7.2.2 Startup Regulator (V_CC)

A startup regulator is integrated within the LM3103. 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 30 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 1 µF. When the voltage on the VCC pin is higher than the under-voltage lock-out (UVLO) threshold of 3.7 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 (≊3.4 V). If V_IN is less than ≊4.0 V, the regulator shuts off and V_CC goes to zero.