A soft-start function is a function that controls the time from when an IC enters the startup start state until the output voltage reaches the set voltage. When the device contains a soft-start function, the output rise time is constant regardless of the output capacity.
A similar function is the inrush current prevention function. The inrush current prevention function only suppresses the inrush current without controlling the startup time.
Voltage regulator startup waveform
(a) Without soft-start XC6216 @VIN=12V, VOUT=5V
(b) With soft-start XC6702 @VIN=12V, VOUT=5V
Having a soft-start function has the following advantages.
(a) Suppresses inrush currents and input voltage decreases
(b) Controls the output voltage rise time and slew rate
(a) Suppresses inrush currents and input voltage decreases
ICs without a soft-start function are generally used in products where the output voltage slew rate during startup is not controlled. In this case, the IC supplies current to the output side until the output voltage reaches the set output voltage, so an excessive inrush current is generated.
When an excessive inrush current is generated, the impedance on the input side reduces the input voltage. If this reduction in input voltage is large, it could an IC startup failure, system reset, etc.
Voltage regulator startup waveform : VIN=4V, VOUT=3V, CL=1μF, RL=20mA/3.0V
(b) Controls the output voltage rise time and slew rate
The soft-start function controls the time from when an IC enters the startup start state until the output voltage reaches the set voltage, so it can be used to adjust the output voltage rise time and slew rate for the startup sequence of downstream devices, etc.
Some products have a fixed soft-start function inside the IC but depending on the product, an externally connected component, etc., can be used to adjust the soft-start time.
Products that can adjust the soft-start time can adjust the output voltage slew rate and reduce the inrush current when a high-capacity capacitor is connected to the output.
When a high-capacity capacitor is connected to the output, the flow of excessive inrush current activates the protective function of the power supply IC, which can cause startup to fail, so generally, this startup failure is prevented by adjusting the soft-start time.
Startup waveform: XC9271 500kHz : VIN=12V/VOUT=5V, CL=22μF x2 + 330μF x 3
