SBFS042 June 2020 AFE3010
It is important to choose proper resistors and capacitors for the VDD pins. Sufficient decoupling capacitance is required to keep the internal 20-V shunt regulator stable during events in which the AFE3010 is driving loads with the ALARM, SCR, or NG_OUT pins. The values recommended in the typical application schematic and Table 5 have shown to maintain a stable VDD even during events when diodes in D1 rectifier were shorted. Additionally, the VDD regulator maintained sufficient voltage even when one of the current limiting resistors (R1 through R4) was disconnected. As a general rule, do not let the VDD regulator drop below 8 V during board failure events, or else the device could reset. If the capacitance at VDD is too large, then the device takes longer to power up, which adds to trip times when device is powered up with a fault current. The recommended values shown in Table 5 have shown to yield power-up trip times compliant with the UL 943 standard.
The current-limiting resistors for VDD (R1 through R4) should have enough resistance to reduce power dissipation, but should not be large enough to affect power-on trip times. To determine the maximum total power rating needed by these resistors, calculate the maximum instantaneous supply current (IVDD) needed by summing the maximum quiescent current and the maximum ALARM and SCR driver currents. The NG_OUT driver current has shown minimal effect on VDD regulation.
Place the decoupling capacitors as close as possible to the VDD pins to generate a short, low-impedance return current path to ground.