SNVSBZ3 June 2021 LM5168-Q1
The LM516x-Q1 buck converter is designed to operate from a wide input voltage range between 6 V and 120 V. The characteristics of the input supply must be compatible with the Absolute Maximum Ratings and Recommended Operating Conditions tables. In addition, the input supply must be capable of delivering the required input current to the fully loaded regulator. Use Equation 33 to estimate the average input current.
If the converter is connected to an input supply through long wires or PCB traces with a large impedance, take special care to achieve stable performance. The parasitic inductance and resistance of the input cables can have an adverse affect on converter operation. The parasitic inductance in combination with the low-ESR ceramic input capacitors form an under-damped resonant circuit. This circuit can cause overvoltage transients at VIN each time the input supply is cycled ON and OFF. The parasitic resistance causes the input voltage to dip during a load transient. If the converter is operating close to the minimum input voltage, this dip can cause false UVLO fault triggering and a system reset. The best way to solve such issues is to reduce the distance from the input supply to the regulator and use an aluminum electrolytic input capacitor in parallel with the ceramics. The moderate ESR of the electrolytic capacitor helps to damp the input resonant circuit and reduce any voltage overshoots. A 10-μF electrolytic capacitor with a typical ESR of 0.5 Ω provides enough damping for most input circuit configurations.
An EMI input filter is often used in front of the regulator that, unless carefully designed, can lead to instability as well as some of the effects mentioned above. The Simple Success with Conducted EMI for DC-DC Converters Application Report provides helpful suggestions when designing an input filter for any switching regulator.