# Can an LED achieve full brightness in 40 µs?

How fast can an LED achieve it's full brightness at it's rated voltage and current?

I need to do multiplexing of LEDs to make a matrix display and I've calculated that each LED can only stay on for 40µs. I don't know if that is enough time for the LED light to be seen however.

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How many LEDs are you lighting? What are you driving it with? –  CMP Dec 28 '11 at 22:12
I am driving 1000 leds with a micro controller –  user3045 Dec 29 '11 at 13:21

(1) LED on times for phosphor LEDs are in the 100'2 of nanoseond range

(2) Turn on times for non-phosphor LEDs are typically in 10's of nanosecond range if driven correctly.

Average current = Peak_Current x time_on / ( time_on + time_off )
Peak current is assumed to be "steady".

(3) Brightness when mutiplexed

     = B_DC x time_on / ( time_on + time_off ) x k


Where B_DC is the brightness when the LED is operated at this PEAK current when DC is used and k = a factor relating to loss of efficiency with current, change of efficincy with die temperature etc.
Initially k=1 is close enough.

or Brightness using average current =

     = B_100% x k     at average current


(4) Modern phosphor LEDs have an allowable peak current 20% to 100% higher than the rated DC current.
ie you cannot usefully multiplex modern phosphor LEDs directly.

(5) SOME modern LEDs MAY allow higher peak/rated current ratios but
you should check the data sheet in EVERY case.

(6) There is a way to multiplex LEDs to allow high peak multiplex currents when the actual LEDS have low allowable peak/rated current ratios.
It takes more circuitry and/or design effort. Few people do this AFAIK

There are various possible implementations but the basic method is to multiplex power (LED drive) to an energy store and then drive the LED from the energy store in such a way that LED current is about constant.

An "energy store" can be a capacitor or an inductor, plus supporting circuitry.

(a) Multiplex into capacitor across LED directly. Input desired average current. LED will stabilise at appropriate voltage for the average current. Energy is lost in the driver due to unavoidable I^R loss.
Capacitor must be large enough to prevent LED current rising above rated value during recharge pulses.
The capacitor increases the turn off time to at least a few multiplex cycle periods and probably 5 to 10 multiplex cycle periods, and maybe much longer at very high multiplex ratios. Turn on time is under the control of the designed but will also usually be slowed to several mutiplex cycle times.

(b) Multiplex into eg inductor in series with LED to ground. Reverse diode from input to ground. This is effectively a buck converter.

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Turn off time for phosphor LEDs appears to be significantly longer than turn on time, regardless of the driver circuitry. Not sure what causes this afterglow. –  morten Oct 29 '12 at 22:33

It does not matter only how long you switch LED on but what is the duty cycle ie. how long it is switched on compared on how long it is switched off. Exact LED response time depends on LED type (color) but these are usually in tens or hundreds of nanoseconds. Your 40µs is more than enough to fully lit it but the average visible light depends on how long it is switched off after that.

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The visibility will be a function of what percentage of the time is the led lit. If I understand correctly, the LED will light essentially instantaneously as current is supplied, so the time to light it is the rising edge time of whatever you are using to drive it. As far as brightness and visibility go, it really depends on how many 40µs bursts each led is getting in a second. As you get around 100Hz or so per led it looks pretty much solid. Some people will say that more or less is necessary, so try it for yourself.

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It will get approx 25 bursts per second; corresponding to persistence of vision. –  user3045 Dec 29 '11 at 13:28

The LED risetime is insignificant compared to delays in drivers and effects of inductance (either actual risetime or the need to actively limit risetime to avoid ringing) on long wires/traces in larger arrays.

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Crap, forgot about inductance. +1 for that. –  user3045 Dec 29 '11 at 13:30

Yes. Most LED's can achieve full brightness in less than 40 uS.

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