Tag: forward voltage

Variations in Vf and “binning”

LTST-C170TBKT SMD blue LED
LTST-C170TBKT SMD blue LED.

LEDs’ forward voltage drop varies from device to device and by more than you might think. This causes a few issues for the user.

The Lite-On LTST-C170TBKT InGaN blue LED datasheet, for example, shows the forward voltage varies between 2.8 V and 3.8 V at 20 mA. This is quite a variation. The lower value would, just about, allow operation with a low value series resistor at 3.3 V whereas the 3.8 V sample might not turn on very brightly. On higher voltages control of the current by series resistor would vary for different LEDs. (See Simple constant-current driver for a work-around for this.)

The problem is likely to be caused by difficulty in controlling part of the manufacturing process.

LTST-C170TBKT-Vf-variation.
LTST-C170TBKT-Vf-variation is rather wide.

Looking at the datasheet we can see that the forward voltage isn’t the only thing affected.

LTST-C170TBKT electrical and optical characteristics.
Table 1. LTST-C170TBKT electrical and optical characteristics.

From Table 1 above we can see that the luminous intensity varies from 28 mcd to 180 mcd at 20 mA. This is more than a factor of six in variation! If a random selection of these parts is connected in series the brightness could vary wildly despite all being fed with the same current.

The solution adopted by the manufacturer is “binning”. This means that the LEDs are tested and graded after manufacture and separated into various “bins” depending on one of the parameters.

LTST-C170TBKT LED bin code list.
LTST-C170TBKT LED bin code list.

In this case the customer has the choice of selecting parts graded by forward voltage, luminous intensity or dominant wavelength. There is no hint about what the relationship is, for example, between various luminous intensities and the forward voltage. The customer is expected to decide what is important for their application – even brightness, forward voltage or dominant wavelength – and order accordingly. The customer will pay a premium for binned LEDs.

When fed by simple resistor current limiters the current through each LED will vary inversely with \(V_f\) as there will be more voltage dropped across the LED and less across the resistor. (With lower V there will be lower I.)
This issue can be solved with a constant current power supply.

‘Resistance’ of an LED

LEDs do not have a linear relationship between current and voltage so they cannot be modeled as simply as a resistor using Ohm’s Law, \( V = IR \). We can, however, make a simplification and model them over a range of currents as a combination of a resistor and a voltage source.

LED IV slope
Figure 1. An LED can be approximated as a resistor with a fixed voltage source.

If we look at a typical LED IV curve we can see that it is approximately linear over much of its useful range. This allows us to model the LED as a resistor and voltage source.

LED equivalent circuit model.
Figure 2. LED equivalent circuit model.

In Figure 1 the grey line is reasonably close to the LED curve from 20 mA to 100 mA. We can work out the resistance that this represents from Ohm’s law \( V = IR \) but in this case we will look at the change in voltage and current in the area of interes.

\( R = \frac{\Delta V}{\Delta I} = \frac{3.5 – 2.0}{100m – 0} = \frac{1.5}{100m} = 15 \mathrm\Omega \).

We can also see that the line crosses the X-axis at Vf = 2.0 V. Our equivalent circuit for this region of interest is (referring to Figure 2) R1 = 15 Ω and V1 = 2.0 V.

Low current approximation.
Figure 3. For the lower end of the IV curve we can create another approximation.

If we are interested in the lower current range of the LED, 1 to 20 mA, for example, we can calculate again. In this case the ends of the grey line have differences of 1.5 V and 40 mA giving a slope of 37.5 Ω. The voltage source becomes 1.5 V.