Forums

Meaning of Vce(sat)?

Started by Daniel Pitts March 9, 2014
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. > > Cheers > > Phil Hobbs >
Check; it is impossible to saturate a darlington.
On Mon, 10 Mar 2014 11:48:26 -0800, Robert Baer
<robertbaer@localnet.com> wrote:

>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. >> >> Cheers >> >> Phil Hobbs >> > Check; it is impossible to saturate a darlington.
The main device... but the driver device saturates. ...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.
Glenn wrote:

>> That's fine for my case. I'm using an 74HCT238 @ 5V to run the thing. >
I was thinking more about the strength of the fet if it is on the high side of the threshold spec. Just something to consider.
On 3/10/14 11:39 AM, Daniel Pitts wrote:
> 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 < $2.0 for 3-in-8-out >>>> (since I'd be able to replace two chips in that case). >>> I should add, I'm talking about price for quantities < 20 at the moment, >>> not including shipping. >>> >> >> You'd think that somebody would make a MOSFET version, but AFAIK they >> don't. > Maybe what I really need is just 8 Enhancment-Mode N-Channel MOSFETs? > > Such as 8 of these? > http://www.digikey.com/product-detail/en/IRLML2060TRPBF/IRLML2060TRPBFCT-ND/2271917
Looking more carefully, I think I might need P-Channel MOSFETS, and then I'd need to invert my logic driver (replace the 74HC238 with a 74HC138). If I used an N-Channel, I don't think I could get the Vgs to be high enough. I was thinking this part: http://www.digikey.com/product-detail/en/SCH1302-TL-E/869-1194-1-ND/2165672 Then I would have the source connected to +5v, the drain connected to to my load, and the gate connected to the output of the 1-of-8. The load is some parallel LEDs (each in series with a constant-current-sink), so the drain voltage could vary between +1.8v up to +5.0v, The current can vary from 10mA to 1200mA. Depending on how many parallel LEDs are enabled. Does this sound like it would work? Thanks, Daniel.
On 11/03/14 19.58, Daniel Pitts wrote:
...
> The load is some parallel LEDs (each in series with a > constant-current-sink), so the drain voltage could vary between +1.8v up > to +5.0v, The current can vary from 10mA to 1200mA. Depending on how > many parallel LEDs are enabled. > > Does this sound like it would work? > > Thanks, > Daniel.
You could control the constant-current-sink directly? E.g. by sorting the gate/source or base/emitter by a small MOSFET (2N7002)? Double 2N7002: 2N7002DWA-7DICT-ND (digikey) /Glenn
In article <71JTu.1700$KZ7.1308@fx32.iad>, 
newsgroup.nospam@virtualinfinity.net says...
> Looking more carefully, I think I might need P-Channel MOSFETS, and then > I'd need to invert my logic driver (replace the 74HC238 with a 74HC138). > If I used an N-Channel, I don't think I could get the Vgs to be high > enough. > > I was thinking this part: > http://www.digikey.com/product-detail/en/SCH1302-TL-E/869-1194-1-ND/2165672 > > Then I would have the source connected to +5v, the drain connected to to > my load, and the gate connected to the output of the 1-of-8. > > The load is some parallel LEDs (each in series with a > constant-current-sink), so the drain voltage could vary between +1.8v up > to +5.0v, The current can vary from 10mA to 1200mA. Depending on how > many parallel LEDs are enabled. > > Does this sound like it would work? > > Thanks, > Daniel. >
Ok, looking at that part it appears it would do exactly what you need. But I've noticed they have only 1 in stock? Sometimes when you see low stock counts like that, it's a sign of stay away... ;) Jamie
On 3/11/14 2:35 PM, Glenn wrote:
> On 11/03/14 19.58, Daniel Pitts wrote: > ... >> The load is some parallel LEDs (each in series with a >> constant-current-sink), so the drain voltage could vary between +1.8v up >> to +5.0v, The current can vary from 10mA to 1200mA. Depending on how >> many parallel LEDs are enabled. >> >> Does this sound like it would work? >> >> Thanks, >> Daniel. > > You could control the constant-current-sink directly? > > E.g. by sorting the gate/source or base/emitter by a small MOSFET (2N7002)?
I'm multiplexing 24*8 LEDs. The "current sink" is 3 8-bit shift-register constant-current-sink (TLC5196) which power between 0 to 24 LEDs in parallel (depending). The MOSFET is a "row enable" to choose between 1 of 8 sets of 24 LEDS. So in total, I'm powering 196 LEDs, just 24 at a time.
> > Double 2N7002: > > 2N7002DWA-7DICT-ND > (digikey) > > /Glenn
On 3/11/14 4:39 PM, Maynard A. Philbrook Jr. wrote:
> In article <71JTu.1700$KZ7.1308@fx32.iad>, > newsgroup.nospam@virtualinfinity.net says... >> Looking more carefully, I think I might need P-Channel MOSFETS, and then >> I'd need to invert my logic driver (replace the 74HC238 with a 74HC138). >> If I used an N-Channel, I don't think I could get the Vgs to be high >> enough. >> >> I was thinking this part: >> http://www.digikey.com/product-detail/en/SCH1302-TL-E/869-1194-1-ND/2165672 >> >> Then I would have the source connected to +5v, the drain connected to to >> my load, and the gate connected to the output of the 1-of-8. >> >> The load is some parallel LEDs (each in series with a >> constant-current-sink), so the drain voltage could vary between +1.8v up >> to +5.0v, The current can vary from 10mA to 1200mA. Depending on how >> many parallel LEDs are enabled. >> >> Does this sound like it would work? >> >> Thanks, >> Daniel. >> > Ok, looking at that part it appears it would do exactly what you need. > > But I've noticed they have only 1 in stock? Sometimes when you see low > stock counts like that, it's a sign of stay away... ;)
Ah, didn't notice that, thanks... Here's a "DUAL P-CHANNEL ENHANCEMENT MODE FIELD EFFECT TRANSISTOR" in stock. <http://www.digikey.com/product-detail/en/DMP2240UDM-7/DMP2240UDMDICT-ND/1774243> About .1&Omega; Rds(on) @ 4.5v, which is good for me. The pinouts also look like they'll be convenient for my application. More than the single-fet packages.
Den onsdag den 12. marts 2014 03.10.34 UTC+1 skrev Daniel Pitts:
> On 3/11/14 4:39 PM, Maynard A. Philbrook Jr. wrote: >=20 > > In article <71JTu.1700$KZ7.1308@fx32.iad>, >=20 > > newsgroup.nospam@virtualinfinity.net says... >=20 > >> Looking more carefully, I think I might need P-Channel MOSFETS, and th=
en
>=20 > >> I'd need to invert my logic driver (replace the 74HC238 with a 74HC138=
).
>=20 > >> If I used an N-Channel, I don't think I could get the Vgs to be hig=
h
>=20 > >> enough. >=20 > >> >=20 > >> I was thinking this part: >=20 > >> http://www.digikey.com/product-detail/en/SCH1302-TL-E/869-1194-1-ND/21=
65672
>=20 > >> >=20 > >> Then I would have the source connected to +5v, the drain connected to =
to
>=20 > >> my load, and the gate connected to the output of the 1-of-8. >=20 > >> >=20 > >> The load is some parallel LEDs (each in series with a >=20 > >> constant-current-sink), so the drain voltage could vary between +1.8v =
up
>=20 > >> to +5.0v, The current can vary from 10mA to 1200mA. Depending on how >=20 > >> many parallel LEDs are enabled. >=20 > >> >=20 > >> Does this sound like it would work? >=20 > >> >=20 > >> Thanks, >=20 > >> Daniel. >=20 > >> >=20 > > Ok, looking at that part it appears it would do exactly what you need=
.
>=20 > > >=20 > > But I've noticed they have only 1 in stock? Sometimes when you see lo=
w
>=20 > > stock counts like that, it's a sign of stay away... ;) >=20 > Ah, didn't notice that, thanks... >=20 >=20 >=20 > Here's a "DUAL P-CHANNEL ENHANCEMENT MODE FIELD EFFECT TRANSISTOR" in sto=
ck.
>=20 >=20 >=20 > <http://www.digikey.com/product-detail/en/DMP2240UDM-7/DMP2240UDMDICT-ND/=
1774243>
>=20 >=20 >=20 > About .1=CE=A9 Rds(on) @ 4.5v, which is good for me. >=20 >=20 >=20 > The pinouts also look like they'll be convenient for my application.=20 >=20 > More than the single-fet packages.
just remember that now that you have turned things up side down the Vgs is = you supply - Vol of your controller So if you supply your LEDs with 1.8V it'll be at most 1.8V =20 -Lasse -Lasse
On 3/12/14 9:33 AM, Lasse Langwadt Christensen wrote:
> Den onsdag den 12. marts 2014 03.10.34 UTC+1 skrev Daniel Pitts: >> On 3/11/14 4:39 PM, Maynard A. Philbrook Jr. wrote: > > just remember that now that you have turned things up side down the Vgs is you supply - Vol of your controller > So if you supply your LEDs with 1.8V it'll be at most 1.8V
As I wrote earlier:
>newsgroup.nospam@virtualinfinity.net says... >> Looking more carefully, I think I might need P-Channel MOSFETS, and then >> I'd need to invert my logic driver (replace the 74HC238 with a 74HC138).
Now, my source is VCC(+5v) always, The Vgs will be 0v (off) when the gate voltage is VCC(+5v). The Vgs will be -5v (on) when my gate voltage is at GND. The 74HC138 will select exactly one of these, the rest will be near VCC. The one that is "logic-enabled" will be near GND. My schematic: <http://virtualinfinity.net/electronics/LED.sch.pdf> My board layout: <http://virtualinfinity.net/electronics/LED.brd.pdf>