Driving the ACULED® VHL™ Introduction ® TM Excelitas’ new ACULED VHL , with its superior four-chip design and smallest footprint, gives customers the most flexible multi-chip LED on the market. The product family contains various products from UV via VIS to IR with a variety of chip configurations, including sensors and thermistors. Excelitas’ ACULED® DYOTM even enables customers to put together their own configuration.
Table of Contents General Remarks and Construction of the ACULED VHL 3 The Electrical Principle of LEDs 4 Connecting the ACULED 6 Influence of Current 9 Pulse Width Modulation 11 Influence of Temperature 12 Using an NTC with the ACULED DYO 16 Using a Photo Diode with the ACULED DYO 17 ESD Protection 21 Standard Drivers for the ACULED 22 Symbols and Units 25 Abbreviations 27 Notes 29 www.excelitas.
General Remarks and Construction of the ACULED VHL The ACULED VHL board is based on an insulated metal core substrate (IMS) made from copper and a highly sophisticated isolation material with a low thermal resistance between the copper and chip pads. This package provides excellent heat dissipation and thermal management from the chip to the substrate’s backside. The thermal resistance Rth JB of the entire package is quite low, depending on the chip configuration.
The Electrical Principle of LEDs LED dies are semiconductors and hence different in their electrical characteristics from conductors such as incandescent light bulbs. In the latter the electrical resistance R will increase with rising temperature and thus with increasing electrical current, which gives a kind of limitation of the electrical current flow. The direction of the current usually does not matter.
such as the ACULED must never be used directly to a voltage source like batteries or voltage supply source. Calculation of current limiting resistor The resistance R of a circuit as shown in figure 3 is given by Ohm’s law R = V I (1) where I is the current in the circuit that is equal to the forward current IF that we want to adjust, and R is the overall resistance of the circuit given by R = RRL + RLED (2) with the internal LED’s resistance RLED and the wanted current limiting series resistor RRL.
Connecting the ACULED The electrical circuit of a specific ACULED VHL can be found in the specific datasheet. Figure 1 shows the layout with the chip pads called Cn and the eight electrical connections labelled Pin n. The assignment from the chip pads to the pins can be found in the ACULED datasheet for the ACULED VHL and in table 1 for the ACULED DYO. Figure 4 is showing an example that represents most of the visible ACULED VHLs.
forward voltages VF of the chips on the one hand and the dependency of the temperature and other parameters of the dies from the current IF on the other hand. Please note the basic rules for parallel and serial connections: • • In parallel circuits, the voltage is the same in each track, whereas the current flow is divided according to the power consumption of the components in the tracks.
Figure 6 Left: Circuit of parallel connection of the ACULED VHL chips, which is highly discouraged. Right: Circuit with common anode. Common anode / cathode If the number of connections is limited and the ACULED is not cascaded with other ACULEDs (see next section), a common anode or cathode can be used. Figure 6, right side shows the diagram of such a circuit. Though each channel can still be driven individually, only five instead of eight connections have to be used.
depends on the particular circuit. Remember that voltages have to be added in serial circuits and remain the same in parallel connection, whereas currents have to be calculated vice versa. Figure 8 Circuit diagram of cascaded ACULEDs. The corresponding chips of each color are connected serially.
• • flux [Φ e and Φ V] wavelength [λ] resp. color [x2° / y2°] and color temperature (TCT). The influence of the current on these parameters is often similar to the impact caused by temperature which we will see later. Influence on lifetime Overdriving an LED chip, i.e. exceeding its forward current IF over the allowable maximum, will damage the chips within a short time.
show that driving the ACULED by changing the current is not a suitable solution. A much better way is to use pulse width modulation which will be explained in the next chapter. Figure 11 Typical wavelength vs. current characteristic of an LED shown by the example of the ACULED VHL RGYB at 25°C board temperature. The color of the curves responds to the emitted color of the chips.
Excelitas ’ LED drivers utilize PWM for a safe and stable operation of the ACULED VHL and DYO products. In the next chapter we will learn about the influence of the temperature. Due to the temperature effects on the one hand and non-perfect slope of the pulses on the other, the dependency of the flux according to relation 7 differs in practice as shown in figure 13. Figure 12 current Principle of PWM: Square wave pulses are modulated in their duty cycle.
driving the ACULED. The parameters influenced by temperature are quite similar to those impacted by current, particularly • • • • • lifetime [tLife] forward voltage [VF] maximum forward current [IF max] flux [Φ e and Φ V] wavelength [λ] resp. color [x2°/ y2°] and color temperature (TCT). A big advantage of the ACULED VHL and DYO versus other similar products on the market is minimized thermal crosstalk between the pads.
Figure 14 Relative Forward Voltage Relative forward voltage VF versus ACULED VHL board temperature TB for red, green and blue chips. 1,06 1,04 VVV F / VF (25°C) 1,02 1 0,98 0,96 0,94 0,92 10 20 30 40 50 60 70 80 T B [°C] Influence on flux and intensity The flux, Φe and ΦV , and their deducted values, such as luminance, radiance, luminous intensity or radiant intensity, decreases with increasing temperature.
Figure 15 Relative Luminous Flux = f(T B) Change of relative luminous flux ΦV vs. board temperature TB for the RGYB chips of the ACULED VHL. 140,00 130,00 120,00 in % 90,00 / 100,00 25°C 110,00 80,00 70,00 60,00 50,00 40,00 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 T B [°C] Besides a changing mixed color ratio due to the different intensity changes, each chip also changes color as a result of wavelength drift caused by temperature.
Figure 16 Peak wavelength λ peak = f(T B) Change of peak wavelength λpeak vs. substrate temperature TB for the IR ACULED VHL. 875,0 870,0 peak [nm] 865,0 860,0 855,0 850,0 845,0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 T B [°C] Using an NTC with the ACULED DYO The safest way of controlling the heat is to measure it. As shown in the previous chapter, we can do this by controlling TB since we can calculate the corresponding junction temperature TJ of the used chips.
you calibrate the NTC used with your ACULED in your specific application. To achieve accurate measurements of the resistance, a four-terminal sensing circuitry (4T) is the best choice. 20 Figure 17 18 Typical curve of resistance R vs. temperature T of an NTC: 16 Left: typical SMD NTCs. 14 Right: ACULED chip level NTC in the typical ACULED TB range for different currents IF of the chips (RGBN-ACULED DYO).
Figure 18 shows the principle of how to drive the PD with the ACULED. The standard PD that is used with the ACULED DYO is a silicon-based PIN-type photo diode and thus should be driven at a reverse bias voltage of VR = -5 V. This voltage has nearly no influence on the photo current but on the bandwidth in fast transient applications. Please note that the cathode of the PD is on the even numbered pads whereas with most of the LED chips the cathode is on the ACULED’s odd pads.
The output voltage is proportional to the photo current and thus to the irradiance Ee or the illuminance EV at a given spectral distribution of the radiation or light reaching the photo diode; an example is given in table 4. Since the sensitivity of a PD depends on the wavelength as shown in figure 21, the resistance RN has to be adjusted according to the amount of light and its spectral distribution.
PD measurement to the LED drive (PWM) signal to know the amount of light coming from the ambient and ambient plus LED. By signal processing you can easily subtract the environmental influence from your PD signal to get the pure LED light. Illuminance EV [lx] Table 4 Photo Current IPh [µA] 500 -4 1000 -8 2000 - 16 Photo current of PD @ VR = - 5 V induced only by ambient light of a certain illuminance at the ACULED. A CAL-2000 Mercury Argon calibration light source was used for the illumination.
ESD Protection We have learned before that the ACULED is a highly sophisticated semiconductor product which is sensitive to electro-static discharge (ESD). Though all LED chips are generally sensitive to ESD, some chips are even more sensitive than others. Therefore Excelitas protects the most sensitive chips with ESD protection diodes up to 2 kV according to the circuit diagram in figure 23. Please refer to the specific datasheet to check whether your ACULED VHL has ESD protection or not.
voltage of the PSU - the current flows over the Z-diode but not the LED chip. Please note that the Z diodes can withstand high power for a short time period only. This is why they do not always prevent the ACULED from electrical damage. Standard Drivers for the ACULED Excelitas provides different standard four-channel drivers and power supply units (PSU) for the ACULED.
Parameter PSU-ACL-01-350 PSU-ACL-01-700 Current per channel 350 mA 700 mA Total power 16 W 38 W Power per channel 4W 9.5 W 11.4 V 13.5 V 5 (Red - IR) 4 (Yellow - Amber) 3 (UV - Green) 5 (Red - IR) 4 (Yellow - Amber) 3 (UV - Green) Max. voltage per channel Max. LED per channel Table 6 Overview over ACULED standard PSUs. Four-Channel DMX PSU LED driver According to the DMX-512 light communication protocol Excelitas provides DMX PSU to drive the ACULED.
Figure 26 ACULED Designer Kit for easy testing of ACULEDs. Pin 1 2 3 4 Chip position C4 C3 5 6 7 8 C2 C1 www.excelitas.com PSU channel VIS ACULED VHL Channel 3 - PSU channel IR ACULED VHL Channel 4 + Channel 3 + Channel 4 - Channel 2 - Channel 1 + Channel 2 + Channel 1 - Channel 1 - Channel 2 + Channel 1 + Channel 4 - Channel 2 Channel 3 + Channel 4 + Channel 3 - Table 7 Assignment of chip positions to the channels of the PSU as used with the ACULED Designer Kits.
Symbols and Units The following terms and their typical units are used in the application notes and datasheets of the ACULED. Please note that not all of these are used in this particular note.
Ra [-] CRI (average value of testing colors R1 to R8) RLED [Ω] (internal) LED resistance RNTC [Ω] NTC resistance (function of T) RPD [Ω] series resistance of PD RRL [Ω] current limiting resistance Rth [K/W] thermal resistance (general) [Kelvin per Watt] Rth BA [K/W] thermal resistance from base (B) backside to ambient surrounding (A) Rth JA [K/W] thermal resistance from junction (J) to ambient air or surrounding (A) Rth JB [K/W] thermal resistance from junction (J) to base (B) back
VR [V] reverse voltage VRL [V] voltage at current limiting resistor VS [V] voltage of (constant voltage) source or battery VZ [V] Zener or break through voltage of Z-diode xn° [-] x coordinate in CIE color space for n-degree observer (usually n = 2 is used with light sources like LEDs: x2°) yn° [-] y coordinate in CIE color space for n-degree observer (usually n = 2 is used with light sources like LEDs: y2°) 2ψ [°] viewing angle (usually at half of maximum intensity) Abbreviations The f
PIN diode Positive Intrinsic Negative diode; diode with undoped intrinsic semiconductor between positive (p) and negative (n) regions PMMA Polymethyl methacrylate, transparent thermoplastic; in optical grade used for lenses pn junction Layer in the LED chip, where positive (p) and negative (n) charged carriers recombine to light respectively radiation. PPA Polyphtalamide (plastic) PSU Power Supply Unit PT100 Thermistor made from platin with 100 Ω at 0 °C.
Notes 1. 2. 3. 4. 5. 6. 7. 8. 9. Excelitas maintains a tolerance of ± 5% on flux and power measurements. Excelitas maintains a tolerance of ± 2 nm for dominant wavelength measurements. Excelitas maintains a tolerance of ± 1 nm for peak wavelength measurements. Excelitas maintains a tolerance of ± 2 K/W for thermal resistance measurements depending on chip properties.
