"Kevin McMurtrie" wrote in message
...
In article ,
"Ian Field" wrote:

Some parts of the AN170 PDF for the 555/556 timer are poor quality and
barely readable - it just so happens that the automotive regulator app is
very blurred.

Also; I have my doubts about the design shown as it shunts the output
(can't
read the resistor value!) and takes its output from a 68 Ohm resistor
feeding the Vcc pin.

Does anyone have any automotive regulator apps for the 555?

The one in AN170 isn't a PWM as such - its just used as a window
comparator
with latch, at least its not a self oscillating PWM, it switches on the
field winding when the battery drops to the low threshold and off when it
reaches the high thr.

Thanks.

Driving transistors off the power supplies to a chip is an old trick.
It has no voltage offset and much less crossover distortion compared to
an emitter follower on the output.

The circuit isn't self-oscillating so it will generate a triangle wave
on the output. This is probably a replacement for very old voltage
regulating relays.

The 555 can be used for real PWM regulation by creating your classic
oscillator circuit then applying feedback current to pins 2/6. The
switching speed is awful compared to modern switching power supply chips
but it's great as a quick hack.

Which in real terms means using a 556. I'm not sure but I have this strange
feeling in my bones that striving for 20kHz might be a very bad idea - maybe
400Hz tops.

I still prefer the simplicity of using a window comparator to trip a
bistable on min/max thresholds.

Isn't that sort of almost a hysteretic regulator?