The LM3524D

The Texas Instruments LM3524D is a beautiful tool that I’ve been looking for for some time now. I saw a few designs based on them in school but I couldn’t recall the number or the right name when I got back into electronics. I’d search for “voltage dependent PWM generator” and the like, but I couldn’t find the right chip.

That right chip is the LM3524D. A beautiful design incorporating just about all of the sub circuits one needs to produce a controlled PWM signal. Its got an astable oscillator to serve as the signal source, tunable with your choice of timing resistor and capacitor. This replaces a 555. Its got two separate voltage differential feedback amplifiers. One is set to operate in mV range for use with current sense resistors, and this one overrides the other which is typicaly used to set a target output voltage. Finally, it permits override of both feedback controls with a single pin voltage analog input that can be used to directly set the PWM duty cycle which is linear with respect to the applied voltage. These features replace a slew of opamps and logic circuits.

The output of the circuit isn’t a regular PWM wave, interestingly enough. Rather, the internal PWM is fed into the base of a BJT, and the inverse of the PWM is fed into the base of a second BJT. This permits a certain level of direct low power usage, and more importantly permits darlington pairing with higher power BJTs, and of course, the transistors can be used to reproduce a PWM voltage signal for distribution to FET gates.

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