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>
Reply by Lasse Langwadt Christensen●March 12, 20142014-03-12
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$KZ7.1308@fx32.iad>,
>=20
> > newsgroup.nospam@virtualinfinity.net says...
>=20
> >> Looking more carefully, I think I might need P-Channel MOSFETS, and th=
en
>=20
> >> I'd need to invert my logic driver (replace the 74HC238 with a 74HC138=
).
>=20
> >> If I used an N-Channel, I don't think I could get the Vgs to be hig=
>=20
> >>
>=20
> >> Then I would have the source connected to +5v, the drain connected to =
to
>=20
> >> my load, and the gate connected to the output of the 1-of-8.
>=20
> >>
>=20
> >> The load is some parallel LEDs (each in series with a
>=20
> >> constant-current-sink), so the drain voltage could vary between +1.8v =
up
>=20
> >> to +5.0v, The current can vary from 10mA to 1200mA. Depending on how
>=20
> >> many parallel LEDs are enabled.
>=20
> >>
>=20
> >> Does this sound like it would work?
>=20
> >>
>=20
> >> Thanks,
>=20
> >> Daniel.
>=20
> >>
>=20
> > Ok, looking at that part it appears it would do exactly what you need=
.
>=20
> >
>=20
> > But I've noticed they have only 1 in stock? Sometimes when you see lo=
w
>=20
> > stock counts like that, it's a sign of stay away... ;)
>=20
> Ah, didn't notice that, thanks...
>=20
>=20
>=20
> Here's a "DUAL P-CHANNEL ENHANCEMENT MODE FIELD EFFECT TRANSISTOR" in sto=
>=20
>=20
>=20
> About .1=CE=A9 Rds(on) @ 4.5v, which is good for me.
>=20
>=20
>=20
> The pinouts also look like they'll be convenient for my application.=20
>=20
> More than the single-fet packages.
just remember that now that you have turned things up side down the Vgs is =
you supply - Vol of your controller
So if you supply your LEDs with 1.8V it'll be at most 1.8V
=20
-Lasse
-Lasse
Reply by Daniel Pitts●March 11, 20142014-03-11
On 3/11/14 4:39 PM, Maynard A. Philbrook Jr. wrote:
> In article <71JTu.1700$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 .1Ω 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.
Reply by Daniel Pitts●March 11, 20142014-03-11
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.
Reply by Maynard A. Philbrook Jr.●March 11, 20142014-03-11
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
Reply by Glenn●March 11, 20142014-03-11
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
Reply by Daniel Pitts●March 11, 20142014-03-11
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.
Reply by miso●March 11, 20142014-03-11
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.
Reply by Jim Thompson●March 10, 20142014-03-10
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
--
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Reply by Robert Baer●March 10, 20142014-03-10
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
>