CN5711 trouble getting the correct current limiting setting - Page 1
Forum section: electronics / beginners

author: Wnef
author_rank: Newbie
timestamp: December 11, 2023, 05:45:08 pm
content: Hello all, I'm usualy the kind of "read forum but don't post" type of guy but I'm so clueless on this one !!!So here is my situation.I'm designing a circuit to control a 3W RGBW led via I2CI'm using fourcn5711chip ( datasheet link ) to control each channel.PWM signal is coming from aPCA8632to each one of them.Power is from a two paralleled 18650 battery ( 4000mah total protected on low side )And here is the problem.Datasheet of the CN5711 specifies that the LED pin current is set by a resistor R-iset wich value is calculated by this formulaILED = 1800V／R-isetthey give this example :R-iset = 1800V/1A = 1.8kΩI want 750ma so 1800/0,75 = 2.4 kΩBut mesuring with a multimeter I only get around 380maIf I put a 1.8kΩ resistor insted like in the example I only get around 800maI'm pretty sure my problem is here (at the Riset) but just as sanity check I've verified the followings :- I've use two different meters for mesure- Use psu rather than battery- I've feed 3.4V at 750ma with bench psu to the LED to check if they would take it ( the did and happyly blinded me ! )- I've cheked the compatibility of the pca9632- outputs a 1.5khz pwm ( mesured with scope ) => in spec with the 2khz max of the CN711- are supposed to deliver 25ma wich is above the 1 uA required for CE pin ( chip enable PWM input of CN5711 )I even taken the PCA9632 out of the equation and shorted CE to VCC on the chip and still I cant get it to output current it is suppose to give me with the ILED = 1800V／R-iset formula.- Parts are supplied by lcsc some of them soldered by jlcpb or myself for the same result.Is this normal ? Doe someone has the same experience with cn5711I've seen CN5711 mounted on pcb with a potentiometer in place of R-isetso I guess lot of people don't even bother to read the datasheet and just turn the pot until power requirements are metshould I just change the resistor value until i get the desired output and forget the ILED = 1800V／R-iset formula ?I'm fairly new to electronics so I'm kind of not confortable to step aside from the datasheetJust for context.I've chose the cn5711 because it is cheap an combines the role of a current limiting resistor and mosfet to drive the led.My part count and size of thoses parts is limited because the pcb need to fit in circle of 30mm diametre in order to fit in alumimun housing I have. the spx3819 is just a regulator for the PCA9632, and I'm considering add pullup resistors to the I2C SDA and SCL but I'm have notread this yetI've linked schematic and PCB layout (4 layers- Ground on inner 1&2 layers ) even if it does not really matter in this case since I've tried outside of the pcb.So yeah ... do you have the same issue ?Am I missing something ?Should I just forget the formula ?Hope to get so feedback !

author: Wnef
author_rank: Newbie
timestamp: January 01, 2024, 01:06:54 am
content: Ok I think I understand where is the issueIt seems the voltage drop of the CN5711 is way more than I expectedfar from what I mesures with Riset set at 2.7kΩVolt R G B W4,6 738 747 738 7404,5 738 747 738 7404,4 738 745 738 7364,3 738 742 731 7324,2 738 738 707 7254,1 738 729 666 7254,0 738 687 620 6803,9 738 642 574 6273,8 738 600 527 5733,7 738 561 486 5253,6 738 518 440 4723,5 737 475 394 4203,4 733 432 348 3653,3 718 395 309 318Since red led has lower foward voltage than the other colors it seems to coroborate a voltage drop around 1.2v trougth the CN5711So maybe it is a normal behavour, and I just need to live with it !If anyone has a suggestion for a replacement of the CN5711 it would be greatly apreciated !

author: LinuxHata
author_rank: Frequent Contributor
timestamp: January 01, 2024, 08:42:41 am
content: ONSEMI has the similarly featured chips (in fact, CN5711 is clone of one of them) and I have used them for high power RGBW led control, but I was using 12VDC supply, so really do not care that much about voltage drop.