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Meaning of Vce(sat)?

Started by Daniel Pitts March 9, 2014
On 09/03/14 22.15, Daniel Pitts wrote:
> On 3/9/14 2:07 PM, miso wrote: >> G >>> DMN6040SSD: >>> http://www.diodes.com/datasheets/DMN6040SSD.pdf >>> >>> /Glenn >> >> That could be OK, but note the threshold voltage could be as high as 3V. >> > That's fine for my case. I'm using an 74HCT238 @ 5V to run the thing.
Then how about these: FDN5630 24$/100 pc 2$/8 pc: http://dk.mouser.com/ProductDetail/Fairchild-Semiconductor/FDN5630/?qs=%2fha2pyFadugc2Dlaoqmg90WO7hkA083UCazTAXEWxXk%3d - The PICs can be bought flashed: DIY :-) 3-to-8 decoder/multiplexer 12 I/O. PIC16F505-I/SL-ND 0,66$/pc @ 10 pc http://www.digikey.com/product-detail/en/PIC16F505-I%2FSL/PIC16F505-I%2FSL-ND/613196 DIY 3-to-8 (or 4-to-13?) decoder/multiplexer 17 I/O (one ADC). PIC16F527-I/SO 0,66$/pc @ 10 pc http://dk.mouser.com/ProductDetail/Microchip-Technology/PIC16F527-I-SO/?qs=sGAEpiMZZMu9ReDVvI6ax1uG3bwFTCkkDdXFBsH0xj8%3d /Glenn
On Sun, 09 Mar 2014 12:21:50 -0700, Daniel Pitts wrote:

> I'm going to be using ULN2803APG in a design of mine, and I just noticed > on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma. > > Does that mean that the voltage drop across the Vce will typically be > 1.3 at that current? Or does that mean the voltage for Vbe needs to be > that high to have saturation?
It does not mean that Vce will be 1.3V at that current -- you could have 350mA at 50V, for a little while. It DOES mean that the lowest that the transistor will pull the voltage down when 350mA is going through it is 1.3V. "Saturation" in a BJT means that the Vce is as low as it's going to ever get at that collector current. Basically, the collector isn't acting like a pretty good current source any more -- instead, it's acting like a pretty good short circuit. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
One more - cheaper and way bigger:

DIY 3-to-8 (or 4-to-16) decoder/multiplexer 24 I/O (one ADC, two 
comparators, two op-amps).
PIC16F570-I/SO
0,47$/pc @ 10 pc
http://dk.mouser.com/ProductDetail/Microchip-Technology/PIC16F570-I-SO/?qs=sGAEpiMZZMvqv2n3s2xjsRHo%2fT9byYHcVLblOvIhXYs%3d

/Glenn
On Sun, 09 Mar 2014 17:23:35 -0500, Tim Wescott
<tim@seemywebsite.please> wrote:

>On Sun, 09 Mar 2014 12:21:50 -0700, Daniel Pitts wrote: > >> I'm going to be using ULN2803APG in a design of mine, and I just noticed >> on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma. >> >> Does that mean that the voltage drop across the Vce will typically be >> 1.3 at that current? Or does that mean the voltage for Vbe needs to be >> that high to have saturation? > >It does not mean that Vce will be 1.3V at that current -- you could have >350mA at 50V, for a little while. > >It DOES mean that the lowest that the transistor will pull the voltage >down when 350mA is going through it is 1.3V. > >"Saturation" in a BJT means that the Vce is as low as it's going to ever >get at that collector current. Basically, the collector isn't acting >like a pretty good current source any more -- instead, it's acting like a >pretty good short circuit.
"Saturation" in a Darlington is equal to a Vbe + Vce-sat of one NPN, thus the odd-looking number. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 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.
On 3/9/2014 3:26 PM, Jim Thompson wrote:
> On Sun, 09 Mar 2014 17:23:35 -0500, Tim Wescott > <tim@seemywebsite.please> wrote: > >> On Sun, 09 Mar 2014 12:21:50 -0700, Daniel Pitts wrote: >> >>> I'm going to be using ULN2803APG in a design of mine, and I just noticed >>> on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma. >>> >>> Does that mean that the voltage drop across the Vce will typically be >>> 1.3 at that current? Or does that mean the voltage for Vbe needs to be >>> that high to have saturation? >> >> It does not mean that Vce will be 1.3V at that current -- you could have >> 350mA at 50V, for a little while. >> >> It DOES mean that the lowest that the transistor will pull the voltage >> down when 350mA is going through it is 1.3V. >> >> "Saturation" in a BJT means that the Vce is as low as it's going to ever >> get at that collector current. Basically, the collector isn't acting >> like a pretty good current source any more -- instead, it's acting like a >> pretty good short circuit. > > "Saturation" in a Darlington is equal to a Vbe + Vce-sat of one NPN, > thus the odd-looking number. > > ...Jim Thompson >
There are other chips in the same series that have pregain and a single output stage. I seem to have misplaced the number, but just threw a bunch of them away. Great chips. -- Many thanks, Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com Please visit my GURU's LAIR web site at http://www.tinaja.com
Why use a Darlinton at all ? You need to sink that much ? If you need to sink 2 amps and have a regular transistor with hfe of 100, all you need to do is supply 20 mA to the base. Off the top of my head, 220 ohm 1/8th watt should do it from a 5 volt line. 
On Sun, 09 Mar 2014 16:43:46 -0400, Phil Hobbs wrote:

> On 3/9/2014 4:33 PM, Daniel Pitts wrote: >> On 3/9/14 1:25 PM, Daniel Pitts wrote: >>> On 3/9/14 12:39 PM, Phil Hobbs wrote: >>>> On 3/9/2014 3:21 PM, Daniel Pitts wrote: >>>>> I'm going to be using ULN2803APG in a design of mine, and I just >>>>> noticed on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma. >>>>> >>>>> Does that mean that the voltage drop across the Vce will typically >>>>> be 1.3 at that current? >>>> >>>> 'Fraid so. It's a Darlington, intended for high voltage outputs. >>> >>> That was what I was afraid of. I should still have enough headroom, >>> but that is wasting a bit of power... >>> >>> Any suggestion on a replacement part? I need to enable/disable 8 lines >>> with logic level inputs. >>> >>> It can even be a 1-of-8 decoder I just need it to support up to 1A, >>> and I'd like it to be relatively cheap. My load is going to 24 >>> parallel LEDs (3.3v Vf for 16 of them, 1.8 Vf for 8), each with a >>> constant current sink in line with them. >>> >>> Cheap, for me, is < $1.25 for 8-in-8-out, or < $2.0 for 3-in-8-out >>> (since I'd be able to replace two chips in that case). >> I should add, I'm talking about price for quantities < 20 at the >> moment, >> not including shipping. >> >> > You'd think that somebody would make a MOSFET version, but AFAIK they > don't.
Not quite what you asked for, but here's a pin-compatible mosfet equivalent to the ULN2003: http://www.ti.com/product/tpl7407l Regards, Allan
On 3/9/14 1:43 PM, Phil Hobbs wrote:
> On 3/9/2014 4:33 PM, Daniel Pitts wrote: >> On 3/9/14 1:25 PM, Daniel Pitts wrote: >>> On 3/9/14 12:39 PM, Phil Hobbs wrote: >>>> On 3/9/2014 3:21 PM, Daniel Pitts wrote: >>>>> I'm going to be using ULN2803APG in a design of mine, and I just >>>>> noticed >>>>> on the datasheet [1] that Vce(sat) typ is 1.3 at 350ma. >>>>> >>>>> Does that mean that the voltage drop across the Vce will typically be >>>>> 1.3 at that current? >>>> >>>> 'Fraid so. It's a Darlington, intended for high voltage outputs. >>> >>> That was what I was afraid of. I should still have enough headroom, but >>> that is wasting a bit of power... >>> >>> Any suggestion on a replacement part? I need to enable/disable 8 lines >>> with logic level inputs. >>> >>> It can even be a 1-of-8 decoder I just need it to support up to 1A, and >>> I'd like it to be relatively cheap. My load is going to 24 parallel >>> LEDs (3.3v Vf for 16 of them, 1.8 Vf for 8), each with a constant >>> current sink in line with them. >>> >>> Cheap, for me, is < $1.25 for 8-in-8-out, or < $2.0 for 3-in-8-out >>> (since I'd be able to replace two chips in that case). >> I should add, I'm talking about price for quantities < 20 at the moment, >> not including shipping. >> > > You'd think that somebody would make a MOSFET version, but AFAIK they > don't.
Maybe what I really need is just 8 Enhancment-Mode N-Channel MOSFETs? Such as 8 of these? http://www.digikey.com/product-detail/en/IRLML2060TRPBF/IRLML2060TRPBFCT-ND/2271917
On 3/9/14 3:08 PM, Glenn wrote:
> On 09/03/14 22.15, Daniel Pitts wrote: >> On 3/9/14 2:07 PM, miso wrote: >>> G >>>> DMN6040SSD: >>>> http://www.diodes.com/datasheets/DMN6040SSD.pdf >>>> >>>> /Glenn >>> >>> That could be OK, but note the threshold voltage could be as high as 3V. >>> >> That's fine for my case. I'm using an 74HCT238 @ 5V to run the thing. > > Then how about these: > > FDN5630 24$/100 pc 2$/8 pc: > http://dk.mouser.com/ProductDetail/Fairchild-Semiconductor/FDN5630/?qs=%2fha2pyFadugc2Dlaoqmg90WO7hkA083UCazTAXEWxXk%3d > > > - > > The PICs can be bought flashed: > > DIY :-) 3-to-8 decoder/multiplexer 12 I/O. > PIC16F505-I/SL-ND > 0,66$/pc @ 10 pc > http://www.digikey.com/product-detail/en/PIC16F505-I%2FSL/PIC16F505-I%2FSL-ND/613196
This doesn't gain me any more than the 74HCT238 decoder, which I already have a bunch of.
> > > DIY 3-to-8 (or 4-to-13?) decoder/multiplexer 17 I/O (one ADC). > PIC16F527-I/SO > 0,66$/pc @ 10 pc > http://dk.mouser.com/ProductDetail/Microchip-Technology/PIC16F527-I-SO/?qs=sGAEpiMZZMu9ReDVvI6ax1uG3bwFTCkkDdXFBsH0xj8%3d
AFAICT, this has absolutely nothing to do with my post.
On 3/9/14 8:26 PM, jurb6006@gmail.com wrote:
> Why use a Darlinton at all ? You need to sink that much ? If you need to sink 2 amps and have a regular transistor with hfe of 100, all you need to do is supply 20 mA to the base. Off the top of my head, 220 ohm 1/8th watt should do it from a 5 volt line. >
I was trying to avoid 8 times transistor+resistor in my original design. I wanted high impedance input high side driver. So I'm thinking discrete parts are the way to go now. In the past, I was uncomfortable with SMD parts, and I was trying to make this as small as possible. Since I started this project, I've gotten far more comfortable with SMD and circuit-board layout.