The LM5185 flyback controller operates over a wide input voltage range from 4.5 V to 100 V. The characteristics of the input supply must be compatible with the design specification ranges. 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 36.

where

• η is the efficiency

If the converter is connected to an input supply through long wires or PCB traces with a large impedance, special care is required to achieve stable performance. The parasitic inductance and resistance of the input cables can have an adverse effect 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. If the regulator 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 capacitors helps damp the input resonant circuit and reduce any voltage overshoots. A capacitance in the range of 22 µF to 100 µF is usually sufficient to provide input damping and helps to hold the input voltage steady during large load transients. A typical ESR of 200 mΩ 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 application report Simple Success with Conducted EMI for DC-DC Converters provides helpful suggestions when designing an input filter for any switching regulator.