The amplitude of the wake-up signal at the VS input must be high enough to be detected. This signal, which originates on the secondary winding, is limited by the impedances of the wake-up signal driver and the L-C resonant tank of the transformer windings. The signal is further attenuated by the VS divider resistors. To maximize the wake-up signal amplitude, the pulse width, t_WAKE, of the wake-up signal should be at least 1/4-wavelength of the switched-node resonant frequency, f_RES. The resonant frequency depends on the primary magnetizing inductance and the total equivalent capacitance at the switching node, that is, the primary-side MOSFET drain node. The switched-node capacitance, C_SWN, includes the MOSFET C_OSS, the transformer winding capacitance, and all other stray circuit capacitance attached to the MOSFET drain. Use Equation 31 to determine f_RES. Conversely, if f_RES is known by experience or measurement, C_SWN can be derived from Equation 31.

Equation 31.

Since the wake-up pulse width is typically fixed by the driver device, such as the UCC24650, maximum signal strength is obtained when Equation 32 is true. Since L_P is generally fixed by other system requirements, only C_SWN can be reduced to increase f_RES, if necessary.

Equation 32.

Equation 33 is used to ensure that there is sufficient amplitude at the VS input to reliably trigger the wake-up function, where R_{WAKE_TOT} is the total secondary-side resistance of the wake-up signal driver and any series resistance. An over-drive of 15 mV is added to the wake-up threshold level for margin.

Equation 33.