BQ25570: need for clarification

Hi Stefan,

Please see my responses embedded below:

• Can I connect a big Lithium Ion Capacitor (LIC) to Vbat pin (10-20F)? What are the drawbacks there? (except a long start-up…)

• Yes to capacitance and no drawbacks except long startup, especially during cold start

• What about the in-rush current to charge a fully depleted LIC? Can the device manage it?

• Small in-rush as the boost charger turns off high side FET when inductor current hits peak current limit

• What happens if there is no sun at all and Vbat>Vbat_uv: is the load on buck output still connected or not?

• Buck output is available as long as VSTORE=VBAT>VBAT_UV

• Why do I have to connect a load on Vstore? Isn’t the load supposed to be on buck output?
  • You can optionally have load at VSTOR and/or buck VOUT.
• Chapter 7.3.2: For the VBAT_UV feature to function properly, the system load should be connected to the VSTOR pin while the storage element should be connected to the VBAT pin
  What does this mean? The feature doesn’t work if I connect my load on buck output?
  • The under-voltage protection feature turns off the PFET between VBAT and VSTOR to prevent a load on VSTOR from deeply discharging the battery at VBAT.  The body diode allows current to flow from VSTOR to VBAT for charging.

• Chapter 7.4.2: “An inverted VBAT_OK signal provided by VB_SEC_ON can be used to drive the gate of this system-isolating, external PFET”
  I can’t find the signal VB_SEC_ON...I cannot find also the pin named VBAT_SEC

• This is a datasheet typo.  VB_SEC_ON is a pin on the BQ25505.  The concept can be implemented on BQ25570 but inverting the VBAT_OK signal with an NFET, which then tries a PFET to isolate a load on VSTOR that might slow or prevent charge of the battery at BAT.

Regards,

Jeff