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AB20-3 Datasheet(PDF) 10 Page - Lumileds Lighting Company |
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AB20-3 Datasheet(HTML) 10 Page - Lumileds Lighting Company |
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10 / 37 page  10 The number of LED emitters per string also affects the total supply current. For a fixed number of LED emitters longer string lengths result in fewer total strings and thus a lower total supply current. Figure 3.3 shows the total supply current for the series string configurations in Figure 3.2. Figure 3.3 assumes a total of 60 LED emitters, thus 2 LED strings would require 30 strings, 3 LED strings would require 20 strings, etc. Note that the total supply current is much higher for series strings with 2 and 3 emitters. As shown by Figures 3.2 and 3.3, the choice of the number of emitters per string is a tradeoff between the regulation of forward current due to input voltage variations and the total supply current for the LED array. Small string lengths give excellent forward current regulation, but require a higher supply current. Long string lengths provide poor forward current regulation but require less supply current. For these reasons, most 12 V resistive limited designs use three or four LED emitters per series string (y = 3 or 4). Variations in the forward voltage characteristics of the individual LED emitters can lead to forward current variations within the LED array. These forward current variations directly affect the luminous flux output of each LED emitter and can cause noticeable luminous intensity variations within the LED array. These random variations in forward voltage characteristics affect the three circuits shown in Figure 3.1 differently. The “series string” circuit shown in Figure 3.1a is least affected by random forward voltage variations between the LED emitters because the forward voltages of all LED emitters in a given string are averaged together. In many cases, one emitter with a high forward voltage can cancel out another emitter in the same series string with a low forward voltage. In addition, the voltage drop across the current limiting resistor, R, is much higher than the combined voltage drops across the series resistors, yR S. Thus, small variations in R S only cause small variations in the forward current through the series string. Because SuperFlux and SnapLED 70 emitters are categorized for forward voltage at 70 mA (150 mA for the SnapLED 150), the smallest Figure 3.2 Forward Current Through String of HPWA-xHOO LED Emitters versus Applied Voltage. Figure 3.3 Total Supply Current versus Supply Voltage for Sixty HPWA-xHOO Emitter LED Signal Lamp Array Driven in Series Strings with Two, Three, Four, Five, or Six Emitters per String. |
Similar Part No. - AB20-3 |
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Similar Description - AB20-3 |
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