Lossless LED current limiting - what would it take? - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: May 31, 2012, 12:50:54 am
content: What aboutthis?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 31, 2012, 07:20:26 am
content: I can't see that, but what you need is a DC-DC converter with current feedback instead of voltage. There are a number of chips designed for exactly this purpose.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 31, 2012, 07:24:57 am
content: You need a buck regulator with a low reference voltage (typically 0.2v) , so the loss across the current-sense resistor is low. There are many devices specifically designed for this to drive higher power LEDs.For low power LEDs, it's harder to achieve as the quiescent current draw of a switching regulator canbe come significant.

author: hlavac
author_rank: Frequent Contributor
timestamp: June 02, 2012, 12:09:01 am
content: For those that don't see the Java simulation:I proposed driving LED from microcontroller output directly thru an inductor, with a waveform like this:_______ _ _ _ _ _ _ _ _ _ _ __| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_________First part is ramp-up, calculated for the current thru inductor to rise from zero to the desired LED current.Second phase is "wiggling" around the desired point, trying to stay about the same.Since the current will drift up or down from the desired value based on the PWM ratio and LED characteristics, there is the third phase of ramp down to zero to allow open loop operation by allowing the next cycle to start from a known position (zero current).The only challenge here is to find an inductor that is cheap and small enough to replace a resistor yet with high enough inductance to keep the wiggling frequency low enough to be practical. For a 10uH/1R inductor in the 1206 package i have seen and simulated, the required frequency is still too high to be practical (~20-25MHz).Has anyone seen better inductor available?

author: hlavac
author_rank: Frequent Contributor
timestamp: June 02, 2012, 01:33:40 am
content: Found a 33uH/1.05R 1206 inductor, that brings us to a usable 8MHz wiggling frequency. The PWM ratio required to stay level seems to be V(LED)/V(drive) here. For a 1,8V drop LED driven by 5V high the PWM should be 0,36. That is pretty close to 3/8, so a PWM driven with 64MHz clock like the one in ATtiny25 could do it!At the cost of little efficiency, a small resistor could be added in series to round up the voltage drop precisely to some easier to do ratio, like 50%...So here is my little super low power efficient design challenge:Flash alternately two identical LEDs with identical brightness with a microcontroller for 3seconds each (enough time to measure current), one with standard current limiting resistor, other with this silly inductor/PWM scheme, and measure supply current needed. How much lower power consumption can you get while preserving the same brightness? Does the microcontroller eat up more power due to complexity than is saved by not having the power loss at the current limiting resistor?This is intended to give Dave inspiration on how to drive the 6 7segment LED displays on the USB power supply efficiently so more power is left for the supply itself.But he will probably blast the full voltage thru the LEDs in a PWM like scheme without any current limiting resistors I bet to make us forget about the USB power negotiationCrazy aussie...

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2012, 09:00:52 am
content: I think you need to lower the switching frequency. At 8MHz, the switching losses would be too high, unless you implement a resonant converter with zero voltage switching.