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LM3421: Peak Current Limit Setting

Omacitin
Prodigy 40 points
Other Parts Discussed in Thread: LM3421

Hi,


The LM3421 reference designs have a peak current limit, set with an R(lim).  It's discussed in section 7.3.5 (page 15) of the datasheet.  It says there that

The LM34xx-Q1 achieves peak current mode control using a comparator that monitors the main MOSFET (Q1) transistor current, comparing it with the COMP pin voltage as shown in Figure 15. Further, it incorporates a cycle-by-cycle overcurrent protection function. Current limit is accomplished by a redundant internal current sense comparator. If the voltage at the current sense comparator input (IS) exceeds 245 mV (typical), the on cycle is immediately terminated.

There are two possible methods to sense the transistor current. The RDS-ON of the main power MOSFET can be used as the current sense resistance because the IS pin was designed to withstand the high voltages present on the drain when the MOSFET is in the off state. Alternatively, a sense resistor (RLIM) located in the source of the MOSFET may be used for current sensing; however, TI suggests a low inductance (ESL) type. The cycle-bycycle current limit (ILIM) can be calculated using either method as the limiting resistance (RLIM):

I(lim) = 245mV/R(lim)

From that, it sounds like R(lim) is not involved in the converter control system, it is simply a protection feature.  Is this correct?


If that's right, are R(lim) and the IS pin needed for the LM3421 to function properly?  It seems like R(lim) would dissipate a lot of heat.


And finally, what should I(lim) be set to, in either case?  Is it designed to keep the inductor from saturating, or something else?  The LED current is independent of this limit, correct?


Thank you for your time.

 

P.S. This question here is similar, but I think the person that answered misunderstood.  R(lim) doesn't appear to be linked to the LED current.

  • 0
    TI__Mastermind 25500 points
    The IS pin is needed for proper operation. That is why it is attractive that it is a high voltage pin so that you can use Rds(on) sensing with the FET since it will be there burning power anyway. However, Rds(on) can vary a bit so it isn't as accurate for setting current limit as a resistor would be. But the LED current you set will be completely independent of where you set the current limit as long as you don't set it too low. Generally the current limit is to keep the inductor from saturating. If it does the peak currents could get too high and destroy the FET and possibly the diode.