Post Go back to editing

Use the ADP2118 tracking input to minimize current-sense resistor value?

I'm contemplating the ADP2118 for a high-power white LED driver.  The usual issue with buck regulators in this application is the [relatively] high reference voltage into the error amplifier.  Getting 0.6 volts from a current-sense resistor means lots of wasted power in I^2*R loss.

From the datasheet, it looks like I could drive the TRK input to something like 50 mV, making that the effective reference voltage for the error amplifier.  Then I could use a small sense resistor and everything is wonderful.  Are there any problems with this scenario, or should it just work?  (I do realize the FB/TRK offset voltage means the fractional uncertainty will be larger.)

Thanks,

Mark

  • As far as using the TRK input goes in the manner you described, it will work. However, there are other factors to consider specific for the application you just described.

    1) ADP2118 has a fixed internal compensation. This means that the external component values should be chosen in accordance with that to avoid loop instability.

    2) ADP2118 is primarily a buck regulator. During the soft-start period, the FB voltage is allowed to ramp up in a controlled manner, but when you have an LED followed by a sense resistor as your load, things are slightly different. You don't have any voltage on FB until the LED starts to conduct (which would require the output cap to charge up to the forward voltage of the LED) before that would occur. If there is too much output capacitance, then there is a chance that overcurrent detect circuit may get activated. The optimum capacitance value should be determined analytically to avoid this problem. If you need further assistance, please contact us and we can help you. 

  • This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin