AD558 output boost

[Sceadwian]

I picked up a couple DAC chips earlier today at a surplus store in town that had them for like 4 bucks cheaper than digikey. I was browsing the PDF and noticed since the feedback to the internal buffer amp is available on one of the pins you can use a current boost transistor to increase output current, they use the venerable 2n2222 as an example and I do have a few around if I want to use one, but I was wondering if I could just substitute say one of the 4 amp mosfets I have instead to use it as basically a digital variable linear voltage regulator. Bellow is a capture of the part of the PDF that shows the boost setup and a link to the full PDF. It doesn't contain much information aside from the simple example.
https://www.analog.com/UploadedFiles/Data_Sheets/AD558.pdf#xml=https://search.analog.com/search/pdfPainter.aspx?url=http://www.analog.com/UploadedFiles/Data_Sheets/AD558.pdf&fterm=ad558jn&fterm=ad558jn&la=en

 

[mneary]

I would expect the AD558 to be able to overcome the threshold voltage of a logic driven MOSFET, if Vcc is high enough. I wouldn't expect an ordinary MOSFET to work quite as well.

 

[Sceadwian]

The only power FET's I have are IRF510's. I'm concerned about oscilation problems since a FET's active regioin (especially a logic fet) is so small, and non linear.

 

[Hero999]

It doesn't matter, I've used power MOSFETs in their non-linear region before without any problems but you need to ensure the gate is sufficiently above the source or the voltage drop will be enourmous.

 

[Sceadwian]

The DAC is 10 volts out, should be plenty.

 

[Hero999]

But if you're using a source follower, then the maximum output will be about 5V depending on what current you're drawing. This is why you need a voltage doubler and op-amp to to shift the voltage up above the supply to get a decient dropout voltage.

 

[Sceadwian]

I thought the opamp on the DAC is what did the voltage following, simply connecting the sense/gain pins to the point where you want the voltage to follow and the VOUT should go from any voltage from 0-VCC in order to get the sense voltage to coincide with the DAC's currently set value?

 

[Hero999]

I don't think you understand what I'm saying, I might not be explaining it very well.

With an emitter follower you loose 0.6V + the saturation losses, with a MOSFET you loose the threshold voltage + the conduction losses. Since the threshold voltage on a MOSFET is so high you'll need to power your circuit from >15V to get 10V at any reasonable current. To get round this you use a smaller power supply for the gate driver circuitry.

 

[Sceadwian]

Never mind Hero, sorry I wrote that post in a hurry I realized what was wrong with it right after I left =)