SLVSE26B November 2017 – April 2018 TPSM84824
Leaving SS/TR pin open enables the internal soft-start time interval of approximately 1.25 ms. Adding additional capacitance between the SS pin and AGND increases the soft-start time. Increasing the soft-start time reduces inrush current seen by the input source and reduces the current seen by the device when charging the output capacitors. To avoid the activation of current limit and ensure proper start-up, the SS capacitor may need to be increased when operating near the maximum output capacitance limit.
|SS Time (ms)||1.25||2.4||3||3.8||6.8|
During soft-start, the output voltage increases from its starting voltage and rises into regulation. The device is allowed to skip pulses as needed whenever the application conditions exceed the minimum on-time of the device. This behavior is a function of input voltage, output voltage, switching frequency, and load current. During the initial rise of the output voltage, adding an additional non-ceramic output capacitor in parallel with the required ceramic capacitance will improve the output voltage ramp-up.
When testing soft start performance with an electronic load, the output voltage noise can be exaggerated due to the control loop of the load. Testing with a pure resistive load is a better way to quantify the device performance.