I have a battery-powered wireless sensor system that sends sensor information periodically. One requirement is that it has to be functional at very low temperatures. Powering this system is a Tadiran TL-5903 lithium battery, which are batteries made to power systems at very low temperatures and supplies 3.6V.
When the sensor sends out a transmission by LoRa radio, it can consume a peak current of around 100mA+. At low temperatures, the battery cannot provide enough current to supply the load during transmissions and the battery voltage drops from normal operation of 3V to below the minimum operational voltage of the microcontroller (2.2V).
The current power supply IC is a buck-boost chip TPS63802. It takes in the battery's voltage range (2.0V to 3.6V) to an operating voltage of 3.0V. However, the IC doesn't have an input current limiter that can limit the current so that it doesn't take too much peak current from the battery. At low temperatures, we can't take in too much current from the battery or else the voltage drops substantially.
simulate this circuit – Schematic created using CircuitLab
We tried putting a super cap at the output of the switching regulator but it doesn't seem to make much difference in the voltage drop when doing radio transmissions.
Is there an IC or a simple circuit that can limit the current coming into the regulator without dropping the voltage at the input of of the switching regulator?
Best Answer
As others have said, a current limiter is not what you need here. In fact, this is the very cause of your problem: a current limiter will decrease/block the voltage as it approaches the current limit, which is exactly what the battery is doing already.
The Tadiran TL-5903 indicates it can supply up to 200mA pulses, but it will not maintain the rated voltage for this. The discharge plot shows that the output sags to 3.4V@5mA and 3.1V@31mA. Working backward from your 100mA transmit, a transmit duty cycle beyond about 30% will be entirely impossible.
This all hinges on the average current, which you have already started to attack with the supercap. A cap will filter the current spikes, and ideally it will prevent the spikes from affecting the battery at all. This is not possible at the regulator output, though; the capacitor delivers current in response to a change in voltage, but the regulator's job is to make sure the output does not change. By the time the capacitor engages, the regulator will have already given up after bringing the battery to its knees.
Moving to the regulator input, the cap will engage as soon as the battery starts to sag. The question is now: will the supercap have enough charge to hold this for the entire transmit?
Wireless transmit will typically be short, but LoRa is not particularly helpful here. Since you emphasized using the maximum transmit power, I assume you also use the minimum rate to maximize your range; this may hurt your power even more than the 100mA. At 0.3kbps, just the packet preamble and sync symbols take 160ms; adding 16 bytes of user data, your transmit pulse will last over half a second at the slowest rate.
Since \$I = C\frac{dV}{dt}\$, we can see that your voltage will drop by \$dV=\frac{I}{C}dt\$, or approximately (100mA / 470mF * 0.5s) = 0.11V.
So. If you keep the transmit duty cycle below 30%, limit your transmit length to half a second (16 usable bytes at 0.3kbps), and move the supercap to the regulator input, this might actually be physically possible... at room temperature, with a fresh battery.
You may want to add a second cell.