SNVSA47D February 2016 – December 2022 LM5165
The LM5165 has a flexible and easy-to-use soft-start control pin, SS. The soft-start feature prevents inrush current impacting the LM5165 and the input supply when power is first applied. Soft start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up. Selectable and adjustable start-up timing options include minimum delay (no soft-start), 900-µs internally fixed soft start, and an externally programmable soft start.
The simplest way to use the LM5165 is to leave the SS pin open. The LM5165 employs the internal soft-start control ramp and starts up to the regulated output voltage in 900 µs. In applications with a large amount of output capacitance, higher VOUT, or other special requirements, extend the soft-start time by connecting an external capacitor CSS from SS to GND. Longer soft-start time further reduces the supply current needed to charge the output capacitors and supply any output loading. An internal current source ISS of 10 µA charges CSS and generates a ramp to control the ramp rate of the output voltage. Use Equation 8 to calculate the CSS capacitance for a desired soft-start time tSS.
CSS is discharged by an internal FET when VOUT is shutdown by EN, UVLO, or thermal shutdown.
It is desirable in some applications for the output voltage to reach its nominal setpoint in the shortest possible time. Connecting a 100-kΩ resistor from SS to GND disables the soft-start circuit, and the LM5165 operates in current limit during start-up to rapidly charge the output capacitance.
As negative inductor current is prevented, the LM5165 is capable of start-up into prebiased output conditions. With a prebiased output voltage, the LM5165 waits until the soft-start ramp allows regulation above the prebiased voltage and then follows the soft-start ramp to the regulation setpoint.