Author Topic: 4026 Max Current Output  (Read 1605 times)

0 Members and 1 Guest are viewing this topic.

Offline JonPyroTopic starter

  • Regular Contributor
  • *
  • Posts: 50
  • Country: gb
4026 Max Current Output
« on: February 03, 2019, 10:22:17 am »
Hi all,

Looking at the cd4026b datasheet for max output current to drive my led 7seg all I can find is output source minimum of 1mA @ 5v.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.ti.com/lit/ds/symlink/cd4026b.pdf&ved=2ahUKEwiq2Ya9q5_gAhU1sXEKHRDdBQcQFjAMegQIBxAB&usg=AOvVaw2sxqS5kuCGFQoooETF1JPy

Googling it I have found many answers ranging from 1mA to 15mA. Even their example at the bottom of the datasheet shows them driving an led display through transistors!

I remember years ago at school we used to use max 15mA per segment as a rule of thumb, however, can anyone tell me how I can find out from the datasheet? I notice they give output wattage of the transistors, could you work this backwards?
 

Offline Benta

  • Super Contributor
  • ***
  • Posts: 6420
  • Country: de
Re: 4026 Max Current Output
« Reply #1 on: February 03, 2019, 11:53:11 am »
Look at Fig. 6, 7, 8 and 9 in the data sheet.
 
The following users thanked this post: JonPyro

Offline JonPyroTopic starter

  • Regular Contributor
  • *
  • Posts: 50
  • Country: gb
Re: 4026 Max Current Output
« Reply #2 on: February 03, 2019, 12:06:04 pm »
Look at Fig. 6, 7, 8 and 9 in the data sheet.

So if I am reading figure 8 correctly, you should limit the output to 4mA at 5V and about 19/20mA at 10V and 29/30mA at 15V?
 

Offline Ian.M

  • Super Contributor
  • ***
  • Posts: 13216
Re: 4026 Max Current Output
« Reply #3 on: February 03, 2019, 12:43:24 pm »
Its worse than that.  If you look closely at the static electrical characteristics on page 2 and analyse the available data, you'll see that you need to derate those curves  by 3% of the current for every 10 deg C temperature rise + the power dissipation heats the die significantly if you push the current too far along those curves from the origin.  You  certainly wont get a sustained 30mA at 15V Vcc from more than one output without destroying the chip, as that will breach the abs. max. dissipation limits on page 1.
« Last Edit: February 03, 2019, 12:46:38 pm by Ian.M »
 
The following users thanked this post: JonPyro

Offline JonPyroTopic starter

  • Regular Contributor
  • *
  • Posts: 50
  • Country: gb
Re: 4026 Max Current Output
« Reply #4 on: February 03, 2019, 12:54:26 pm »
They don't make this easy!

Ignoring your 3% for a second. Looking at the first page max output wattage is 100mW......so 30mA @ 15v is 450mW  :o
« Last Edit: February 03, 2019, 12:58:32 pm by JonPyro »
 

Offline Ian.M

  • Super Contributor
  • ***
  • Posts: 13216
Re: 4026 Max Current Output
« Reply #5 on: February 03, 2019, 01:12:01 pm »
It doesn't quite work like that.  Its not simply Vcc x Iout.  You need to multiply the voltage drop, measured from the output to the rail its pulling towards, by the current through it.   The voltage drop will increase as the die warms up, increasing the dissipation and further warming the die. 

Its a lot of work to even roughly calculate the limits at your max intended operating temperature, and it may be simpler to set up a heated test jig to take some measurements of a few chips,  derate by 50% then check you aren't too close to the package or individual output dissipation limits.
 
The following users thanked this post: JonPyro

Offline rdl

  • Super Contributor
  • ***
  • Posts: 3667
  • Country: us
Re: 4026 Max Current Output
« Reply #6 on: February 04, 2019, 03:21:52 am »
LEDs are vastly more efficient now than they were years ago. Unless you have a special situation or are using really old displays, you probably only need a couple of mA. The 4026B have 1k resistors already, but you may need more just to keep the brightness down to a reasonable level. Some blue 7 segments I bought a few years back were too bright to look at with just 7 mA driven directly from the 4026B outputs.
 
The following users thanked this post: JonPyro

Offline JonPyroTopic starter

  • Regular Contributor
  • *
  • Posts: 50
  • Country: gb
Re: 4026 Max Current Output
« Reply #7 on: February 04, 2019, 10:05:28 am »
LEDs are vastly more efficient now than they were years ago. Unless you have a special situation or are using really old displays, you probably only need a couple of mA. The 4026B have 1k resistors already, but you may need more just to keep the brightness down to a reasonable level. Some blue 7 segments I bought a few years back were too bright to look at with just 7 mA driven directly from the 4026B outputs.

That's interesting to know but where on the datasheet does it mention this?
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20357
  • Country: gb
  • 0999
Re: 4026 Max Current Output
« Reply #8 on: February 04, 2019, 11:45:30 am »
Modern pure green and cool white are the brightest of all LEDs. Most LED datasheets give a brightness in mcd vs forward current. One thing to note is the viewing angle, which is often quite small, meaning the LED will appear to be very bright when one is looking straight at it, but very dim when viewed from a different angle.

The voltage drop vs current characteristic is common to all CMOS ICs in the 4000 family.
https://assets.nexperia.com/documents/design-support/familyhef4000specification.pdf
http://www.huergo.edu.ar/tcweb/pdf/DS.pdf
 
The following users thanked this post: JonPyro

Offline rdl

  • Super Contributor
  • ***
  • Posts: 3667
  • Country: us
Re: 4026 Max Current Output
« Reply #9 on: February 04, 2019, 02:16:46 pm »
That's interesting to know but where on the datasheet does it mention this?

It's something I learned from the CMOS Cookbook actually, though my old TI data sheet from 2003 does mention it in a footnote.

Quote from: Don Lancaster - CMOS Cookbook, page 93, 4026
Fluorescent and newer LED displays may be driven directly by the outputs. High current light-emitting diodes and neon displays require external drivers.

Quote from: Texas Instruments 4026/4033 data sheet (acquired from Harris Semiconductor SCHS031B – Revised July 2003)
The interfacing buffers shown, while a necessity with the 4026A and 4033A, are not required when using the "B" devices (which) can drive most display devices directly, especially above 10 volts.


 
The following users thanked this post: JonPyro

Offline Doctorandus_P

  • Super Contributor
  • ***
  • Posts: 4004
  • Country: nl
Re: 4026 Max Current Output
« Reply #10 on: February 04, 2019, 02:55:26 pm »
As you have discovered the 50 year old technology of the cd4000 chips is not very well suited for driving led's.
octave:1> 2018-1968
ans =  50
https://en.wikipedia.org/wiki/4000-series_integrated_circuits

But https://duckduckgo.com/?q=don%27t+panic&iax=images&ia=images  Technology has improved in the meantime.
« Last Edit: February 04, 2019, 02:57:50 pm by Doctorandus_P »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf