UCC14241-Q1: Question about Rlim at single output, SW connected Rcharge and Rdischarge

Answers below in blue:

(1)  On page 30 of the datasheet, there is a recommended circuit diagram for a single output, and the resistor connected to the Rlim terminal is marked as (optional).

- Is it necessary to connect the Rlim resistor?

• Not necessary in most cases. In single output voltage configuration, the RLIM resistor connected to VDD helps to discharge VDD-VEE voltage rail faster through the internal RLIM function. This can help to set the conditions to clear a fault quicker as well as assuring the load is quickly discharged so the next restart will not attempt to start into a pre-biased load. If this is not a concern, it is acceptable to leave the RLIM pin as open and save the PCB routing and cost of a single resistor. Therefore, from a functional point of view is is optional to include the RLIM resistor when UCC14241-Q1 is configured for a single output voltage.

- If so, what is the recommended resistance value and why?

• If RLIM is used, test with something in the range of 500Ω-1kΩ. The purpose (reason why) is to protect the internal RLIM switches by limiting the current

(2)  Please advise on the operation of the SW connected to Rcharge and Rdischarge on page 20 of the datasheet.

When using with single output, the FBVEE terminal must be shorted to the FBVDD terminal, so I think that a voltage of about 2.5V is basically applied to each terminal. However, if a fluctuation of 2.5V±α occurs due to a fluctuation in VDD, I think that the SW connected to Rcharge and Rdischarge built into the IC will switch according to the output of the comparator. Is this a problem in terms of the IC specifications?

In the case of single output, since the RLIM terminal is connected to VDD, if the SW connected to Rdischarge is turned on, there is a concern that the IC will heat up because the current flows through the path VDD→RLIM→SW→Rdischarge→VEE.

• RLIM internal switches will not turn on during normal steady state operation. RLIM considers the difference between FBVDD and FBVEE to determine if the RLIM pin should source/sink current. Since FBVDD=FBVEE (FB pins are shorted)

Steve