If the input decoupling capacitor is not ceramic (and has an ESR greater than  20 mΩ), then at the instant the power transistors switch on, the voltage at the input pins falls momentarily. If the voltage falls below approximately 4 V, the device detects an undervoltage condition and switches the internal drive circuits to a momentary off state. This detection is carried out as a precaution against a genuine low input voltage condition that could slow down or even stop the internal circuits from operating correctly. A slow-down or stoppage results in the drive transistors being turned on too long, causing saturation of the transformer and destruction of the device.

Following detection of a low input voltage condition, the device switches off the internal drive circuits until the input voltage returns to a safe value, at which time the device tries to restart. If the input capacitor is still unable to maintain the input voltage, shutdown recurs. This process repeats until the input capacitor charges sufficiently to start the device correctly.

Normal start-up should occur in approximately 1 ms after power is applied to the device. If a considerably longer start-up duration time is encountered, it is likely that either (or both) the input supply or the capacitors are not performing adequately.

For 5-V to 15-V input devices, a 2.2-μF, low-ESR ceramic capacitor ensures good startup performance. For 24-V input voltage devices, 0.47-μF ceramic capacitors are recommended. Tantalum capacitors are not recommended, since most do not have low-ESR values and will degrade performance. If tantalum capacitors must be used, close attention must be paid to both the ESR and voltage as derated by the vendor.