• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 245 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3423 Questions

Gerry Kurz
Intellectual 685 points

Can someone explain to me the relationship between the duty cycle and cycle off time with regard to the PRO control, the current sense resistor and the peak current limit sensing in the buck boost application of this part. Specifically, I am trying to maximize the efficiency and I want to know if I can eliminate or severely reduce the peak current sense resistor.

In addition, the datasheet states "to minimize the effect of the high-side amplifier voltage offset on LED current accuracy, the minimum VSNS is suggested to be 50 mV." Is there some quantitative information about how the accuracy would be effected by going below 50 mV?

  • 0
    TI__Intellectual 2590 points

    Gerry,

    Some background first:  The PRO control is as follows...The FET is turned on and remains on until the peak inductor current reaches the threshold set by the closed loop error amp output COMP.  This peak is determined by the amount of energy necessary to regulate the current at the output.  This peak will change depending on the input voltage, output voltage, inductance, and switching frequency.  Once the FET is turned off, it remains off until the off-timer expires (when RCT pin reaches its threshold).  The off-timer is proportional to the swithc node voltage during the off time (which is VOUT in a boost converter or VIN+VOUT in a buck-boost) through the RC network that is external.  The off-timer is inversely proportional to VIN (reference is VIN/25 internally).  This creates a pseudo-fixed frequency operation (it's in the math :)).

    So to answer your question about the sense resistor:  The IS pin is also used for cycle by cycle current limit.  The threshold is 250mV, so you have to set over current limit via 250mV/Ris where Ris is the sense resistor.  To maximize efficiency you can make the overcurrent limit further from your operational maximum so that in operation you are at a lower threshold.  This is OK if your OC limit does not exceed your component ratings, however if you make the limit too high then your operational range may be too far in the system noise.  This can only be verified in the lab.

    As for the offset, that has to do with the LED sense resistor at HSP and HSN.  The part is designed to be operated above 50mV in steady state (not analog dimmed).  The offset of the amplifier is sufficiently small to statistically be irrelevant at those levels.  As you decrease from there it starts to contribute the innacuracy of the sense.

     

    -Jim