SNVS788H January 2012 – August 2021 LM5019
A soft-start feature can be implemented with the LM5019 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 exceeds the reference voltage (1.225 V) and switching is therefore 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 follows. Note that the effect of R1 has been ignored to simplify the calculation shown in Equation 5.
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 from the 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.