Why wast majority of LED driving IC's are current sinks instead of source? - Page 1
Forum section: electronics / beginners

author: jimon
author_rank: Regular Contributor
timestamp: January 07, 2017, 07:01:13 pm
content: I never really asked myself this question until I ended up in need for current source IC because I want to drive a HDSP-2000 LED display (more or less museum piecesimilar was featured on eevblog youtube channel not so long ago) you can find adatasheet here. Basically it's plain stupid array of LED's with shift register ONLY for cathodes, aka all column anodes are wired together and exposed as 5 pins .. in high brightness each this pin can sink like 20-30 mA, so 5 pins and total can be 100 mA per display, and mul. it by 3+ displays - viola we have half an amp!Because I'm lazy and don't want to spend 5 pins of MCU with a buffer array IC in between, I was looking for something like serial interface + current sources in one IC, but almost everything I found was serial + current sinks! Like this one:So I was thinking, why is that? Is making high power NMOS is that much easier/cheaper than PMOS? Or is there something else? Or is it just basic pragmatic practicality of IC companies?PS. Is there non-obscure i2c + current source IC out there?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 07, 2017, 07:17:10 pm
content: Probably easier to make, and then it becomes the convention.It does have some disadvantages, like making it harder to use lower supply voltage for red in big muxed displays to reduce heat.Once company has a "revolutionary" common-cathode drive system for use in video walls :http://www.silicon-core.com/common-cathodehttps://youtu.be/Jm0aVF6_y_c

author: shteii01
author_rank: Frequent Contributor
timestamp: January 07, 2017, 08:05:47 pm
content: Because sophisticated miniature circuits and high currents don't play well together.

author: r4tch3t
author_rank: Newbie
timestamp: January 07, 2017, 08:16:23 pm
content: NMOS is easier to manufacture, I did a quick search and found this crazy chipL T3595 - 16 Channel Buck Mode LED DriverThe pca9685 can source 10mA power channel if that's enoughSent from my ALE-L02 using Tapatalk

author: danadak
author_rank: Super Contributor
timestamp: January 09, 2017, 12:01:34 pm
content: If you look at a "typical" CMOS output it is an NMOS and PMOS structurebetween the rails. One of the principal issues in a simple non bootstrappedoutput stage is source bulk effect on P devices (that connects output to Vddrail) ability to turn on hard.So the NMOS is easiest to turn on hard, which connects the output to ground,Vss.Look at typical device output source and sink current ratings, you will see this.To offset this issue typically the PMOS device is sized with greater channel capability.This is more than you want to know -http://bwrcs.eecs.berkeley.edu/Classes/icdesign/ee141_f01/Notes/chapter5.pdfRegards, Dana.

author: djococaud
author_rank: Regular Contributor
timestamp: January 10, 2017, 01:39:28 pm
content: Simply because you can then add more leds in series on one output with a higher supply voltage...(within limits...)

author: jimon
author_rank: Regular Contributor
timestamp: January 10, 2017, 04:15:15 pm
content: danadak, wow thanks! just checked datasheets for some parts and indeed - current rating for LO state are like 100 mA, while HI are like 5-10 mA ...

author: bson
author_rank: Unknown
timestamp: January 11, 2017, 07:58:24 pm
content: It's also much easier to design circuits around a common ground rather than common supplies. Even the example shows dual supplies - 3.3V for the logic and 5V for the current source; with a common supply the device would either need to operate at 5V or you'd need multiple offset grounds. Supplies are easier to have many of since they only need to source; ground needs to both source and sink, which is more complex, and makes it easier to only have one of.

author: brabus
author_rank: Frequent Contributor
timestamp: January 11, 2017, 08:31:03 pm
content: The problem with PMOS is also the lower electron mobility in the P-silicon, if I well remember.In other words, a PMOS has to be 3x larger to perform as the equivalent NMOS.You could say: why don't we use an NMOS to switch the positive rail? That would be good, but the gate driver voltage would have to swing over the supply... a real PITA, basically only done on switching power electronics applications, using a bootstrap capacitor.

author: danadak
author_rank: Super Contributor
timestamp: January 12, 2017, 01:33:49 am
content: The current per output, sink, is one of two ratings you have to pay attention to.The other is total current of all outputs flowing in supply rail. This tends to be a greaterproblem if driving to Vdd vs sink which is driving to Vss. Reason is where the currentis concentrated, in later case substrate. Vdd rail more sensitive because of the generalway its routed and the large number of drops to circuits off the rail.So look for the total supply pin allowed rating to manage the number of outputs thatcan carry max current. Pay attention also to special notes, does the rating include overallchip load or is it just I/O loads.Regards, Dana.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 12, 2017, 02:58:45 pm
content: Quote from: danadak on January 12, 2017, 01:33:49 amThree things - you also need to watch total power dissipation. Many 16 or more channel drivers can't deliver their rated maximum output on all channels if the drop across them is much more than the dropout voltage, though they do typically shut down or reduce current rather than melting.