High Wattage white led with LM2576 & variable intensity problem - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 04, 2017, 04:38:37 pm
content: 1. I have to drive a white led. Input voltage is 6V-8.4V. I need to vary the intensity using a potentiometer.2. Using the ckt as in attached image. Not using optional output ripple filter. Ckt shows use of HV version, but I am using normal version of LM25763. Directly connected the outout to white led. Max output voltage set to 3.7V. Instead of using 1N5822, I have used SS34 diode.R1 = 5K fixedR2 = 10k(pot)Calculated using formula:Vout = Vref(1 + R2/R1)Vref = 1.23V, R1 between 1K & 5KBy fixing r1=5K, R2 value comes out to be 10KProblem:1. When I rotate the pot from one end to another, led brightness smoothly decreased.2. But when it is at max end, i.e output voltage at 3.7V, & slighlty rotate the pot in another direction, suddenly the output voltage drops to 3.05V & there is noise starts from regulator also. Which causes sudden dim in ledAnd if further move down the pot, the voltage now rises to 3.5V & after that voltage decreases smoothly.This sudden drop/noise problem only occur when I slightly drop the value from 3.7Vout.

author: DmitryL
author_rank: Regular Contributor
timestamp: January 04, 2017, 04:51:06 pm
content: In brief:* LED needs to be driven with a current, not voltage.* build a proper LED driver that supplies a given current, not a given voltage.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2017, 10:22:52 pm
content: Quote from: DmitryL on January 04, 2017, 04:51:06 pmYes. The LM2576 isn't suitable for this. I started designing a circuit using the LM2576 with a transistor to implement the current limiting, then decided it was a bad idea because: the only way to dim it is to PWM the shut down pin which might not be fast enough to eliminate flicker, the current regulation would be poor and it might be unstable.EDIT:I missed a pull-down resistor from the BJT collector (pin 4 on the LM2576) to 0V.

author: strawberry
author_rank: Super Contributor
timestamp: January 05, 2017, 12:23:34 am
content: Add bipolar tranzistor paraled it to R2 and feed load current to it.

author: mariush
author_rank: Super Contributor
timestamp: January 05, 2017, 03:32:36 am
content: LED Drivers dude...Example:LM3405AXMY/NOPB Led Driver, 1 Output, Buck (Step Down), 3V-22V input voltage , 1.6MHz switch, up to 20V/1A out, HVSSOP-8 :http://uk.farnell.com/texas-instruments/lm3405axmy-nopb/led-driver-buck-1-6mhz-hvssop/dp/2492180Can control current through feedback resistor ( recommended for at least 200mA and up), and through pwm / enable pin when below 200mAON SEMICONDUCTOR NCL30100SNT1G Led Driver, 1 Output, Buck (Step Down),6.35V-18V in, 700kHz switch,1.2A out, TSOP-6 :http://uk.farnell.com/on-semiconductor/ncl30100snt1g/ic-led-driver-buck-cntrl-6tsop/dp/1830588ON SEMICONDUCTOR NCP3065DR2G Led Driver, 1 Output, Buck, Boost, 3V-40V in, 250kHz switch,1.5A out, SOIC-8 :http://uk.farnell.com/on-semiconductor/ncp3065dr2g/ic-dc-dc-converter/dp/1652395TEXAS INSTRUMENTS TPS92512DGQT LED Driver, Buck (Step Down), 4.5V to 42V input, 557 kHz switch-freq., 40V/2.5 A out, HVSSOP-10 :http://uk.farnell.com/texas-instruments/tps92512dgqt/led-driver-buck-557khz-hvssop/dp/2471141TEXAS INSTRUMENTS TPS92691PWP LED Driver, Buck-Boost, Flyback, SEPIC, 4.5V to 65V in, 700 kHz Switch. Freq., 65V/5A out, HTSSOP-16 :http://uk.farnell.com/texas-instruments/tps92691pwp/led-driver-mutli-topology-htssop/dp/2522587etc etc ....

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2017, 08:41:31 am
content: If you must use the LM2576, then your best bet is to set the output voltage to 300mV or so above the LEDs' maximum rating and power the LEDs from a low drop-out linear current regulator.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 07, 2017, 08:04:03 am
content: At 3.7V, where led works ok, is around 300mA as checked by digital multimeter.I will place a 1ohm series resistor & check if it resolves the problem.Otherwise I will use PWM ckt.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 07, 2017, 08:19:14 am
content: As others have said, you're doing it wrong. You need to be regulating current, not voltage.Most voltage regulatorscanbe used as current regs by putting a current-sense resistor in the -ve leg of the load and connecting the reg's feedback pin to the top of this resistor, however the typical 1.2 or 0.8v reference voltage of most voltage regs makes this inefficient due to the loss in the sense resistor. purpose-designed LED drivers typically have a reference of about 0.1v to reduce losses in the sense resistpr

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2017, 06:50:16 pm
content: Quote from: mikeselectricstuff on January 07, 2017, 08:19:14 amOne way round that is to add an amplifier across the sense resistor to reduce the voltage across it but that may cause problems with oscillation. I agree, it's a bad idea, just use a proper constant current LED driver IC.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 12, 2017, 12:09:24 pm
content: THis is one circuit, I found in old local market which works fine.It works exactly fine, on varying pot value, led intensity varies accordingly.Which IC it could be:1. Pin1: Input2. Pin4: Ground3. Pin7: led output conectedIts not LM2593, since LM's 7 pin is N_SSI am making its reverse ckt right now.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 12, 2017, 03:22:23 pm
content: Its most probably is this as 6 pin is grounded in pic in my last post:http://www.digikey.com/product-detail/en/infineon-technologies/TLE4242GATMA3/TLE4242GATMA3TR-ND/4854895or it could be:http://www.digikey.com/product-detail/en/infineon-technologies/TLE4309G/TLE4309GTR-ND/2081744or it could be any other ic?

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2017, 11:19:47 pm
content: If it works fine, then why not use it?There doesn't appear to be a part number on the IC.If you want to build your own LED driver, then why not use the correct IC? All the main semiconductor manufacturers make them. Look at the links below:http://www.linear.com/products/led_driver_icshttp://www.st.com/en/power-management/led-drivers.htmlhttp://www.ti.com/lsds/ti/power-management/led-driver-overview.page

author: jazer
author_rank: Contributor
timestamp: January 14, 2017, 02:25:24 am
content: Slightly off topic, but what's the general experience with LM2576 robustness? I've killed one inadvertently shorting the output briefly. They're supposed to handle that according to the specs. Did I just get a bad counterfeit or something?