SLUAAO7 March   2023 BQ24640 , BQ25173 , BQ25713 , BQ25798

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Supercapacitors Charging Key Care-Abouts and Implementations
    1. 2.1 Supercap Charge Profile
    2. 2.2 Linear Based Supercap Charger
    3. 2.3 Switch-Mode Buck Converter Based Supercap Charger
    4. 2.4 Modifying Li-ion Chargers to Charge Supercaps
    5. 2.5 Using a Li-ion Buck-Boost Integrate FET Charger to Charge a Supercap or Li-ion Battery
    6. 2.6 Using a Buck-Boost Controller with External FETs to Charge a Supercap
  5. 3Summary
  6. 4References

2.2 Linear Based Supercap Charger

A step-down only linear regulator-based charger is best for applications where

  • The input voltage is higher than VREG.
  • ICHG is low, typically < 1.0A
  • The supercap's capacitance is low or charging time is not critical
  • The system load can be connected directly to the supercap

The BQ25173 is a linear regulator based supercap charger. The FB pin resistors set the CV voltage, VREG, and the ISET resistor sets the CC current, ICHG.

GUID-20211012-SS0I-0HZS-DV4G-WDGM2PSRBZXP-low.png Figure 2-2 BQ25173 Supercap Charger Block Diagram

A complete charge cycle of the BQ25173 charging a 10F supercap to 5 V with ICHG=800mA and V(IN)=5.5 V is shown in Figure 2-3.

GUID-20230111-SS0I-SGZ9-95S7-FLXKPSHFTK54-low.png Figure 2-3 BQ25173 Charge Cycle of a 10 F Supercap to 5.5 V with 800 mA Charge Current

Note the thermally reduced charge current at the beginning of the cycle. The STAT pin goes low when the supercap voltage reaches 98% of the regulation voltage. The charger stays in CV to maintain the supercap voltage at 5 V.