LM317 Adjustable Regulator Question

[mike]

Actually the 30-40 ohm channel output resistance of the HC(T) works out to be about right for a 2.5V drive and the most common red/green bicolor LEDs with Vf in the 2.0-2.2V range for If~10mA, meaning no external resistors are required. There will probably be more uncertainty in light output due to the LEDs than the 595s making it kind of pointless to do any more than that. Power dissipation-wise, the 5V square is no more efficient than the 2.5V rail splitter, all it does is spread the waste energy to a mass of resistors that are otherwise unnecessary.

I prefer more robust designs. Putting a bunch of stuff with
exponential V-I curves in series to a power supply makes me nervous.
I'm partial to designs that use components in their intended/specified
mode, can
breeze thru the production process and are never heard from again.

 

[MFudalla]

On Monday, June 18, 2012 10:06:21 PM UTC-4, MFudalla wrote:
Wonder if anyone here can offer some insight on this issue. Googling
didn't
turn up much.

I have some 2 lead bicolor LED's that I'm controlling from the outputs
of
an
74HC595 shift register. Outputs are tri-state. I'm using one LM317 set
up
for 5 volts out as the supply to the 595. A second LM317 is set up for
2.5
volts out and was supposed to act as a voltage reference for the other
side
of the LED's. Everything works great with shift register outputs at
"0"
or
"low" - the 2.5 volt LM317 is sourcing and the LED's appear as load
and
everything's good. With shift register outputs at "1" or "high" the
2.5
volt
output is being pulled up to close to 3 volts. I'm thinking this is a
sinking scenario for the LM317 which is not how it was intended to
operate
or was designed for.

I thought I had a pretty simple circuit until I noticed this issue.
Any
comments would be appreciated.

This will work much better- slight problem with placement of Rcs in the
first one:

Please view in a fixed-width font such as Courier.

.
.
.
. LM317
. -------
. 5V>--+-------+-[Rcs]-+-------+---|I O|----+---+->2.5V
. | | | | | A | | |
. | | [R1] --- ------- [100] |
. | >| | --- | | |
. | PN3906 |-----+ 0.1u| +--------' |
. | /| | | | |
. | | [R2] | [100] |
. | | | | | |
. | | | | | |
. GND>--|-------| ------+-------+-------+--------+---|-->
. | | | | | |
. | | [Rc] | --- |
. | | | | 100u--- |
. | | Cc --- | +| |
. | | --- | '---+
. | | | | |
. '-[100]-+-------+-------|---------+----[100]-+
. | | |
. | ------- |
. | | A | |
. '-----|I O|------+
. -------
. LM337
.
.
.
.
.

What is the Rc and Rcs? I have some 337's on order and will be trying
this
along with some of the other ideas suggested.

Mike

Your idea of operating the logic from the +/-2.5 supplies and GND'ing the
common junction of the LEDs is the least parts intensive and most reliable
thus far.

Thanks! I'm happy to hear someone likes that idea. I'm just a hobbiest with
some electronics schooling 35 years ago. I really appreciate that comment!

Mike

 

[mike]

You'll be quoting the Bible next with all that 'intended/specified mode' nonsense.

Why don't you try reviewing the concept of load lines, then you might
see driving the LEDs

directly with logic outputs is nothing to get in a panic over.

No panic here. If you've done the math and it works for you, use it.
You'll have no worries from me until you invite me to one of your design
reviews.
That's not likely to ever happen.

Second notice:
Please stop emailing me stuff.

 

[mike]

---
The 595 can only sink or source a maximum of 35mA per pin or a total
of 70 mA through Vcc or GND, so if you have 8 LEDs being driven per
package, that means the current through them should be limited to 8.75
mA each.

That's an excellent point. What is the limiting factor?
Depending on the output characteristic.
If you load it with a square wave at twice the peak current,
You can get twice the average internal power dissipation due to
output loading.

 

[MFudalla]

---
The 595 can only sink or source a maximum of 35mA per pin or a total
of 70 mA through Vcc or GND, so if you have 8 LEDs being driven per
package, that means the current through them should be limited to 8.75
mA each.

In one of your posts you mentioned an LED current of 8mA, so I guess
you figured that out already.

For 70 LEDs, that would be a total of 613 mA from the supply, and with
2V across each of the LEDs, the series resistors would be about 343
ohms each.

360 ohms is a standard value and would allow 8.3mA through the LEDs,
with a total drain on the 5V supply of 581mA.

In order to keep from having to split the supply, someone mentioned
using an oscillator driving a half-bridge totem pole at a rate high
enough to prevent flicker as your display changed.

Something like this:

Version 4
SHEET 1 1596 748
WIRE -128 -816 -256 -816
WIRE 160 -816 -128 -816
WIRE 464 -816 160 -816
WIRE 752 -816 464 -816
WIRE 880 -816 752 -816
WIRE 752 -784 752 -816
WIRE 880 -688 880 -816
WIRE 624 -672 560 -672
WIRE 736 -672 688 -672
WIRE 752 -672 752 -720
WIRE 752 -672 736 -672
WIRE 832 -672 752 -672
WIRE 1120 -576 1072 -576
WIRE 1216 -576 1184 -576
WIRE 1312 -576 1280 -576
WIRE 1424 -576 1376 -576
WIRE 560 -560 560 -672
WIRE 608 -560 560 -560
WIRE 736 -560 736 -672
WIRE 736 -560 688 -560
WIRE -128 -528 -128 -816
WIRE 160 -480 160 -816
WIRE 208 -480 160 -480
WIRE 528 -480 432 -480
WIRE 880 -464 880 -592
WIRE 912 -464 880 -464
WIRE 1072 -464 1072 -576
WIRE 1072 -464 992 -464
WIRE 208 -416 32 -416
WIRE 496 -416 432 -416
WIRE 560 -368 560 -560
WIRE 608 -368 560 -368
WIRE 736 -368 688 -368
WIRE -128 -352 -128 -448
WIRE 32 -352 32 -416
WIRE 32 -352 -128 -352
WIRE 64 -352 32 -352
WIRE 160 -352 128 -352
WIRE 208 -352 160 -352
WIRE 560 -352 560 -368
WIRE 560 -352 432 -352
WIRE 1072 -352 1072 -464
WIRE 1120 -352 1072 -352
WIRE 1216 -352 1184 -352
WIRE 1312 -352 1280 -352
WIRE 1424 -352 1424 -576
WIRE 1424 -352 1376 -352
WIRE -128 -320 -128 -352
WIRE 880 -320 880 -464
WIRE 464 -288 464 -816
WIRE 464 -288 432 -288
WIRE 560 -240 560 -352
WIRE 624 -240 560 -240
WIRE 736 -240 736 -368
WIRE 736 -240 688 -240
WIRE 752 -240 736 -240
WIRE 832 -240 752 -240
WIRE 1424 -224 1424 -352
WIRE -256 -208 -256 -816
WIRE -128 -208 -128 -240
WIRE 64 -208 -128 -208
WIRE 160 -208 160 -352
WIRE 160 -208 128 -208
WIRE 160 -192 160 -208
WIRE 496 -192 496 -416
WIRE 496 -192 160 -192
WIRE 752 -192 752 -240
WIRE 160 -176 160 -192
WIRE -256 -96 -256 -128
WIRE 160 -96 160 -112
WIRE 160 -96 -256 -96
WIRE 528 -96 528 -480
WIRE 528 -96 160 -96
WIRE 752 -96 752 -128
WIRE 752 -96 528 -96
WIRE 880 -96 880 -224
WIRE 880 -96 752 -96
WIRE 1424 -96 1424 -144
WIRE 1424 -96 880 -96
WIRE -256 -32 -256 -96
FLAG -256 -32 0
SYMBOL Misc\\NE555 320 -384 M0
SYMATTR InstName U2
SYMBOL cap 176 -176 M0
WINDOW 0 -33 32 Left 2
WINDOW 3 -39 58 Left 2
SYMATTR InstName C2
SYMATTR Value 1µ
SYMBOL diode 64 -336 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 128 -224 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL res -112 -544 M0
SYMATTR InstName R8
SYMATTR Value 4.7K
SYMBOL nmos 832 -320 R0
SYMATTR InstName M2
SYMATTR Value FDS6930A
SYMBOL voltage -256 -224 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
SYMBOL res -112 -336 M0
SYMATTR InstName R1
SYMATTR Value 4.7K
SYMBOL pmos 832 -592 M180
SYMATTR InstName M1
SYMATTR Value FDS6685
SYMBOL voltage 1424 -240 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
WINDOW 3 24 96 Invisible 2
SYMATTR Value PULSE(0 5 0 1u 1u 100ms 200ms)
SYMATTR InstName V2
SYMBOL res 1008 -480 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 180
SYMBOL diode 688 -256 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D5
SYMATTR Value 1N4148
SYMBOL res 704 -384 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL cap 736 -192 R0
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL diode 624 -656 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D7
SYMATTR Value 1N4148
SYMBOL res 704 -576 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL cap 736 -784 R0
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL diode 1120 -560 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 1216 -560 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 1312 -560 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D6
SYMATTR Value 1N4148
SYMBOL diode 1184 -368 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D8
SYMATTR Value 1N4148
SYMBOL diode 1280 -368 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D9
SYMATTR Value 1N4148
SYMBOL diode 1376 -368 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName D10
SYMATTR Value 1N4148
TEXT -32 -56 Right 2 !.tran .5 startup uic
TEXT 440 -496 Left 2 ;1
TEXT 440 -432 Left 2 ;2
TEXT 440 -368 Left 2 ;3
TEXT 440 -312 Left 2 ;4
TEXT 184 -304 Left 2 ;5
TEXT 184 -368 Left 2 ;6
TEXT 184 -432 Left 2 ;7
TEXT 184 -496 Left 2 ;8

Notice that since the duty cycle is 50%, the currents are allowed to
be twice normal, with 16mA peak available into the LEDs for the same
brightness as 8mA, CW.

A few more components and the brightness of the red and green could be
equalized, if that's important.

BTW, what LEDs are you using/do you have a link to their data sheets?

Thanks for commenting John. The LED's are off Ebay. Unfortunately, no data
sheet. Taking into consideration the 70ma. max of the 595, I did some
testing and came up with 150 ohms for the series resistors. I figured if I
had an overcurrent situation, I could lower the 5V and 2.5V supplies a
little. Here's a link to the LED's.
There diffused and not the greatest but I figured it's a start.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180672639796&ssPageName=ADME:L:OC:US:1123

 

[mike]

I did not email a response to you. Are you sure you don't have some box checked about receiving follow-ups? I notice if I click respond to a post with a gmail account, the little CC box to their address pops up. I unchecked it for that last response to you, so it wasn't anything I did. And the box is unchecked for this response. Responding to non-gmail posts does not bring up the CC box.

I have three emails from you. To: the newsgroup and CC: to me.
You are the ONLY one I get email from.
Not sure what you mean by gmail post. My posts go out the frontier SMTP
server. My return address is indeed gmail.

Doesn't like this post produced a direct email.

 

[[email protected]]

On 6/23/2012 3:09 PM, John Fields wrote:

[on driving LEDs with 74HC595]

That's an excellent point. What is the limiting factor?

The wording of the 70 mA limit in my old TI databook is 'stress ratings only
and functional operation ... is not implied'.

The chosen package (DIP or surface mount), and the size of the solder pads (for heatsinking)
might make a difference. The limiting factor, as I see it, is the crabbiest
critic present at your design review...

The limitation is metal migration, AIUI. *Someone* at the design review
should flag such a violation. This is a no-no.