SLUSDB2A August 2018 – December 2021 UCC28951
The voltage at the DCM pin provided by the resistor divider RDCMHI between VREF pin and DCM, and RDCM from DCM pin to GND, sets the percentage of 2-V current limit threshold for the Current Sense pin, (CS). If the CS pin voltage falls below the DCM pin threshold voltage, then the controller initiates the light load power saving mode, and shuts down the synchronous rectifiers, OUTE and OUTF. If the CS pin voltage is higher than the DCM pin threshold voltage, then the controller runs in CCM mode. Connecting the DCM pin to VREF makes the controller run in DCM mode and shuts both Outputs OUTE and OUTF. Shorting the DCM pin to GND disables the DCM feature and the controller runs in CCM mode under all conditions.
A nominal 20-µA switched current source is used to create hysteresis. The current source is active only when the system is in DCM Mode. Otherwise, it is inactive and does not affect the node voltage. Therefore, when in the DCM region, the DCM threshold is the voltage divider plus ΔV explained in Equation 15. When in the CCM region, the threshold is the voltage set by the resistor divider. When the CS pin reaches the threshold set on the DCM pin, the system waits to see two consecutive falling edge PWM cycles before switching from CCM to DCM and vice-versa. The magnitude of the hysteresis is a function of the external resistor divider impedance. The hysteresis can be calculated using Equation 15:
DCM must be used to prevent reverse current in the output inductor which could cause the synchronous FETS to fail.
The controller must switch to DCM mode at a level where the output inductor current is positive. If the output inductor current is negative when the controller switches to DCM mode then the synchronous FETs will see a large VDS spike and may fail.