The LM25149-Q1 buck controller is designed to operate from a wide input voltage range of 3.5 V to 42 V. The characteristics of the input supply must be compatible with the Section 7.1 and Section 7.3. In addition, the input supply must be capable of delivering the required input current to the fully loaded regulator. Estimate the average input current with Equation 51.

Equation 51.

where

• η is the efficiency.

If the regulator 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 underdamped 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. The best way to solve such issues is to reduce the distance from the input supply to the regulator and use an aluminum or polymer input capacitor in parallel with the ceramics. The moderate ESR of the electrolytic capacitors helps damp the input resonant circuit and reduce any voltage overshoots. A capacitance in the range of 10 µF to 47 µF is usually sufficient to provide parallel input damping and helps to hold the input voltage steady during large load transients.

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.