ZHCSBB6H April 2012 – November 2017 LM34927
PRODUCTION DATA.
A soft-start feature can be implemented with the LM34927 device using an external circuit. As shown in Figure 18, 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 to pull up the FB voltage. 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 will gradually rise 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. The effect of R1 has been ignored to simplify the calculation.
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:
Using component values shown in Figure 19, selecting C1 = 1 µF, R2 = 1 kΩ, R1 = 30 kΩ results in a soft-start time of about 2 ms.