SLUSBL5A February 2015 – June 2019 UCC28730
During voltage regulation, the controller operates in frequency modulation and amplitude modulation modes according to the control law as illustrated in Figure 16 below. The control law voltage VCL reflects the internal operating level based on the voltage-error amplifier output signal. Neither of these signals is accessible to the user, however the approximate VCL may be inferred from the frequency and amplitude of the current sense signal at the CS input. As the line and load conditions vary, VCL adjusts the operating frequency and amplitude as required to maintain regulation of the output voltage. Because the UCC28730 incorporates internal loop compensation, no external stability compensation is required.
The internal operating frequency limits of the device are fSW(max) and fSW(min), typically 83.3 kHz and 32 Hz, respectively. The choice of transformer primary inductance and primary-peak current sets the maximum operating frequency of the converter, which must be equal to or lower than fSW(max). Conversely, the choice of maximum target operating frequency and primary-peak current determines the transformer primary-inductance value. The actual minimum switching frequency for any particular converter depends on several factors, including minimum loading level, leakage inductance losses, switched-node capacitance losses, other switching and conduction losses, and bias-supply requirements. In any case, the minimum steady-state frequency of the converter must always exceed fSW(min) or the output voltage may rise to the overvoltage protection level (OVP) and the controller responds as described in the Fault Protection Section.
The steady-state Control-Law voltage, VCL, ranges between 1.3 to 4.85 V, depending on load, but may occasionally move below 0.75 V or above 4.85 V on load transients. Dropping below 0.75 V shifts the switching frequency to a lower range at light loads, while exceeding 4.85 V enters the constant-current mode of operation. There are 3 lower operating frequency ranges for progressively lighter loads, each overlapping the previous range to some extent, to provide stable regulation at very low frequencies. Peak-primary current is always maintained at IPP(max)/3 in these lower frequency levels. Transitions between levels is automatically accomplished by the controller depending on the internal control-law voltage, VCL.