The converter OFF-time is controlled via the COFF pin. The output voltage charges a capacitor to 1 V through a resistor creating a delay. Deriving the OFF-time from the output voltage creates a ramp representing the inductor current. If the output voltage cannot be used, another voltage fixed source may be implemented to create a truly constant OFF-time. However, this configuration reduces output current accuracy. When the device is first enabled (when VCC rises above the VCC undervoltage lockout threshold) the pull-down on the COFF pin is disabled, allowing a voltage to build up on the COFF capacitor. At the same time, the maximum off timer begins. If the voltage source is sufficiently above the 1-V threshold, the ramp becomes linear and approximates the inductor current. If the 1-V nominal COFF threshold is reached, or the COFF capacitor charge time duration is greater than tOFF(max) (maximum OFF-time timer expires), a switching cycle starts.
The timer reaches the maximum OFF-time during start-up when the output is completely discharged or when shunt FET dimming and the shunt FET shunts the output for the required period.
Equation 7 calculates ROFF for a desired OFF-time.