DESCRIPTION
1. Start-up Circuit And Reference The FAN7601 contains a start-up switch to reduce power loss in the external start-up circuit of conventional PWM converters. The internal start-up circuit charges the Vcc capacitor with a 1mA current source if the line is connected until the soft start is completed as shown in Fig. 2. The soft start function starts when the Vcc voltage reaches the start threshold voltage(typically 12V) and it ends when the LATCH/SS pin voltage reaches 1V. The internal start-up circuit starts charging the Vcc capacitor again if the Vcc voltage is lowered to the minimum operating voltage (typically 8V). In such a case the UVLO block shuts down the output drive circuit and some other blocks to reduce the IC current, and the soft start capacitor is discharged to zero voltage. If the Vcc voltage reaches the start threshold voltage, the IC starts switching again and the soft start capacitor is charged from zero voltage. The internal start-up circuit supplies current until the soft start is completed .
Fairchild Semiconductor
