Current sense resistor hack
This story began when I bought a power RGBW LED to experiment with.
Inside this amazing part you can actually find as many as four independently connected LEDs, requiring a constant current. So this time I decided that there’s no point in reinventing the wheel and proceeded to use of a LED driver IC ZXLD1360 (you can see its standard application circuit below).
The target current is set by means of a current sense resistor, which can be calculated according to this simple equation found in datasheet:
IOUTnom=0.1/Rs, where Rs must be >0.1 Ohm.
Hence, for example, its value for 350mA output current is 0.286 Ohms. It’s obviously non-standard value, you know. Moreover, I guess it can hardly be derived by combining standard value resistors. Nevertheless, its tolerance directly affects output current tolerance. So how can we overcome this difficulty?
Hey, I have a solution that will save the world from this great trouble! Just use any resistor which is greater then the target value, and divide the voltage from it with a potentiometer. Like this:
The driver’s current sense input has a fixed threshold. As soon as voltage on that input reaches this value, IC knows that the current has reached desired limit and stops increasing it. Voltage on the resistor is directly proportional to its current, so the greater resistor value is, the lower current limit is, cause the threshold voltage will be reached while lower current flows through the resistor. But if we divide the voltage by the appropriate number of times, we can freely alter actual current at which IC will sense the threshold with any resistor greater then calculated.
This principle eliminates the need in precise low-value resistors and allows to choose comfortable resistor values.
The resistance of a potentiometer is much greater than a value of a current sense resistor, and input resistance of an IC input is greater than a value of a potentiometer. So the potentiometer
has no significant side influence on the circuit operation.
Wise people who read datasheets can ask me why not using ADJ dimming input to achieve the same. But I have different plans on that pin. And note that there are drivers which have no dimming input, so my method is the only suitable for them (or you have to make that custom resistor yourself).
One note on the schematic: if you’re going to control the driver with PWM, then do not install C2.
This circuit was assembled and tested:
Measured efficacy is 94% and the current is set precisely, so the experiment can be considered succesful.