LT3797 "scaleable" current sensing on LED driver

jstamour802 said:

I'm using the LT3797 LED driver in a SEPIC configuration. It uses a high-side currents sense resistor to set the output current. I'm following this circuit example for my application.View attachment 120053



I would like to be able to "scale" the sense resistor on-demand (from a microcontroller) so that I'm able to drive loads at very low current and high currents without having to physically change the resistor value. This is so I can have better resolution over the current under different conditions.

I experimented with using a p-mosfet to switch the sense resistor, but I had mixed results and I need some more time to test it. I'm concerned that the mosfet RDSon and potentially PCB layout issues may be a problem with this method.

I would like to go between a 0.5ohm and 0.050ohm scale if possible.

Is there some other way to do this? Is it possible to make some kind of "programmable" current sense with an op-amp circuit or similar?

Thanks for any help! I'm really quite stuck on this one...

-Joe

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jstamour802 said:

Yes, I am doing that already. I'm using a DAC to control the output brightness. The problem is I am controlling a brightness range from 0-5000mA and if I choose a sense resistor based on 5000mA, I can't get reliable currents below 125mA or so. The driver has a shut-off point around 200mV on the control pin, so when I get close to this value the output begins to get very flaky (flickering/shuts-off unexpectedly). Basically, I can't adjust with fine resolution at the lower currents since it is right on the stability threshold.

I'm doing this because I need the driver to power both small LED loads (0-100mA) and large loads (up to 5000mA), but I can't replace the sense resistor value.

Thanks for the input!

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