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Design >> Analog Design >> Output limited- SR controled Driver
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Message started by XY-oriented on Jan 20^th, 2009, 6:54am

Title: Output limited- SR controled Driver
Post by XY-oriented on Jan 20^th, 2009, 6:54am

Hello,
is there anybody who knows a structure of a high current capability drvier with:
- Slew rate control
- Output must be limited to Vref+ and Vref- (programmable voltage source basically)
- the driver must source/sink current from/into the power supply and not Vref+/-
I found some circuits ideas for the slew rate control but I couldn´t clip the output at the right Vref+/- "in an elegant way"

any circuits ideas or papers links ?

Title: Re: Output limited- SR controled Driver
Post by thechopper on Jan 20^th, 2009, 6:28pm

Are you trying to drive a large capacitive load?
In this case a simple SR control circuit would involve a current starving inverter, where such current is used for controlling the output SR.
One option for limiting the voltage swing to VREF+/- would be to use active clampers; i.e. two comparators looking at the driver's output voltage and turning on a control signal when hitting VREF+/-. Such control signal could - for example - modulate a Resistance in series with your driver in such a way it keeps the voltage constant if it tries to go beyond VREF.

Regards
Tosei

Title: Re: Output limited- SR controled Driver
Post by XY-oriented on Jan 21^st, 2009, 4:06am

Thanks for your replay Tosei,
Actually the current starving inverter is not an option for me because the SR must be controlled over a wide range of load capacitance.
one idea is to generate a "wave form" with the proper SR followed by a SR-free buffer. my problem is how to limit the output to Vref+/-
- If I use active clippers at the driver output I will burn a lot of current inside the chip.
- Controlling the amplitude of the SR shaping waveform feeding the Buffer in a feedback loop require a huge bandwidth (the driver is supposed to work at 20MHz).

I am designing a substitute chip, and the old one has such a smooth settling behavior that I am convinced that they didn´t use a feedback loop.
It must be something else ! :-/

Title: Re: Output limited- SR controled Driver
Post by thechopper on Jan 21^st, 2009, 6:41pm

XY-oriented wrote on Jan 21^st, 2009, 4:06am:

Thanks for your replay Tosei,
- If I use active clippers at the driver output I will burn a lot of current inside the chip.

I´m not sure about this. If you used a simple comparator which might consume a little current and such comparator could linearly control a RON in series with the output driver once the output voltage hits your threshold that would make it without burning a lot of power.

Regards
Tosei