Figure above shows a voltage controlled LED drive circuit. When VIN is at 0V, the op amp supply current is nominally 20μA. The offset, for example, could be 450µV, appears across R1, inducing a 0.45mA current in the LED. Some applications want a guaranteed zero LED current at VIN = 0, and this is the purpose of R5. R5 forces 2.5μA current through R7, creating a negative 0.6mV sense offset. This offset guarantees a zero LED current.
Indeed, the circuit works nicely. Once the input voltage is near 0, the LED current output is 0 and the total supply current is 20µA. Gain from the input voltage to LED current is 0.022A/V, as can be taken from the R2/R3 voltage divider and the sense resistor value.
LED Current = (VIN / R1) • R3 / (R2 + R3)
The LTC®6258/LTC6259/LTC6260 are single/dual/quad operational amplifiers with low noise, low power, low supply voltage, and rail-to-rail inputs and outputs. They are unity gain stable with or without capacitive loads. They feature 1.3MHz gain-bandwidth product, 0.24V/μs slew rate while consuming only 20μA of supply current per amplifier operating on supply voltages ranging from 1.8V to 5.25V. The combination of low supply current, low supply voltage, high gain bandwidth product and low noise makes the LTC6258 family unique among rail-to-rail input/output op amps with similar supply current. These operational amplifiers are ideal for power efficient applications.
Tools & Simulations
LTspice® is a powerful, fast and free simulation software, schematic capture and waveform viewer with enhancements and models for improving the simulation of analog circuits.
To launch ready-to-run LTspice demonstration circuits for this part:
Step 1: Download and install LTspice on your computer.
Step 2: Click on the link in the section below to download a demonstration circuit.
Step 3: If LTspice does not automatically open after clicking the link below, you can instead run the simulation by right clicking on the link and selecting “Save Target As.” After saving the file to your computer, start LTspice and open the demonstration circuit by selecting ‘Open’ from the ‘File’ menu.