4026 Max Current Output - Page 1
Forum section: electronics / beginners

author: JonPyro
author_rank: Regular Contributor
timestamp: February 03, 2019, 10:22:17 am
content: 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=AOvVaw2sxqS5kuCGFQoooETF1JPyGoogling 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?

author: Benta
author_rank: Super Contributor
timestamp: February 03, 2019, 11:53:11 am
content: Look at Fig. 6, 7, 8 and 9 in the data sheet.

author: JonPyro
author_rank: Regular Contributor
timestamp: February 03, 2019, 12:06:04 pm
content: Quote from: Benta on February 03, 2019, 11:53:11 amSo 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?

author: Ian.M
author_rank: Super Contributor
timestamp: February 03, 2019, 12:43:24 pm
content: 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.

author: JonPyro
author_rank: Regular Contributor
timestamp: February 03, 2019, 12:54:26 pm
content: 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

author: Ian.M
author_rank: Super Contributor
timestamp: February 03, 2019, 01:12:01 pm
content: 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.

author: rdl
author_rank: Super Contributor
timestamp: February 04, 2019, 03:21:52 am
content: 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.

author: JonPyro
author_rank: Regular Contributor
timestamp: February 04, 2019, 10:05:28 am
content: Quote from: rdl on February 04, 2019, 03:21:52 amThat's interesting to know but where on the datasheet does it mention this?

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2019, 11:45:30 am
content: 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.pdfhttp://www.huergo.edu.ar/tcweb/pdf/DS.pdf

author: rdl
author_rank: Super Contributor
timestamp: February 04, 2019, 02:16:46 pm
content: Quote from: JonPyro on February 04, 2019, 10:05:28 amIt'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, 4026Quote from: Texas Instruments 4026/4033 data sheet (acquired from Harris Semiconductor SCHS031B – Revised July 2003)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 04, 2019, 02:55:26 pm
content: As you have discovered the 50 year old technology of the cd4000 chips is not very well suited for driving led's.octave:1> 2018-1968ans = 50https://en.wikipedia.org/wiki/4000-series_integrated_circuitsButhttps://duckduckgo.com/?q=don%27t+panic&iax=images&ia=imagesTechnology has improved in the meantime.