Op-amp based current sensing

[hrishib]

Hi, I’m trying to build a Li-ion charger and finally found a chip that suits my needs (namely low cost, low complexity and one that can charge a 12V or 4-18650 cells in series Li-Ion cells). The LT1513 works great because it can operate above, below or at the battery voltage. This reduces the strain on building a power supply with high voltages. The only issue I’m facing is charge termination. The LT1513 has a shutdown pin that shuts the charger down when pulled low. I looked at some charger protection and controller ICs but they are too complex for my liking and do not give me options to switch battery configurations. The next best thing I thought of was an op-amp based current sensing and using the output of the op-amp to shut down the charger once the charging current has reached a specific level.

I’m fairly new to op-amps and was hoping someone might guide me a bit in the direction of current sensing op-amps and some of the most commonly used ones. The typical charger current is 500mA and the current sensing should be at 10% of this charger current i.e. 50mA.

 

[KrisBlueNZ]

The general idea is that you insert a low-value resistor (called a current shunt) in series with the charge path and use a differential amplifier to measure the voltage across that resistor.

Generally you want this current to produce a voltage that is relative to the 0V rail, so you can easily compare it to an adjustable threshold, or acquire it with a microcontroller.

Depending on how the 0V rail is arranged, you may be able to insert the resistor in the negative connection to the battery; in this case, the voltage across it is already referenced to 0V so the measuring/monitoring circuit can be simpler.

Otherwise you need to measure current in the positive lead; this is called high-side current monitoring. There are devices called high-side current monitors that will measure this current and produce a proportional output current across a resistor in the 0V return path; these are made by Zetex (now Diodes Inc) and others.

You can also use an op-amp, wired as a differential amplifier. The inputs, which connect to the positive charger output, must be within the op-amp's operating input range, so the op-amp must be powered from a supply rail that's at least a few volts higher than the charger output voltage. The type of op-amp is not critical; the only really important parameter is the input offset voltage. I have used the Texas Instruments TLE2022 in this application. For a one-off project it might be worth getting a better (more expensive) part.

As well as Googling some of these terms, you should get some application notes from companies that make charger ICs - Texas Instruments, Maxim, Linear Technology etc. Application notes, and even the applications information in data sheets, are an extremely valuable resource when you're learning about a new topic.

 

[hrishib]

Thanks Kris. This certainly does help!