7.3.2 Start-up Regulator (V_CC)

A start-up regulator is integrated within the LMR24220. The input pin VIN can be connected directly to a line voltage up to 42 V. The V_CC output regulates at 6 V, and is current limited to 65 mA. Upon power up, the regulator sources current into an external capacitor C_VCC, which is connected to the VCC pin. For stability, C_VCC must be at least 680 nF. When the voltage on the VCC pin is higher than the UVLO threshold of 3.75 V, the main MOSFET is enabled, and the SS pin is released to allow the soft-start capacitor C_SS to charge.

The minimum input voltage is determined by the dropout voltage of the regulator and the V_CC UVLO falling threshold (≊ 3.7 V). If V_IN is less than ≊ 4 V, the regulator shuts off, and V_CC goes to zero.