SLUSBL5A February 2015 – June 2019 UCC28730
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, tWAKE, of the wake-up signal should be at least 1/4-wavelength of the switched-node resonant frequency, fRES. 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, CSWN, includes the MOSFET COSS, the transformer winding capacitance, and all other stray circuit capacitance attached to the MOSFET drain. Use Equation 31 to determine fRES. Conversely, if fRES is known by experience or measurement, CSWN can be derived from 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 LP is generally fixed by other system requirements, only CSWN can be reduced to increase fRES, if necessary.
Equation 33 is used to ensure that there is sufficient amplitude at the VS input to reliably trigger the wake-up function, where RWAKE_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.