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 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
>
Check; it is impossible to saturate a darlington.
Reply by Daniel Pitts●March 10, 20142014-03-10
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.
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
Reply by Don Lancaster●March 9, 20142014-03-09
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
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