0
\$\begingroup\$

ILIM (pin 9) on the UC3825B is confusing to me. I know the current limit threshold voltage (typ. 1 V) and the overcurrent threshold voltage (typ. 1.2 V) from the datasheet, but I don't know how that information is useful, or how to hook up this pin.

In the examples I've seen (specifically this application note on pg. 9) it seems there is a resistor (R3) hooked up to the ILIM pin right after the sense resistor(s?) R5-R8 (correct me if I'm wrong). They mention R3 and C8 are a RC network to suppress leading edge from mosfet turn-on.

  1. How do you use the current limiting feature on this chip?

  2. How did they calculate the RC network and why does this ILIM connection work?

I'd greatly appreciate some help, and as always let me know if you need any more information.

\$\endgroup\$
6
  • \$\begingroup\$ Current sense is discussed on p. 3-96 (3) of the appnote ("Sense Resistor R (s)"). I agree the application information is brief, especially lacking in the datasheet. Mechanism appears similar to UC3842 family, if that's any help: they are well documented in comparison. \$\endgroup\$
    – Tim Williams
    Commented Aug 17, 2022 at 21:38
  • \$\begingroup\$ @TimWilliams Could you provide a link to these documents? I cannot find them from a google search. I do understand the sense resistor part fairly well regardless of the fact that they never use the sense resistor value mentioned in the application note. I'm only really confused about the ILIM portion. \$\endgroup\$
    – sarrio
    Commented Aug 17, 2022 at 22:58
  • \$\begingroup\$ Sure, ti.com/lit/an/slua143/slua143.pdf ti.com/lit/ds/symlink/uc3842a.pdf The RC network to ILIM reduces leading-edge triggering, evidently they felt some was necessary in addition to the internal blanking feature. \$\endgroup\$
    – Tim Williams
    Commented Aug 18, 2022 at 0:17
  • \$\begingroup\$ @TimWilliams So basically you just connect the pin to the sense resistor (disregarding the RC network) and whatever voltage is going through it will be fine for the pin (since I already have the sense resistor calculated)? I'm sure you have to add at least one resistor right? \$\endgroup\$
    – sarrio
    Commented Aug 18, 2022 at 1:49
  • 1
    \$\begingroup\$ That extra \$RC\$ network is there to filter out any spurious pulses happening at turn-on. These spikes can wrongly trip the current sense comparator and stop operations erratically. See my answer on SE here. These extra elements should be located very closely to the control IC. \$\endgroup\$
    – Verbal Kint
    Commented Aug 18, 2022 at 6:51

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.