12 leads). Pricing begins at $2.25 (1000 pieces).
For more information: contact Linear Technology Corp., http://www.linear.com.♦