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BQ24640: Does capacitor reach 104% or 100% capacity?

Lauren Mayfield
Expert 7935 points
Part Number: BQ24640

Hi BCP, 

Please see a question below from a customer. Thanks!

I am finally testing the BQ24640 and have encountered a specification problem: From the datasheet

 

Feedback regulation voltage (bottom of sheet 5)

Feedback regulation voltage = 2.1V

 

Out Overvoltage Comparator (middle of page 6)

Overvoltage rising threshold = 104% of VFB

Overvoltage falling threshold = 102% of VFB

 

This suggests that the capacitor should/could/will eventually reach 104% of what was programmed by the feedback network and voltage reference.

 

Looking further through the datasheet:

 

Figure 8 – Typical Charging Profile (page 10)

Voltage versus time plot strongly suggests that constant-current charging ceases prior to V_OREG (PROG) and that taper current finally brings voltage up to V_OREG (PROG)

 

That is the only use of “OREG” in the entire document. I suspect it is the same value as VFB, which is used in several places.

 

This suggests that the capacitor should/could/will eventually reach 100% of what was programmed by the feedback network and voltage reference.

 

Could you please help me resolve this dilemma?

  • 0
    TI__Mastermind 24075 points

    Hi Lauren,

        OREG is referring to VFB. VFB is internally regulated to 2.1V. The resistor divider that the user sets from actual battery voltage to ground, with the midpoint tied to VFB sets the charge regulation voltage.

    • Voltage versus time plot strongly suggests that constant-current charging ceases prior to V_OREG (PROG) and that taper current finally brings voltage up to V_OREG (PROG)
      • Behavior shown above is transition from CC to CV phase of charging. During CC phase charge current is regulated, and battery voltage is allowed to rise. During CV phase, battery voltage is regulated and charge current tapers down until termination current level is hit. This allows accurate charging of battery to maximize run time and battery capacity after consecutive charge discharge cycle.

    Battery OVP is a protection mechanism to ensure that battery voltage does not rise above 104% of programmed charge regulation voltage. 104% would be the rising threshold, and 102% would be the falling threshold for hysteresis. BQ24640 does not charge to this 104% threshold point. BQ24640 has +/- 0.5% charge regulation accuracy.

    During battery OVP: The converter will not allow the high-side FET to turn-on until the output voltage goes below 102% of the regulation voltage. This allows one-cycle response to an overvoltage condition – such as occurs when the load is removed.