When powering up the MAX5974, the IC will detect the voltage at the SS pin. The internal soft-start circuit will not work if the detected SS voltage is larger than 0.15V. After reviewing the MAX5969A+MAX5974AEVKIT schematic, there are three components that have direct connections to the SS pin, namely C3, R29 and Q2, as shown below:
C3 is the external capacitor from SS to GND that programs the soft-start time, R29 and Q2 are components that make up the COMP soft-start circuit. B ecause a high voltage at COMP pin charges the soft-start capacitor C3 through Q2, the SS voltage can be higher than 0.15V before the IC initiates the soft-start. With the presence of Q2, MAX5974 cannot start successfully at its first start-up attempt. T he internal soft-start circuit is enabled when the SS voltage is lower than 0.15V and after 1024 clock cycles delay. The bleeding resistor R29 also helps to discharge the soft-start capacitor voltage to a value lower than 0.15V. Another straight forward option is to increase the soft-start capacitor value. When the capacitance value is increase the soft-start voltage will be less after 1024 cycles to ensure its value is less than 0.15V.