A buck switch mode supercap charger is needed when the

• The input voltage is higher than V_REG.
• I_CHG is high, typically > 1.0A
• The supercap's capacitance is high or fast charging time is required
• The system load can be connected directly to the supercap

The BQ24640 is a supercap charge controller with external FETs as shown below. The VFB pin resistors set the CV voltage, V_REG, and the ISET resistors set the CC current, I_CHG. The optional TS pin senses the supercap temperature from a NTC thermistor placed on the supercap and disables charge if the supercap temperature is outside a set window.

Figure 2-4 BQ24640 Supercap Charger Block Diagram

A complete charge cycle using the BQ24640 to charge a 10 F supercapacitor to 5 V with I_CHG=4A and V (adapter) = 24 V is shown in Figure 2-5.

Figure 2-5 BQ24640 Charge Cycle of a 10 F Supercap with ICHG = 4 A

Note that to protect the power FETs, while the supercap voltage is less than 2.1 V, the ISET pin is clamped to 600 mV. For a 10 m ohm current sense resistor (the resistor between SRP and SRN pins), this clamp limits the max charge current to 3 A even if the ISET pin sets I_CHG higher. Termination is disabled on the BQ24640 so the STAT pin (not shown above) does not change state when the supercap reaches full charge. The converter operates in CV mode indefinitely to keep the supercap charged to 5 V.