How are PWM IC's able to provide so many features in small footprints ?
Transistors are about 10 micron * 10 micron, allowing for aluminum metallization connections to other transistors or resistors or capacitors or diodes or to bondpads.

An analog comparator, to detect some flawed mode of operation, needs a diffpair (two transistors very close together, to have nearly the same doping and the same temperature), several current mirrors (two transistors very close together), and some method to set the operating current to 10microAmps or 100microAmps. The more current, the faster the analog comparator will operate. This analog comparator needs diffpair + (call it) 4 more current-mirrors, a total of 10 transistors. Its range of operation likely will not be rail-to-rail; double the # transistors (at least) to achieve rail-to-rail input operation.

So how did big is this rail-to-rail analog comparator? 20 transistors, each with 10*10micron area, or 2,000 square microns.

On older die, the bondpads (where the tiny 25 micron gold wires attach) are 100*100 = 10,000 square microns. Thus 5 analog comparators will fit into the area of where ONE bond wire goes.

These small transistors will NOT BE WELL matched (well, maybe if bipolar they will be matched) at that size, if old-CMOS process is used. But you may not need super-precision threshold detection, since you have to perform all the error-budgets, and you get to make all the tradeoffs.

Logic? CMOS NAND gates are about 10*10micron; Flipflops are about 10*50micron, in old processes.

Big power devices? High voltage devices (even if low-power)? very dependent on the process.

Here is schematic of a very simple analog comparator







simulate this circuit &ndash; Schematic created using CircuitLab

Notice the 4rth terminal of the transistor symbol (MOS body, bulk, well, substrate, backside; BIPOLAR has its collector tub) is not shown.

This simple analog comparator will not function down to GROUND on the input pins; that is, a precise comparison cannot be performed if both inputs are near or at or below GROUND. This is because both of the diffpairs will be turned OFF when their gates are taken that low.

Notice the extreme robustness of this design: you can replace the FET version with the bipolar version, using the exact same schematic. Your mileage will vary, your specs will vary.

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One of the rules for signal processing is: if the analog signal does not contain the INFORMATION, then the ADC/digital-signal-processing will not be able to behave as needed BECAUSE THE INFORMATION IS GONE.

Hence attention to Electric Field interference, to Magnetic Field interference, to Power Supply trash, to Ground System trash, to bandwidth, to differential sensing, to Bolzmann Random Noise ----- become your design task.