LT3797 "scaleable" current sensing on LED driver

[jstamour802]

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.



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

 

[aruipksni]

re: LT3797 "scaleable" current sensing on LED driver

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

just from a look in datasheet it looks like you can control LED current by changing the CTRL voltage input.
so you can connect it to D/A and cpu, and control current.

 

[jstamour802]

re: LT3797 "scaleable" current sensing on LED driver

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!

 

[aruipksni]

re: LT3797 "scaleable" current sensing on LED driver

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!

you can combine it (CTRL) with PWM input, or just PWM.
you can generate PWM pulses from your cpu (can get to 1:3000).

 

[D.A.(Tony)Stewart]

Are two shunt resistors and a jumper allowed?