I am interested in understanding how to place the LT3741-1 into current-sink mode. The datasheet mentions that this device can also sink current, but not under what conditions. Is it possible to use the LT3741-1 to force it into sinking a constant current set by the sense resistor and the voltage on CTRL1 pin?
LT3741-1 is not able to sink current. You should use LT3741 for your application.
In order for LT3741 able to sink current , Vfb voltage should be 1.21V < Vfb < 1.5V . This condition is shown on Graph 3741 G14 on page 6 of the datasheet.
Thank you for the reply. I've looked at the plot but I'm not sure how I would use the FB pin to command the LT3741 to sink current. In my application I would be using the LT3741 to supply a constant current to a large capacitor. The current is set by CTRL1. There is a resistor divider at the FB pin to regulate the voltage once the capacitor is charged to the specified voltage. My application requires discharging the same capacitor with a constant current at some point in time. In order to discharge the capacitor at a constant current, should I override the voltage at the FB pin through an external circuit? Bottom line, how would I supply a constant current to a load while ensuing the output voltage does not exceed a specified amount, and be able to discharge that same load with a constant current?
I'd like to pick-up on this thread once more, now that I have actually built a board and done some testing. I'm charging a capacitor when the LT3741 is in current-source mode. I'm charging at a constant current. At the input voltage node of the regulator, I've placed a resistor to mimic another load on the main supply rails. Monitoring the input power supply's output current, I can see that it is supplying current to the resistor plus the LT3741 regulator as its' charging the output capacitor.
When the capacitor is charged to a specified voltage level, I place the LT3741 in current-source mode. Now, the input power supply does not supply any current since the source is the capacitor and the LT3741 is in boost mode (boost voltage higher than the power supply's). As the capacitor is discharging, I monitor its voltage and I can see that it drops. While its dropping (but still above 1V) the power supply current is zero. But as the capacitor discharges below 1V the power supply starts to contribute load current. Once the capacitor voltage reaches about 0.65V, it no longer drops and all of the input load current (for the input resistor) is supplied by the power supply. The entire time that the LT3741 is in current-sink mode, the FB voltage is maintained at 1.33V, even when the capacitor voltage reaches 0.65V and it no longer discharges.
I'd like to know if this is normal behavior or not. According to simulations, the capacitor discharges to zero volts. Obviously, the simulation is as good as the models used, but I would have expected less of a discrepancy between the simulation and the real circuit. Any insight would be appreciated.
Thanks for the answers. I have some follow-up questions. Assume the LT3741 is set to source current and the output load is a large capacitor. If that capacitor has a negative voltage across it already, say between -1V and -0.5V, will the LT3741 be able to provide a constant output current in order to transition the negative voltage into a positive one? This appears to be possible according to simulation. Can the voltage at the FB pin be negative, and if so, at what maximum level?
Please refer at this E-zone entry for the mean time: