Operational considerations for LED lamps and display devices

A thorough understanding of how to interpret datasheet information is the basis for achieving the optimum drive circuit design for an LED lamp, LED light bar, or LED seven-segment display. This article presents an in-depth discussion on the use of the optical and electrical information contained in LED device datasheets.

 

Typical datasheet information

For LED lamps, the first table in the datasheet is usually the selection guide. This presents information about the typical and miminum axial luminous intensity (Iv) and the viewing angle.

The next table contains the absolute maximum ratings at an ambient temperature (T_A) of 25°C. These maximum ratings are typically the peak, DC and average currents; transient current primarily; operating and storage temperature ranges; and the absolute maximum LED junction temperature.

The third table, contains the electrical and some optical characteristics, at an ambient temperature of 25°C, which are used to determine the operating conditions for the device. The forward voltage (V_F) and device thermal resistance (Rθ_J-PIN), used in the calculations for the operating condition, are listed in this table.

The following graphs are also included in lamp datasheets and are used to determine the operational conditions:

• Relative intensity vs. wavelength.
• Forward current vs. forward voltage. (see Figure 1)
• Relative luminous intensity vs. DC forward current. (see Figure 2)
• Relative efficiency vs. peak current. (see Figure 3)
• Maximum forward DC current vs. ambient temperature. (see Figure 4)
• Maximum average current vs. peak forward current. (see Figure 5)
• Relative luminous intensity vs. angular displacement. (Not shown here)

 

 

Design criteria

The two criteria that establish the operating limits are the maximum drive currents and the absolute maximum LED junction temperature, T_JMAX. The maximum drive currents have been established to ensure long operating life. The absolute maximum LED junction temperature is a device package limitation that must not be exceeded.

 

Thermal resistance

The LED junction temperature (T_J), in degrees Centigrade, is the sum of the ambient temperature, (T_A) and the temperature rise of the LED junction above ambient (ΔT_J) which is the product of the power dissipated within the LED junction, (P_D) in Watts (W), and the thermal resistance LED junction-to-ambient, (Rθ_J-A) as °C/W.

The cathode leads (pins) of a typical LED device are the primary thermal paths for heat dissipation from the LED junction to the surrounding environment. The exceptions are TS AlGaAs lamps, that use flip-chip technology (anode die attach), where the anode lead is the primary thermal path. The datasheet lists the thermal resistance of the LED junction-to-pin, (Rθ_J-PIN) for each device. This device thermal resistance is added to the PCB mounting assembly thermal resistance-to-ambient
(Rθ_PC-A) to obtain the overall thermal resistance LED-junction-to-ambient (Rθ_J-A) in °C/W.

Rθ_J-A is on a per LED chip basis for lamps, light bars, and sevensegment displays, and on a per device basis for displays with onboard ICs. For reliable operation, it is recommended that the value of Rθ_PC-A be designed low enough to achieve the lowest possible Rθ_J-A to ensure that the LED junction temperature does not exceed the absolute maximum value when the device is operated in the maximum surrounding ambient temperature.

 

Maximum power calculation

The maximum allowed power that may be dissipated within an LED junction (P_MAX) is determined by multiplying the maximum rated DC current by the forward voltage for that current, determined from Figure 1.

 

Derating vs. temperature

The drive current derating vs. temperature, Figure 4, is a function of drive current, T_JMAX, and Rθ_J-A. Typically derating curves are given from two ambient temperatures: 50°C (solid line) and 70°C (dashed line). The derating curves are lines of T_JMAX with slopes equal to the specific maximum Rθ_J-A values indicated, intersecting the temperature axis at the maximum LED junction temperature point with zero current. Operation of the LED device at a particular drive current should be at or below a derating curve with a thermal resistance-toambient at, or less than, the maximum value indicated for that curve.

 

Current limiting

An LED is a current-operated device and, therefore, requires some kind of current limiting incorporated into the drive circuit.

This typically takes the form of a current limiter resistor (R) placed in series with the LED. The forward voltage characteristic of Figure 1 is used to calculate the value of the series current limiter resistor.

Where:

• V_CC = power supply voltage.
• V_SAT = saturation voltage of driver transistor(s).
• V_F = forward voltage of the LED at I_PEAK.
• I_PEAK = the peak drive current through the LED.

 

Light output

The luminous intensity at T_A = 25°C for a particular DC drive condition is determined using the relative luminous intensity factor from Figure 2.

• I_V (dc) = [I_V (25°C)] [relative intensity factor]

Where:

• I_V (25°C) is obtained from the datasheet.

For pulsed drive conditions, the time average luminous intensity is determined from the relative efficiency characteristic (ή_V) presented in Figure 3. Not all datasheets, however, include relative efficiency data.

• I_V (time average) = [I_V (25°C)] [I_AVG/I_F] [ή_V]

Where:

• IV (25°C) = datasheet luminous intensity value.
• I_AVG = the time average operating current.
• I_F = the current where the datasheet luminous intensity is specified.
• ή_V = relative efficiency factor for the peak drive current (I_PEAK).

The calculated luminous intensity value at TA = 25°C can be adjusted for a different operating ambient temperature by the following exponential equation, and using the k factor for the specific LED.

• I_V (T_A) = I_V (25°C)e^{[k(T_A - 25°C)]}

 

 

Pulsed operation vs. DC operation

When operating an LED device under DC drive conditions, the LED junction temperature is a linear function of the DC power dissipation multiplied by Rθ_J-A. The light output is proportional to the DC drive current by the luminous intensity factor of Figure 2 and as expressed in the above equation for luminous intensity at T_A = 25°C.

For best pulsed operation and overall lightoutput performance, a rectangular current waveform with a refresh rate equal to, or greater than, 100Hz is strongly recommended. Sinusoidal waveforms are not generally recommended, as the Root Mean Square (RMS) power will exceed that of a rectangular current waveform with the same peak current value. If a sinusoidal current waveform is used, the peak current should not exceed the maximum DC current rating. Sinusoidal waveforms produce less than twothirds of the light output of an equivalent rectangular pulse, and at 50Hz or 60Hz, are not fast enough to prevent observable flicker.

When operating an LED device in pulsed current mode, it is the peak junction temperature, not the average junction temperature, that governs the performance of the device. At refresh rates below 1000Hz (the number of times per second a device is pulsed), the peak junction temperature is higher than the average junction temperature. As a result, the allowed time average currents for refresh rates between 100Hz and 1000Hz are less than those permitted at 1000Hz, as can be seen by the 100Hz and 300Hz curves of Figure 5.

 

 

Design steps

In order to determine the derated drive conditions from the datasheet for an elevated ambient temperature, the value for Rθ_J-A must be determined. The required current derating can then be determined for safe operation at the elevated temperature directly from Figure 4. The basic design steps are:

1. Determine Rθ_J-A.
2. Calculate the required value for Rθ_PC-A for the PCB-mounting configuration.
3. Determine the maximum allowable DC drive current for the operating ambient temperature.
4. Calculate the LED-chip power dissipation to be sure it will not cause T_J to exceed the absolute maximum value.
5. Calculate the value of the current limiting resistor.
6. Determine the luminous intensity at 25°C and at the elevated ambient temperature.