In applications where a high frequency variable PWM generator is needed, the LT1715 provides a small and simple solution, as shown in the circuit of Figure 10. The top comparator, U1A, acts as a ramp generator. Biasing resistors R1 and R2 and a 7:1 positive feedback ratio through R3 set the positive input to approximately 2.5V±250mV. The inverting RC feedback creates a relaxation oscillator by tripping the comparator into the opposing output state each time it passes through the ±250mV positive input voltage. The 880kHz triangle wave created on C1 is then passed to the inverting input of the second comparator, U1B. The input signal to be modulated, referenced from 0V to 5V,is first divided to match the 2.5V ±250mV ramp input, and then the duty cycle is set with the second comparator acting as a threshold detector.
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.