Can an LED achieve full brightness in 40 µs?

Question

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.

Answer 1

(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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

Bare LEDs will reach the brightness for its current much faster than 40 µs. Even LEDs that re-emit light thru phosphors can handle that, like white LEDs. Bare LEDs can be eaily used for digital communcation exceeding a MHz (much more in some cases), so this is not pushing it.

However, you have a different problem. Let's say the LED is rated for 20mA, and that's the effective brightness you want it to have. You can pulse the LED on for 40 µs at 40 mA and then off for 40 µs to get close to the same brightness, but you can't keep using more current and shorter time. Each LED will also have a maximum instantaneous current spec, not just a average current spec. For some IR LEDs that are generally meant to be pulsed as part of a digital communication scheme, the steady to maximum current ratio can be as high as 10. For most ordinary LEDs its less than that. For high power lighting LEDs its only a little above 1.

Let's say that your LED has a maximum to average current ratio of 10. That means you need to figure out a multiplexing scheme so that the LED is on for at least 1/10 the time else you won't be able to run it at full brightness. In fact, the brightness as a function of current falls off a bit at high currents, so if you're pushing the limits pulsing will always be a little less than steady at the maximum current.