| Author |
Message |
Spehro Pefhany
Guest
|
 Posted:
Mon Dec 12, 2005 5:35 pm Post subject:
Re: multi-led hi-side constant-current question |
|
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On Mon, 12 Dec 2005 09:32:36 -0700, the renowned J W <nospam@pat7.com>
wrote:
| Quote: | meow2222@care2.com wrote in sci.electronics.design:
Jim Thompson wrote:
On Thu, 08 Dec 2005 17:35:50 -0800, Bob Monsen ... wrote:
On Tue, 06 Dec 2005 12:57:59 -0800, JIW wrote:
[Re http://pat7.com/jp/2313q1.png ]
...
If you arranged things a bit differently, and added a couple of PNP
transistors + a resistor, things would be much better:
vbatt----------o---------.
| |
68R |
| |
e e
.-b .----b (PNP Transistors
| c | c
| | | |
'--o---' '--- LEDS & PORTS
|
'------------- YOUR CURRENT SINK
...
Note that if you make the current sink supply about 1mA, that
will allow the LEDs to have 10mA. The 68 ohm resistor makes that happen.
[snip]
Until the temperature changes ;-)
...Jim Thompson
Its the temp diff between the 2 trs that would be the issue, and the
LED's tr is only dissipating 75 mW or so max. @50C/watt thats 3.75C. If
we subtract the 10% temp rise of the other tr, thats apx 3.4C temp
diff. How much variation will that cause?
Each Vbe would shift about 2 mV/K (for silicon near room temperature
obeying (kT/q)*ln(I/Is) equation) so a 3.4K difference gives about 7
mV or 1% deviation. I'm not concerned about variation that small, but
if I were would use a 2-transistor die.
-jiw
|
How much do you think the *load current* will change with a 3.4K
temperature delta? It's also not all that stable wrt ambient
temperature. Maybe good enough for LEDs.
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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Jim Thompson
Guest
|
| Posted:
Mon Dec 12, 2005 5:35 pm Post subject:
Re: multi-led hi-side constant-current question |
|
|
On Mon, 12 Dec 2005 09:32:36 -0700, J W <nospam@pat7.com> wrote:
| Quote: | meow2222@care2.com wrote in sci.electronics.design:
Jim Thompson wrote:
On Thu, 08 Dec 2005 17:35:50 -0800, Bob Monsen ... wrote:
On Tue, 06 Dec 2005 12:57:59 -0800, JIW wrote:
[Re http://pat7.com/jp/2313q1.png ]
...
If you arranged things a bit differently, and added a couple of PNP
transistors + a resistor, things would be much better:
vbatt----------o---------.
| |
68R |
| |
e e
.-b .----b (PNP Transistors
| c | c
| | | |
'--o---' '--- LEDS & PORTS
|
'------------- YOUR CURRENT SINK
...
Note that if you make the current sink supply about 1mA, that
will allow the LEDs to have 10mA. The 68 ohm resistor makes that happen.
[snip]
Until the temperature changes ;-)
...Jim Thompson
Its the temp diff between the 2 trs that would be the issue, and the
LED's tr is only dissipating 75 mW or so max. @50C/watt thats 3.75C. If
we subtract the 10% temp rise of the other tr, thats apx 3.4C temp
diff. How much variation will that cause?
Each Vbe would shift about 2 mV/K (for silicon near room temperature
obeying (kT/q)*ln(I/Is) equation) so a 3.4K difference gives about 7
mV or 1% deviation. I'm not concerned about variation that small, but
if I were would use a 2-transistor die.
-jiw
|
Your math is flawed.
WITHOUT even a differential temperature, the variation from 20°C to
60°C (both devices at same temperature) is 21.34%
The dominating effect is delta-VBE = (kT/q)*ln(Iright/Ileft)
NOT the delta-VBE hand-waving rule-of-thumb you quote, which is valid
only for devices at the same current.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
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J W
Guest
|
| Posted:
Mon Dec 12, 2005 5:35 pm Post subject:
Re: multi-led hi-side constant-current question |
|
|
meow2222@care2.com wrote in sci.electronics.design:
| Quote: | Jim Thompson wrote:
On Thu, 08 Dec 2005 17:35:50 -0800, Bob Monsen ... wrote:
On Tue, 06 Dec 2005 12:57:59 -0800, JIW wrote:
[Re http://pat7.com/jp/2313q1.png ] |
....
| Quote: | If you arranged things a bit differently, and added a couple of PNP
transistors + a resistor, things would be much better:
vbatt----------o---------.
| |
68R |
| |
e e
.-b .----b (PNP Transistors
| c | c
| | | |
'--o---' '--- LEDS & PORTS
|
'------------- YOUR CURRENT SINK
....
Note that if you make the current sink supply about 1mA, that
will allow the LEDs to have 10mA. The 68 ohm resistor makes that happen.
[snip]
Until the temperature changes ;-)
...Jim Thompson
Its the temp diff between the 2 trs that would be the issue, and the
LED's tr is only dissipating 75 mW or so max. @50C/watt thats 3.75C. If
we subtract the 10% temp rise of the other tr, thats apx 3.4C temp
diff. How much variation will that cause?
|
Each Vbe would shift about 2 mV/K (for silicon near room temperature
obeying (kT/q)*ln(I/Is) equation) so a 3.4K difference gives about 7
mV or 1% deviation. I'm not concerned about variation that small, but
if I were would use a 2-transistor die.
-jiw
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