Any multiple EMITTER FOLLOWER in-a-single pack? - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: November 29, 2016, 09:10:31 am
content: Dear forum.I know it sounds strange but I try.In order to buffer the output of a 8-digit LED display driver (MAX7219) , I am using a common collector configuration for each anode output of the chip.A resistor of 10k connect each anode output to a BJT 547 and the emitter is going into the anode of the led.; the cathode of the led goes then into the cathode output of my chip.This is done in order to wire far away enough from the MAX7219 the LED matrix and it works.Since I have to mount 128 BJT in order to drive my whole 256 leds I was asking if there is a more convenient way to buffer the output avoiding using all this components.Best regards and thank you

author: RoGeorge
author_rank: Super Contributor
timestamp: November 29, 2016, 11:57:52 am
content: Try these ones:http://www.ti.com/lit/ds/symlink/uln2003a.pdfFrom memory, UDN2981 are similar but for common cathode.

author: spudboy488
author_rank: Regular Contributor
timestamp: November 29, 2016, 12:39:01 pm
content: To add to RoGeorge's response, the ULN2803 is the same part but with 8 drivers (vs. 7 in the ULN2003). A better fir for 256 LEDs.

author: Houseman
author_rank: Regular Contributor
timestamp: November 29, 2016, 02:17:56 pm
content: Thanks Thanks thanks and again thanks!!!!

author: mikerj
author_rank: Super Contributor
timestamp: November 29, 2016, 05:30:28 pm
content: Both the ULN2003 and the ULN2803 are common emitter devices, not emitter followers.

author: Gyro
author_rank: Super Contributor
timestamp: November 29, 2016, 06:58:06 pm
content: You're very unlikely to find an Emitter Follower chip because power dissipation can quickly become too high. ULN2xxx devices are intended for switching purposes rather than linear operation, so their output transistors are either on or off. An emitter follower is inherently a linear device which could easilly generate a lot of heat in the output transistor. The best you are likely to find is a common emitter PNP device but I can't think of one off hand. They will be less common because their maximum supply voltage will be very limited unless they also include level-shifting circuitry to allow low voltage logic level inputs.

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2016, 07:02:30 pm
content: According to Digikey, the UDN2981 is obsolete.http://www.digikey.com/product-detail/en/allegro-microsystems-llc/UDN2981A/UDN2981A-ND/1006543Use the, MIC2981/2982, which is still in production and is similar:http://ww1.microchip.com/downloads/en/DeviceDoc/mic2981.pdfhttp://www.digikey.co.uk/product-detail/en/microchip-technology/MIC2981-82YN/576-1158-ND/771627

author: edavid
author_rank: Super Contributor
timestamp: November 29, 2016, 07:59:20 pm
content: The only common collector transistor array I know of was the RCA CA3082, but it's long obsolete.OP, what is your anode current and voltage drop requirement?

author: Houseman
author_rank: Regular Contributor
timestamp: November 30, 2016, 10:30:16 am
content: Well according to the MAX7219 data sheet each output drives in a multiplexed way 1 LED with Vf=2V and If=20mA.I suppose also 1mt. length of ribbon flat cable from the emitter of the BJT and the ANODE - KATODE of the LED should be added in order to have some fraction of voltage drop down. Am I right?Can this info help you?Best regards and thanks