SWCS142 July 2018 TPS650861
PRODUCTION DATA.
The buck controllers (BUCK1, BUCK2, and BUCK6) have inductor-valley current-limit architecture and the current limit is programmable by an external resistor at the ILIMx pin. Equation 1 shows the calculation for a desired resistor value, depending on specific application conditions. ILIMREF is the current source out of the ILIMx pin that is typically 50 µA, and RDSON is the maximum channel resistance of the low-side FET. The scaling factor is 1.3 to take into account all errors and temperature variations of RDSON, ILIMREF, and RILIM. Finally, 8 is another scaling factor associated with ILIMREF.
where
where
The buck converter limit inductor peak current cycle-by-cycle to IIND_LIM is specified in Section 4.8.
The current limit circuit also protects against reverse current going back into the low side FET from the load. When operating in Force PWM mode, the inductor current is expected to go negative so it is important to ensure that the RILIM value is sufficient to account for this. If operating in PFM, this can be neglected. The equation for Force PWM minimum RILIM value is:
where
where
If RILIM is too low for the chosen inductor and voltage conditions, then the ripple current at no load will trigger the negative current limit, forcing the low side FET to turn off. This will eventually result in the output voltage increasing above target regulation point due to irregular duty cycle created by current limit being triggered.