7.3.12 Soft Start

A soft-start feature can be implemented with the LM34925 device using an external circuit. As shown in Figure 11, 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 V_OUT voltage. Capacitor C₁ is discharged and diode D is thereby forward biased to pull up the FB pin 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. V_OUT 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 5. Please note that the effect of R₁ has been ignored to simplify the calculation.

Equation 5.

C1 is charged after the first start up. Diode D1 is added to discharge C1 when the input voltage experiences a momentary drop to initialize the soft-start sequence.

To achieve the desired soft start, the following design guidance is recommended:

(1) R₂ is selected so that V_FB is higher than 1.225 V for a V_CC of 4.5 V, but is lower than 5 V when V_CC is 8.55 V. If an external V_CC is used, V_FB should not exceed 5 V at maximum V_CC.

(2) C₁ is selected to achieve the desired start-up time which can be determined from Equation 6.

Equation 6.

(3) R₁ is used to maintain the node B voltage at zero after the soft start is finished. A value larger than the feedback resistor divider is preferred. Note that the effect of resistor R1 is ignored in Equation 5.

Based on the schematic shown in Figure 11, selecting C₁ = 1 uF, R₂ = 1 kΩ, R₁ = 30 kΩ results in a soft-start time of about 2 ms.

Figure 11. Soft-Start Circuit