SLUSBL5A February 2015 – June 2019 UCC28730
PRODUCTION DATA.
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.
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 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.