intentional Slew rate limiting

I've made multiple similar circuits requiring precision slewing ramp rates, using high-speed amplifiers and programmable opamp slew-rate circuits. I doubt you need a high level of accuracy, so a more simple scheme could work for you. There's a concept called a Miller integrator, which you can implement by adding a feedback capacitor from the output MOSFET's drain to gate, plus use a series gate resistor to set the dV/dt integration rate, where Rgate = Vin-Vgs / dVo/dt determines the gate resistor.

Maybe a second FET with the drain & source between the driver output & a cap to ground?

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   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      
  \\|/  \\|/              Perna condita delenda est
---^----^---------------------------------------------------------------

"Mook Johnson"

** Why ?

** That would be a massive source a radio interference on AM and SW bands. ** 5 pF per ft - so it sure ain't co-ax. ** The solutions to crosstalk are well known.

Co-axial cable, metal shielding, twisted pairs etc.

Use one of them.

..... Phil

View in a fixed-width font such as Courier.

. . . . . ---- . | d | . .--------| -- |-------. . | | dt | | . | ---- | . -| | . +V-- --- ---- | . | | + | -- | | \\ | . ----- | | ------| \\ |----------| > ----+---->

. | | | /_ | | / . -V-- - - ---- . . .

[snip]

Below is the downmarket, belt 'n braces suggestion.

+15V---+---+-----+ | |0.1? | | === \\220 \\_|_ | / Zener,5V1 /_\\ | | | | |/e +---+---|pnp 23mA\\|/ _|_ |\\ | /_\\ | \\ | | Linear 430 / '-----+ +1 amp. \\ _|_ | \\_/ -+- +15V 0V-+- | _ | 20mA\\|/Constant. | \\| +------------+--|+1>-->Out 38mA\\|/Switched. | |_/| 3.3V-+-----+------. _|_ | | | | | \\_/ C=== -+- -5V |\\| |/ | | | ---| >---|npn | +--------. | Tr/f = 10*C/19mA |/| |\\e | | _|_ | | | |/ \\| /_\\ | 0V--+- +----| npn |--. | | | |\\e e/| | | | | | | '---+---+-0V | '-+-' | | Diodes are Schottky. \\ \\ 680/ /110 \\ \\ | | -5V --+--------+-----

If C = Cstray = 50pF then Tr/f(min) = 26nS.

If C = (470+50)pF then Tr/f = 274nS.

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Tony Williams.

You ask about 'other' ideas.

Well, there's an OLD idea that might work: an increductor. The inductance of a simple choke will be the slew rate limiter, and the inductance responds to the field in the core, so you make a high-impedance winding with some DC current in it and at high DC current it has low inductance (50 nS slew into load) and at low DC current it has high inductance (resulting in

500 nS slew into load). The signal is sent through a low-impedance winding in the same core.

Because it's current-programmed, it's easy to adjust. Capacitors with voltage-adjust are generally not of such size as to work for 500 nS delay at 500 mA drive.

Its too bad the CA3080 isn't with us today. There was a nifty circuit that used one of them to make slew rate limiting.

But here's an idea

ASCII art:

--------------------------- ! ! ! r-r opamp ---!!---+ ---!+\\ ! ! ! >-+-/\\/\\----+--!-\\ ! ---/\\/\\--+-!-/ ! ! >--+------- Output ! ! GND-----!+/ --!!-- Power op-amp

The r-r opamp limits the slew rate by hitting its power supply rails. Finding a r-r opamp that recovers well from this may be the biggest problem with this.

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kensmith@rahul.net   forging knowledge

The classic slew limiter is a diode bridge...

i+ | | + / \\ a a k k / \\ in---------+ +-------+------out \\ / | a a | k k c \\ / | + | | gnd | i-

where current i into the cap sets the slew rate.

John

Ken Smith a écrit :

AD8036/8037

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Thanks,
Fred.

You are correct the capacitance is actually 10,000pF or 50pf/foot. The cable is twisted shielded quad (4 wires inside a single shield). Wire to wire coupling capacitance is about 10p/ft.

My opinion is that the situatuin is near hopeless for high speed communication. As soon as you get the edges steep enough to look like digital its going to crosstalk like a SOB.

mook Johnson a écrit :

Is it quad or dual pairs? Do you have a ref?

For long cables, quad is worse than twisted pairs since it's coupling the same way all along the path. Twisted pairs cancel much of this.

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Thanks,
Fred.