7.3.12 Soft Start

A soft-start feature can be implemented with the LM34926 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 before the V_OUT voltage. Capacitor C₁ is discharged and diode 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. 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. The effect of R₁ has been ignored to simplify the calculation.

Equation 5.

C1 is charged after the first start up. Diode D1 is optional and can be 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 .

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. The effect of resistor R1 is ignored.

Using the component values shown in Figure 12, 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