- Public FAQ
Reason: Customer has questions for MAX38889, need to support.
Source: Theory analysis
Q: From below drawing, if DC/DC output 4.9V, and then will the boost voltage of MAX38889 to be 4.9V as well? Does the boost voltage adjustable? What is the accuracy for the boost voltage?
A: MAX38889 can adjust the boost voltage. But will up 2.5% lower than the charge voltage due to the hysteresis. If DC/DC output 4.9V, and then the boost voltage for MAX38889 will be up to 4.9*0.975=4.7775V. The accuracy is decided by chip internal threshold & external resistor divider.
Q: The supercap has 2.7V voltage, what is the lowest voltage it can discharge for keep boost voltage?
A: It is listed in the datasheet as above, actually it also determined by internal threshold & external divider.
During back-up, the super capacitor voltage will discharge and the boost converter automatically adjusts its duty cycle to regulate VSYS. When VSYS drops below UVLO threshold (VUVLOF) or VFBCH drops to 10% of the FBCH threshold voltage (VTH_FBCH), the boost regulator stops delivering load current and the super capacitor voltage is preserved with only the current from the resistor divider discharging the super capacitor further.