SLUSCZ1 May 2017 TPS92518-Q1
PRODUCTION DATA.
The converter Off-time is controlled via the LEDx_TOFF_DAC[7:0] register and VLEDx pin. The VLEDx pin voltage is converted to a current that charges an internal capacitor to a voltage set by the LEDx_TOFF_DAC creating a delay. Details of this circuit are shown in the Functional Block Diagram. Deriving the off-time from the output voltage creates a ramp representing the inductor current.
When the TPS92518-Q1 is first enabled (All UVLO levels are cleared) both timer capacitor pull-downs are disabled allowing voltage to increase on the internal timer capacitors. When either capacitor reaches the matching DAC control voltage, the high-side FET is turned on, starting a switching cycle. The maximum off-time timer is always dominant at start-up when the output is completely discharged or when shunt FET dimming and the shunt FET shunts the output for the required period.