SLUSCR9B June 2017 – December 2020 UCC28730-Q1
PRODUCTION DATA
A major feature available at the VS pin is the wake-up function which operates in conjunction with a companion secondary-side wake-up device, such as the UCC24650. This feature allows light-load and no-load switching frequencies to approach 32 Hz to minimize losses, yet wake the UCC28730-Q1 from its wait state (sleep mode) in the event of a significant load step between power cycles. Despite the low frequencies, excessive output capacitance is not required to maintain reasonable transient response. While in the wait state, the UCC28730-Q1 continually monitors the VS input for a wake-up signal, and when detected, responds immediately with several high-frequency power cycles and resumes operation as required by the control law to recover from the load-step transient and restore output voltage regulation.
Because the wake-up feature interrupts the wait state between very low frequency switching cycles, it allows the use of a much lower output capacitance value than would be required to hold up the voltage without the wake-up function. It also allows the controller to drop to extremely low switching frequencies at no-load conditions to minimize switching losses. This facilitates the achievement of less than 5 mW of input power to meet zero-power stand-by requirements. Use of the UCC28730-Q1 controller alone cannot ensure zero-power operation since other system-level limitations are also imposed, however, the UCC28730-Q1 and UCC24650 combination goes a long way to reaching this goal.
The signals illustrated in Figure 7-7 refer to circuit nodes located on the Figure 3-1 diagram on the first page of this datasheet. The wake-up signal, which is provided by a secondary-side driver, must meet certain criteria to be considered valid and recognized by the UCC28730-Q1 at the VS input. To distinguish the signal from the residual resonant ringing that follows a switching power cycle, the resonant ringing amplitude must diminish and remain below the wake-up signal detection threshold, VWU, for a fixed qualification time, tWUDLY.
The UCC28730-Q1 has two such thresholds; one at VWU(low) and one at VWU(high). The lower VWU(low) threshold is used by converters which incorporate a relatively high-impedance driver for the wake-up signal, while the upper VWU(high) threshold may be used in converters with a low-impedance wake-up driver. Both thresholds work exactly the same way. The advantage of the upper threshold is that the UCC28730-Q1 is qualified to accept a strong wake-up signal without waiting additional time for the resonant ringing to diminish below the lower threshold.
Figure 7-8 illustrates the qualification delay period and wake-up response to a low-level wake-up signal. Figure 7-9 illustrates the qualification delay period and wake-up response to a high-level wake-up signal.