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UCC28700: Current Limit

Dan Cristy
Intellectual 2615 points
Part Number: UCC28700
Other Parts Discussed in Thread: UCC24636, UCC28704, PMP15002

Hi Team –

Posting this on customer behalf.

We have a couple clarifying questions on the UCC28700:

We’re trying to limit our output to under 15W.  Is there any problem with continually running this part while hitting the current sense limit?

Also, we’re seeing the current limit drop as the circuit heats up, I think this is related to the resistor we are using, but could it be in the UCC28700? Anything specific that we should be looking at?

  • 0
    TI__Expert 7395 points
    Dan,
    Thanks for your interest here.
    To answer your direct questions:
    1) No-no issue running this part in a power supply while continually hitting the current sense limit as long as you size the external components to withstand the electrical stresses and thermal requirements. As you can see on Figure 17 of the UCC28700 datasheet, it has a constant-current, constant-voltage V-I profile which helps to protect the port or supply.
    2) the current limit should drop as the circuit heats up. There is little variance in the CS threshold of the IC over temperature as you can see in Figure 5 on page 7 but the current sense resistor will experience a change. The resistance will increase with increased temperature so the current limit will go down. If you find a current sense resistor with tighter temp tolerance that could prevent this, or use larger package Rcs to limit self-heating. Please also note that the peak current limit (in each cycle) changes depending on where you are in the control law profile as seen in Figure 15. This is the reason for the 2 CS limits in the electrical table.

    As a seperate note, I would encourage you to use the UCC28704 over UCC28700 as the control law has been adjusted to maximize efficiency and other features have been added to support more robust supplies. UCC24636 Synchronous rectifier controller also pairs well with this controller for higher efficiencies or to meet DOE or CoC efficiency standards if this is for an External Power Supply product. See also our PMP15002 reference design online using the UCC28704 and UCC24636 for 5V, 15W USB-C AC/DC power.

    Regards,
    John