SNVS953G December 2012 – May 2021 LM25018
A soft-start feature can be implemented with the LM25018 using an external circuit. As shown in Figure 7-3, the soft-start circuit consists of one capacitor, C1, two resistors, R1 and R2, and a diode, D. During the initial start-up, the VCC voltage is established prior to the VOUT voltage. Capacitor C1 is discharged and D is thereby forward biased. The FB voltage is pulled up above the reference voltage (1.225 V) and switching is thereby disabled. As capacitor C1 charges, the voltage at node B gradually decreases and switching commences. VOUT gradually rises to maintain the FB voltage at the reference voltage. Once the voltage at node B is less than a diode drop above the FB voltage, the soft-start sequence is finished and D is reverse biased.
During the initial part of the start-up, the FB voltage can be approximated as shown in Equation 8.
C1 is charged after the first start-up. Diode D1 is optional and can be added to discharge C1 and initialize the soft-start sequence when the input voltage experiences a momentary drop.
To achieve the desired soft start, the following design guidance is recommended:
With component values from the applications schematic shown in Figure 8-1, selecting C1 = 1 µF, R2 = 1 kΩ, and R1 = 30 kΩ results in a soft-start time of about 2 ms.