Finding IC LED Driver Non-PWN with External MOSFET - Page 1
Forum section: electronics / beginners

author: kandrey89
author_rank: Contributor
timestamp: April 17, 2020, 07:51:05 pm
content: Hi Folks,I've been searching for a bit on digikey and vendor sites but have had trouble finding the right IC driver.The application is a scientific LED controller where a high voltage supply is used, linear mode driving (the power supply is linear, not switching), the voltage drop to get to the right forward voltage happens on the MOSFET (high heat dissipation and that's OK), a sense resistor provides current measurement to an analog feedback which is controlled by a microprocessor through a DAC to achieve a desired current through the LED.It's important to use a linear mode as opposed to PWN LED dimming because it's intolerable to have flickering light (has nothing to do with whether or not a human can see it), light must be absolutely DC for scientific reasons.I've been trying to find an IC that could drive a large external MOSFET, have a feedback loop with the sense resistor and accept a voltage range on a pin to change the current through a linearly driven LED but have only found ICs with internal MOSFET and/or PWN control to change the "current" and/or fixed LED current w/PWN.Has anyone encountered such an IC? I'd rather not build this IC out of opamps.Thank you

author: langwadt
author_rank: Super Contributor
timestamp: April 17, 2020, 09:15:18 pm
content: Quote from: kandrey89 on April 17, 2020, 07:51:05 pmwhat's wrong with an opamp? and why do you need current control if a microprossor is already doing that?

author: thm_w
author_rank: Super Contributor
timestamp: April 17, 2020, 10:06:49 pm
content: Don't look for LED drivers, as your circuit is not specific to LEDs. What you want is an electronic load, adjusted for current through the mosfet/sense resistor.As langwadt says, use an opamp to achieve this.Many examples online and on this forum.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 18, 2020, 05:19:24 am
content: Give us some informations like: topology and supplies(LED connecteds to +10kV and is driven at 1 A, with a drop of 1.6V, uC supplied at 1.1V, DAC from 1.8V) - you know, the SMALL details...As result, I'm pretty sure, that you will have more useful help than you will ever need...

author: kandrey89
author_rank: Contributor
timestamp: April 18, 2020, 05:54:44 am
content: Zoli: thanks, let me try to address below.thm_w: hmm interesting, never thought of it that way but perhaps.The LED controller I'd like to build would ideally be powered by an off the shelf AC-DC brick, which only come up to 56V max, and I'd like to have up to 4A range in 0.25mA steps or so, ie 14 bit or 16bit. However, I'm still thinking about whether it'd be worth it to plugin an IC that would sort of buck-boost the voltage to the required level and filter the shit out of it before supplying it to the LED and mosfet, bypassing the need for multiple power bricks with different voltages.The uC is not selected yet.LED string can be anywhere from 3V to 54V drop, the power supply will be swapped out accordingly to minimize heat dissipation, for example for a 3.8V LED use a 12V supply (or whatever the Vmin input allows), for a 30V LED use a 36V supply, etc.Topology? Not sure, Linear, all the excess voltage is dropped on the mosfet.Doesn't really matter what the uC is or the DAC voltage, the important part is getting regulation to work properly.In addition, the MOSFET and accompanying driving circuitry must be able to perform up to 10KHz PWM based off external trigger, signal generator. This is not to dim the light, it's to time and strobe the light onto a detector.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 18, 2020, 04:40:11 pm
content: Your problem has to be split in two:Power supplyLED driver1. Power supply:From the typical AC-DC brick you can expect a wideband noise in the 100mVRMSballpark; pay attention to that detail, when you think about the 0.25mA steps in the drive current.To take care of the noise, buy/build a lab supply; take a look to this tread(no affiliation):https://www.eevblog.com/forum/crowd-funded-projects/eez-bench-box-3-sequel-to-eez-h24005/The classic source of inspiration for low noise PS isLinear Technologies appnote 118.2. LED driver:Use the classic opamp-FET combination, but choose the parts carefully: to find an opamp which can drive a medium powered MOS-FET reasonable at 10kHz will take some effort;take a look atLM8261, but I think his offset will exclude it. The MOS-FET part: consider the SOA for the maximum possible dissipation; look at theIXYSparts, but most of them will be overkill..Conclusion: Your project is a custom design from top to bottom: programmable low noise power supply, medium-high speed analog LED drive.Recommandations: reasonable low noise programmmable lab supplies can be bought for development; buy(an used) one and focus on the LED driver and controller part. Once you've get that right, move on the PSU part.