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?
When the PSU is operating at sink mode it allows the inductor current to reverse. The PSU effectively becomes a boost convert at this instance and dumps the energy back at the input. The energy returned to the input then is dissipated by other loads in the system.
OK, so I ran some LTSpice simulations with the LT3741 and I was able to command the device to source or sink current into a large capacitor. I ran many simulations where I changed the capacitor value and the source-current value. The sink current was set to be at least 100% of the source current. In every simulation, the measured source current was always about 90% of the programmed current (well beyond the ±6% current set accuracy of the device), while the sink current was always measured to be about 70% of the measured source current. In order to command the device to sink current, I ensured that the feedback voltage was set to at least 1.27V, but I also tried setting it to 1.30V and 1.35V. According to the plot that was referenced in earlier replies, the sink current should be well above 100% of the programmed current, but I did not get this result in simulation. A sample plot is attached. The red plot is the voltage at the FB pin and the cursors show the values when the device is sourcing and sinking current.
1. Should we expect this sort of performance from the real device, where there is significant difference between the source and sink currents?
2. When the FB pin voltage is set so that the sink current value is at least 100% of the source current, what factors can cause the sink current value to actually be significantly lower, say around 70%?
We would need to test this on bench to answer you accurately. We would put this on queue and provide the data to you next week
For the mean time, can you share with us your estimated annual unit orders for this particular part ?
Probably around 5000 units.