No current sense resistor required for MOSFET switch protection
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From: Jim Thompson on 7 Apr 2010 19:52 On Wed, 07 Apr 2010 16:42:26 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On 7 Apr 2010 16:37:57 -0700, Winfield Hill ><Winfield_member(a)newsguy.com> wrote: > >>Jim Thompson wrote... >>> >>> I love chip processes that have depletion-mode FET's... >>> just great for Bandgap starters. >> >> Don't all fabs have JFETs? Maybe not defined and 'scoped >> out, but aren't they at least able to accommodate them if >> you roll your own? > >Some fabs have an N-well resistor that is sort of a JFET. The problem >is you want one that cuts off sharply enough that start-up goes away >completely. I've only seen a few useable for low voltage circuits, >such as in RFID tags. > > ...Jim Thompson Here's one from an old AMIS process... ..model jpx pjf ( ;level = 1 +beta = 3.449e-6 lambda = 0 vto = -18.6937 +rd = 10 rs = 10 ;bex = -1.2759 +cgs = 1e-14 cgd = 1e-14 pb = 0.7 ) * ..model ppx pmos ( ;version = 3.1 +level = 7 ;intcap=1 +tnom = 21 ;templev = 2 +mobmod = 1 +nqsmod = 0 capmod=2 xpart = 0 +;acm = 2 +noimod = 1 ;hdif = 8e-7 +tox = 2.65e-8 +xj = 1.5e-7 nch = 5e16 k1 = 0.944 +k2 = -5.095e-5 vth0 = -1.562 u0 = 202.436 +rsh = 90 rdsw = 1e3 lint = 4.209e-7 +wint = 1.844e-7 js = 5e-3 cgdo = 3e-10 +cgso = 2.5e-10 cgbo = 3e-10 cj = 4.02e-4 +cjsw = 2.65e-10 +ua = 3.699e-9 ub = 1e-21 +uc = -5.688e-11 k3 = 27.517 k3b = -6.989 +w0 = 4.735e-6 dwg = -8.738e-9 dwb = 2.389e-8 +nlx = 0 dvt0 = 0.105 dvt1 = 0.027 +dvt2 = -0.106 prwg = -2.068e-4 prwb = -0.433 +wr = 1 voff = -0.049 nfactor = 0.75 +cdsc = -1.802e-4 cdscd = 6.693e-6 cdscb = -2.478e-5 +cit = 0 vsat = 1.892e5 a0 = 0.558 +ags = 0.076 a1 = 0 a2 = 1 +b0 = 4.855e-6 b1 = 1.205e-5 eta0 = -0.137 +etab = -0.105 dsub = 1.999 pvag = 2.177e-3 +pclm = 2.379 pdiblc1 = 0.06 pdiblc2 = 1.968e-3 +pdiblcb = -0.276 drout = 0.32 pscbe1 = 1e9 +pscbe2 = 3e-8 keta = 4.992e-3 delta = 0.01 +wwl = -6.117e-20 lwl = -4.892e-20 +prt = 355.913 +ute = -1.472 kt1 = -0.538 kt1l = -5.849e-8 +kt2 = -0.067 ua1 = -3.078e-11 ub1 = -4.705e-18 +uc1 = -9.696e-11 at = 1.473e5 n = 1.83 +mj = 0.41 pb = 0.8 mjsw = 0.199 +pbsw = 0.8 elm = 5 cgsl = 0 +cgdl = 0 ckappa = 0.6 cf = 0 +clc = 1e-7 cle = 0.6 ) * *----------------------------------------------------------------------------* Of course AMIS just recently got eaten up, and fabs shuttered, by ON Semi :-( ...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 | The only thing bipartisan in this country is hypocrisy
From: Phil Hobbs on 8 Apr 2010 10:36 Jim Thompson wrote: > On Wed, 07 Apr 2010 17:42:45 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> On 4/6/2010 12:31 PM, John Larkin wrote: >>> On Mon, 5 Apr 2010 17:23:01 -0700 (PDT), "oparr(a)hotmail.com" >>> <oparr(a)hotmail.com> wrote: >>> >>>> No relatively expensive high side current monitor, comparator or latch >>>> ICs either. You're wasting your time and money using these items if >>>> all you need is a fast electronic "fuse". A single BJT, in conjunction >>>> with the MOSFET, is all that is required for UVLO, overcurrent and >>>> short circuit protection. Throw in a voltage reference, like the >>>> TL431, if the switch has to cover a wide voltage range. >>> I sort of doubt it. >>> >>> John >>> >> He can do most of that (if he's prepared to be crappy enough) by using >> one of those beta-dependent BJT circuits of yours--stick a BJT in series >> with the source of the MOSFET, and give it just enough base current to >> almost saturate at the highest current you want to allow. >> >> NOT how you'd want to do it in real life, but it'd limit the current and >> so on. >> >> Cheers >> >> Phil Hobbs > > Yep. Larkin's magic bag of tricks can certainly give one heartburn > ;-) > > ...Jim Thompson I've got a lot of mileage out of sleazier tricks than that one! Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Jim Thompson on 8 Apr 2010 10:50 On Thu, 08 Apr 2010 10:36:03 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >Jim Thompson wrote: >> On Wed, 07 Apr 2010 17:42:45 -0400, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> On 4/6/2010 12:31 PM, John Larkin wrote: >>>> On Mon, 5 Apr 2010 17:23:01 -0700 (PDT), "oparr(a)hotmail.com" >>>> <oparr(a)hotmail.com> wrote: >>>> >>>>> No relatively expensive high side current monitor, comparator or latch >>>>> ICs either. You're wasting your time and money using these items if >>>>> all you need is a fast electronic "fuse". A single BJT, in conjunction >>>>> with the MOSFET, is all that is required for UVLO, overcurrent and >>>>> short circuit protection. Throw in a voltage reference, like the >>>>> TL431, if the switch has to cover a wide voltage range. >>>> I sort of doubt it. >>>> >>>> John >>>> >>> He can do most of that (if he's prepared to be crappy enough) by using >>> one of those beta-dependent BJT circuits of yours--stick a BJT in series >>> with the source of the MOSFET, and give it just enough base current to >>> almost saturate at the highest current you want to allow. >>> >>> NOT how you'd want to do it in real life, but it'd limit the current and >>> so on. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Yep. Larkin's magic bag of tricks can certainly give one heartburn >> ;-) >> >> ...Jim Thompson > >I've got a lot of mileage out of sleazier tricks than that one! > >Cheers > >Phil Hobbs One-offs can be tweaked to work. My stuff has to work 100's of thousands of times out of the box. I do prefer current-limited games, as opposed to loops which can be real squirrelly to get stable. But these can be done quite precisely if one plans ahead. ...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 | The only thing bipartisan in this country is hypocrisy
From: John Larkin on 8 Apr 2010 11:31 On Thu, 08 Apr 2010 10:36:03 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >Jim Thompson wrote: >> On Wed, 07 Apr 2010 17:42:45 -0400, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> On 4/6/2010 12:31 PM, John Larkin wrote: >>>> On Mon, 5 Apr 2010 17:23:01 -0700 (PDT), "oparr(a)hotmail.com" >>>> <oparr(a)hotmail.com> wrote: >>>> >>>>> No relatively expensive high side current monitor, comparator or latch >>>>> ICs either. You're wasting your time and money using these items if >>>>> all you need is a fast electronic "fuse". A single BJT, in conjunction >>>>> with the MOSFET, is all that is required for UVLO, overcurrent and >>>>> short circuit protection. Throw in a voltage reference, like the >>>>> TL431, if the switch has to cover a wide voltage range. >>>> I sort of doubt it. >>>> >>>> John >>>> >>> He can do most of that (if he's prepared to be crappy enough) by using >>> one of those beta-dependent BJT circuits of yours--stick a BJT in series >>> with the source of the MOSFET, and give it just enough base current to >>> almost saturate at the highest current you want to allow. >>> >>> NOT how you'd want to do it in real life, but it'd limit the current and >>> so on. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Yep. Larkin's magic bag of tricks can certainly give one heartburn >> ;-) >> >> ...Jim Thompson > >I've got a lot of mileage out of sleazier tricks than that one! > >Cheers > >Phil Hobbs Is designing to the min/max specs on a datasheet a "sleazy trick"? Is adhering to a rule somebody taught you 50 years ago a virtue? John
From: oparr on 8 Apr 2010 11:35
>That's certainly true, but keeping those secrets safe usually involves >maintaining a low profile and not blabbing about them in public. So this is your crusade to keep a tight lid on this one? >"Loose lips sink ships", you know, so, again, what was the purpose of >the original post? Maybe I'm a life jacket salesman. On Apr 6, 10:28 pm, John Fields <jfie...(a)austininstruments.com> wrote: > |