Problem with current mirrors
in [Design]
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From: Hammy on 23 Feb 2010 11:56 On Tue, 23 Feb 2010 14:39:27 +0100, Remco Poelstra <remco.poelstra+albasani(a)duran-audio.com> wrote: >Hi, > >I'm trying to operate MOSFET devices with current mirrors. I want a >maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >the other hand vary a lot, so I decided to use current sources to >develop the required Vgs across a resistor. >See http://remco.beryllium.net/Sheet1.pdf for a schematic > >Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >switch the current source off has been removed. M2 is switched perfectly >using this construct. >Q6 uses the current source reference output to generate another current >of about 1mA. >The other BJT's are current mirrors to copy the same current to R48, to >switch M6. >The problem is that the current through R37 is way larger than the >current through R27. Q12,9 and 8 also do not seem to be able to settle >at the correct value. >I'm wondering why it doesn't work correctly. >Can someone help? > >Kind regards, > >Remco Poelstra That is a pretty elaborate cct. to just switch a Highside PFET and a low side NFET. If your willing to use all those components just to switch a HS PFET you might as well use a NFET on the HS and bootstrap the drive. The whole reason you would use a PFET on the HS is to reduce the component count and complexity you defiantly aren't doing that. What is its purpose? Active inrush suppression?
From: Jim Thompson on 23 Feb 2010 12:04 On Tue, 23 Feb 2010 11:56:42 -0500, Hammy <spam(a)spam.com> wrote: [snip] > >That is a pretty elaborate cct. to just switch a Highside PFET and a >low side NFET. If your willing to use all those components just to >switch a HS PFET you might as well use a NFET on the HS and bootstrap >the drive. The whole reason you would use a PFET on the HS is to >reduce the component count and complexity you defiantly aren't doing >that. > Odd! I didn't find the OP's post to be "defiant" ;-) But he indeed didn't ask for alternate circuits, he asked _why_ his mirrors were inaccurate. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Tim Wescott on 23 Feb 2010 12:09 Remco Poelstra wrote: > Hi, > > I'm trying to operate MOSFET devices with current mirrors. I want a > maximum Vgs, to make sure they have minimum Rds_on. The supply rails on > the other hand vary a lot, so I decided to use current sources to > develop the required Vgs across a resistor. > See http://remco.beryllium.net/Sheet1.pdf for a schematic > > Q7 and Q5 make up the current source, using R12 to switch M2. Logic to > switch the current source off has been removed. M2 is switched perfectly > using this construct. > Q6 uses the current source reference output to generate another current > of about 1mA. > The other BJT's are current mirrors to copy the same current to R48, to > switch M6. > The problem is that the current through R37 is way larger than the > current through R27. Q12,9 and 8 also do not seem to be able to settle > at the correct value. > I'm wondering why it doesn't work correctly. > Can someone help? > What are you really doing here? What's this circuit driving? How fast are you switching the FETs? Whatever else, you have enough stuff tossed on the board that you may well be able to reach your end goal with an IC or three, with less board space used, and fewer manufacturing techs confused. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Hammy on 23 Feb 2010 12:16 On Tue, 23 Feb 2010 10:04:11 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 23 Feb 2010 11:56:42 -0500, Hammy <spam(a)spam.com> wrote: > >[snip] >> >>That is a pretty elaborate cct. to just switch a Highside PFET and a >>low side NFET. If your willing to use all those components just to >>switch a HS PFET you might as well use a NFET on the HS and bootstrap >>the drive. The whole reason you would use a PFET on the HS is to >>reduce the component count and complexity you defiantly aren't doing >>that. >> > >Odd! I didn't find the OP's post to be "defiant" ;-) > >But he indeed didn't ask for alternate circuits, he asked _why_ his >mirrors were inaccurate. > > ...Jim Thompson I know my spelling has really gone downhill. :-) To much relying on software spell checkers.
From: John Larkin on 23 Feb 2010 12:19
On Tue, 23 Feb 2010 09:09:00 -0800, Tim Wescott <tim(a)seemywebsite.now> wrote: >Remco Poelstra wrote: >> Hi, >> >> I'm trying to operate MOSFET devices with current mirrors. I want a >> maximum Vgs, to make sure they have minimum Rds_on. The supply rails on >> the other hand vary a lot, so I decided to use current sources to >> develop the required Vgs across a resistor. >> See http://remco.beryllium.net/Sheet1.pdf for a schematic >> >> Q7 and Q5 make up the current source, using R12 to switch M2. Logic to >> switch the current source off has been removed. M2 is switched perfectly >> using this construct. >> Q6 uses the current source reference output to generate another current >> of about 1mA. >> The other BJT's are current mirrors to copy the same current to R48, to >> switch M6. >> The problem is that the current through R37 is way larger than the >> current through R27. Q12,9 and 8 also do not seem to be able to settle >> at the correct value. >> I'm wondering why it doesn't work correctly. >> Can someone help? >> >What are you really doing here? What's this circuit driving? How fast >are you switching the FETs? > >Whatever else, you have enough stuff tossed on the board that you may >well be able to reach your end goal with an IC or three, with less board >space used, and fewer manufacturing techs confused. If all he wants to do is switch the fets on and off, he doesn't need current mirrors at all. The whole thing can be done with 4 to 6 parts. John |