how to DRIVE the LED

Basic properties: - LEDs share many of the properties of signal diodes and power rectifiers.  They have a low impedance when forward biased and very high impedance when reverse biased.  These properties are illustrated in the typical Diode Volt / Current Graph below: -

As the LED goes into Forward Bias the relationship between Vf (the voltage across the LED) and If (the current flowing through the LED) is almost linear.  Above the linear region the LED has a very low impedance, thus large amounts of current flow for very little change in Vf. 

As illustrated above, the diode has very high impedance when operated in reverse bias i.e. the diode blocks reverse current flow.  In fact the Ir curve in the above diagram is much magnified; the actual current flowing under reverse bias is A compared to mA in forward bias.  This reverse blocking occurs until the breakdown VBR at this point the diode has very low impedance and is usually destroyed by the heat caused by excessive current flow.

Reverse Bias: - Rectifier diodes are specified with relatively high VBR and are designed to be operated in reverse bias, they are often used to smooth (rectify) AC voltages in AC – DC converters.  LEDs however, have very low VBR and are not designed for rectification; in fact an LED should be protected from negative voltages.

Lifetime: - LED reliability is inversely proportional to Tj (i.e. higher Tj = lower reliability).  The graph below illustrates a typical curve of Tj vs. Lifetime.  It shows that by maintaining Tj at 25°C designer can expect to have a lifetime of 300k hours (>30 years) continuous usage before the LED output intensity will degrade by 30%.  However with a Tj of 50°C the LED will degrade 30%  intensity in 50k hour (~5 years) : -

In similar environmental conditions the junction temperature (Tj) of an LED is directly proportional to If.  In an LED design it is critical to control If.  Hence, an LED driver circuit needs to control the current flowing (If ) through the LED.

Light Output Intensity: - As illustrated below light output (intensity) of an LED is proportional to the Forward Current (If): -. 

As shown above the relative light output intensity will increase proportionally with the Forward Current (If), however the intensity vs. current curve falls off towards the upper limit of Forward Current.  It should also be noted that different colours have different properties.
Basic LED drivers: - In the simplest LED drive design the LED is driven from a constant voltage source i.e. Battery, with a current limiting resistor: -


V supply = 3v
If (forward current) = 10mA
Vf (typ) = 1.8v
R1 calculated to be 1.2 / 0.01 = 120Ω
Using an LED with: -
-Intensity (@10mA)    30mcd (Typ)
Light output intensity can be varied by: -
•    V supply e.g. increasing to 4v: - If = (4 - 1.8)/120 = 18mA = 54mcd
•    R1 e.g. reducing to 82 Ω: = If = (3 - 1.8)/68 = 17mA = 51mcd
•    Selecting a higher intensity LED i.e.  60mcd @10mA
•    Increasing the number of LEDs from the same supply

Driving multiple LEDs: -  As stated above, one method of increasing the light output is to increase the number of LEDs in the system.  One method, but not a recommended one, is as below: -

In this scheme R1 is used to supply current to all LEDs in Parallel.
The scheme is cheapest on component cost as it only uses one Resistor to drive several LEDs.  However, the main issue with this layout is matching the Vf values of the LEDs.  No two LEDs will have exactly the same Vf and any imbalance will result in an unequal current sharing and therefore uneven light output form the LED array.

A better system for driving LEDs is given below: -

In this system each LED is driven through its own current setting resistor, thus the effect of unequal Vf values is reduced.  It is also possible to adapt the Resistor values to variations in Vf and intensity values.

The optimum method of driving multiple LEDs is in series: -

This connection method is immune to any variation in Vf as all LEDs share the same current.  However, it is necessary if even illumination is required to have LEDs with the same intensity bin.

Reverse Voltage protection: - The use in the above diagram of a TVS (Transient Voltage Suppressor) has a dual purpose: -
1.    It protects the static sensitive LEDs (InGaN & GaN) devices from voltage spikes.
2.    It protects the LEDs from negative voltages i.e. being reversed biased

If static protection is not required the expensive TVS can be replace by a low-cost high speed rectifier.  The Vf of most standard rectifiers is <2v, which is well below the 5v max limit Vr of the LED.  

Dimming LEDs

In essence there are two methods of reducing the light output of an LED: -
1.    Reducing the Forward Current of the LED
2.    Switching the LED On and Off very quickly to reduce the perceived light output

These are illustrated below: -

Diagrams (1) and (2) are illustration of modifying the Forward Current (If) to change the light output.  In both, R1 is chosen to give the highest light output (Iv) and resistance is added to dim the LED.  In this way the LED is protected from over-current.
In (1) If is limited by R1 and reduced as resistance of the variable resistor is increased.  A PCB mounted trimpot can be used to adjust the brightness of individual LEDs within an array.

Diagram (2) replaces the Variable resistor in (1) by a Field Effect Transistor (FET).  The resistance of the FET (Rds(on)) is changed by an analogue voltage input.  Both of these methods suffer from non-lineararity as the LEDs starts to turn off.  This is due to the non linear relationship between Intensity (Iv) and Forward Current (If) in the Turn off  / Turn-on region.
Diagram (3) illustrates the light output of the LED being altered by the application of a digital signal to the gate of the FET in (3).  In this scheme the FET is driven fully ‘On’ and ‘Off’.  The ‘On’ value of the FET (Rds(on)) is measured in mΩ and the ‘Off’ ((Rds(off)) is ~ ∞Ω.  Driven in this way there is a direct relationship between the mark-space ratio of the driving signal, such that at a 50% mark-space ratio the light output will appear to 50% of the full on intensity.

The driving signal must be fast enough not to cause the LED to flicker, but not too fast that the LED cannot turn off.  A driving signal in the low KHz range should suffice.