JAJU858 December 2022
The shunt voltage reference (U6_P) sets the final charge voltage to 7.8 V. The NPN transistor (Q4_P) and 4.99-Ω resistor (R38_P) limit the charge current to approximately 120 mA (0.6 V / 4.99 Ω). The NMOS FET (Q3_P) operates in saturation region to maintain the required charge voltage drop. U6_P pulls down the gate of Q3_P as soon as Vbackup (as shown in the schematic) reaches 7.8 V. This way, Vbackup stays constant at the nominal 7.8 V. There are two charging scenarios:
In summary, the worst-case charging time is 1 minute and 21 seconds, while recharging between energy storage releases is 36.46 seconds.