The LTC4425 employs a linear CC-CV architecture with thermal limiting to charge two supercapacitors in series to a programmable output voltage from a Li-Ion/polymer battery, a USB port, or other 2.7 V to 5.5 V current-limited supply.

The LTC4425 features two operating modes: charge current profile (normal) mode and LDO mode. The device’s charge current profile mode charges the top of the supercap stack to the input voltage V_IN with a charge current that varies inversely with the input-to-output voltage differential in order to present excessive heating. The LDO mode charges the stack to an externally programmed output voltage with a fixed charge current that is also externally programmable. Charge current is resistor programmable up to 2 A (3 A peak), and each capacitor is protected against overvoltage by internal shunts (2.45 V/2.7 V selectable). The LTC4425’s onboard current-limited ideal diode features low 50 mOhm on resistance to prevent back-driving of V_IN, making it suitable for a wide variety of high peak power battery and USB-powered equipment, industrial PDAs, portable instruments and monitoring equipment, power meters, supercap backup circuits and PC card/USB modems.

The LTC4425’s automatic cell balancing feature maintains equal voltages across both cells, eliminating the need for balancing resistors while protecting each supercapacitor from overvoltage damage while minimizing current drain on the capacitors. The IC operates with a very low 20 uA quiescent current when the output voltage is in regulation and draws only 2 uA in shutdown from either V_IN or V_OUT, whichever is higher. The basic charging circuit requires only six external components and is highly compact, offered in a tiny 9 mm² package footprint as well as a leaded package. Other key features include a V_IN power fail indicator and continuous monitoring of V_IN to V_OUT current via the PROG pin. Additional protection features include current and thermal limiting that reduces charge current in cases of excessive temperature.

Summary of Features: LTC4425

•    Linear Constant-Current, Constant-Voltage, (CC-CV) Charging of Two Series Supercapacitors
•    50 mOhm Ideal Diode from V_IN to V_OUT
•    Smart Charge Current Profile Limits Inrush Current
•    Automatic Cell Balancing Prevents Capacitor Overvoltage During Charging
•    Programmable Output Voltage (LDO Mode)
•    Programmable V_IN to V_OUT Current Limit
•    Continuous Monitoring of V_OUT Current via PROG Pin
•    Low Quiescent Current: 20 uA
•    V_IN Power Fail Indicator
•    Programmable Charging Current (Up to 2 A Continuous), 3 A Peak Current Limit
•    Selectable 2.45 V or 2.7 V Max Voltage per Cell (4.9V/5.4 V Supercap Max Top-Off Voltage)
•    Tiny Application Circuit, All Components <1mm High
•    Compact, Low-Profile (0.75 mm) 3 mm × 3 mm 12-Lead DFN Package and MSOP-12 Package

Availability and Pricing

The LTC4425 is available in two compact, thermally enhanced packages; a 12-lead, low-profile (0.75 mm) 3 mm x 3 mm DFN package and a 12-lead MSOP package. Operation from -40 to 85 degrees C operating junction temperature is guaranteed. The LTC4425 is in stock, priced starting at 2.25 dollars each in 1,000-piece quantities.

Related link: www.linear.com