I had an interesting idea I wanted to explore. I’ve had my eye on the UCC25800-Q1 for a while which is similar to the MPQ18913. In the datasheet for the former, in regards to the SYNC functionality, it says:
Given that, to ensure the output voltage remains within the normal operation range, the half of the frequency of external synchronization signal needs to be between 15% and 30% (nominal) above the programmed switching frequency with a tolerance of 5% or less, as described in Equation 2. A minimum high and low pulse width of 150 ns is required.
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The transformer driver ignores the external synchronization signal during the 1.5-ms soft-start time. The switching frequency during the soft-start time is based on the RT pin voltage as described in Section 8.3.2. After the soft-start period ends, if an external synchronization signal is present and its frequency and pulse width are within the specified range, the switch node is driven by the SYNC pin signal. The transformer driver also integrates a hand-off algorithm so that when the switching frequency transitions from internal oscillator to the external synchronization signal, the disturbance is minimal and transformer saturation is avoided.
The hand-off algorithm first confirms that the external synchronization signal is within the range. If the frequency is not within the acceptable range, the hand-off doesn’t happen. If the frequency is within the acceptable range, the hand-off algorithm begins to search for the optimal transition point and locks the switching frequency with the external SYNC signal. After the frequency is locked, the hand-off algorithm stops monitoring the SYNC pin frequency.
The datasheet for the MPQ18913 does not specify what happens after the PLL acquires the SYNC clock signal:
Then EN/SYNC pin can also be used for external clock synchronization. Connect a clock with a 400kHz to 1.67MHz frequency to the EN/SYNC pin. The internal clock frequency scales to be 1x or 3x the external clock frequency while continuing to operate at a 50% duty cycle. The scale factor is set by the FREQ resistor (RFREQ).
The idea I wanted to explore was using some of the sigma-delta modulator channels on the real-time MCU I’m using in conjunction with the AMC0336 to implement semi-closed loop control via frequency modulation of the SYNC clock. The application is a half-bridge isolated gate driver power supply. This is obviously impossible with the UCC25800-Q1 as the frequency range is too narrow and does not continuous track SYNC. Is this also true for the MPQ18913?