Datasheet

Secondary Side Soft-Start

www.ti.com

8 Secondary Side Soft-Start

In a typical DC-DC converter with a 3.3 V output the voltage reference for the error amplifier is 1.2V. Prior

to the power supply being turned-on and if there is a pre-bias load, the secondary side soft-start capacitor

(CSS) will be pre-charged to the voltage reference level of 1.2 V (if the pre-bias load > 1.2V), refer to

Figure 9. On start-up the primary side soft-start begins and the output voltage rises from the pre-bias

voltage level to 3.3 V, refer to Figure 8. At the end of the primary side soft-start period the controller will be

at maximum duty cycle and the output voltage will overshoot until the feedback error amplifier has a

chance to respond and reduce the output voltage to the regulation set point.

Figure 8. Pre-bias Secondary Side Soft-Start

AN-1976 LM5027 Evaluation Board SNVA400B–August 2009–Revised May 2013

Submit Documentation Feedback