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ISD2012 Datasheet(PDF) 11 Page - OSRAM GmbH |
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ISD2012 Datasheet(HTML) 11 Page - OSRAM GmbH |
11 / 16 page ISD201X, ISD231X, ISD235X 2006-04-04 11 The small alphanumeric displays are hybrid LED and CMOS assemblies that are designed for reliable operation in commercial and industrial environments. Optimum reliability and optical performance will result when the junction temperature of the LEDs and CMOS ICs are kept as low as possible. Thermal Modeling ISD displays consist of two driver ICs and four 5 x 7 LED matrixes. A thermal model of the display is shown in Figure „Thermal Model“. It illustrates that the junction temperature of the semiconductor = junction self heating + the case temperature rise + the ambient temperature. Equation 1 shows this relationship. Thermal Model See Equation 1 below. The junction rise within the LED is the product of the thermal impedance of an individual LED (37°C/W, DF=20%, F=200 Hz), times the forward voltage, V F(LED), and forward current IF(LED), of 13 – 14.5 mA. This rise averages T J(LED)=1°C. The Table below shows the V F(LED) for the respective displays. The junction rise within the LED driver IC is the combination of the power dissipated by the IC quiescent current and the 28 row driver current sinks. The IC junction rise is given in Equation 2. A thermal resistance of 28°C/W results in a typical junction rise of 6°C. See Equation 2 below. For ease of calculations the maximum allowable electrical operat- ing condition is dependent upon the aggregate thermal resistance of the LED matrixes and the two driver ICs. All of the thermal man- agement calculations are based upon the parallel combination of these two networks which is 15°C/W. Maximum allowable power dissipation is given in Equation 3. Equation 3. For further reference see Figures „Maximum Allowable Power Dis- sipation vs. Temperature“ (page 6) and Figures from page 12 on. Key to equation symbols DF Duty factor I CC Quiescent IC current I COL Column current n Number of LEDs on in a 5 x 7 array P CASE Package power dissipation excluding LED under consideration P COL Power dissipation of a column P DISPLAY Power dissipation of the display P LED Power dissipation of a LED R qCA Thermal resistance case to ambient R qJC Thermal resistance junction to case T A Ambient temperature T J(IC) Junction temperature of an IC T J(LED) Junction temperature of a LED T J(MAX) Maximum junction temperature V CC IC voltage V COL Column voltage V F(LED) Forward voltage of LED Z qJC Thermal impedance junction to case Optical Considerations The light output of the LEDs is inversely related to the LED diode’s junction temperature as shown in „Normalized Luminous Intensity vs. Junction Temperature“ (page 12). For optimum light output, keep the thermal resistance of the socket or PC board as low as possible. Equation 1. Equation 2. Model Number VF Min. Typ. Max. ISD2010 ISD2310 1.6 1.7 2.0 ISD2011/2/3 ISD2311/2/3 ISD2351/2/3 1.9 2.2 3.0 IDDG5321 θ R 1 LED Power LED T1 IC Power Rθ2 IC T2 LED Power Rθ1 LED T1 LED Power LED θ R 1 T1 IC Power θ R 2 IC T2 LED Power R 1 θ LED T1 θ R CA P DISPLAY T JMAX () TA – RθJC RθCA + ---------------------------------------- = P DISPLAY 5V COL I COL n35 ⁄ () DF V CC I CC + = T J LED () P LED Z θJC PCASE RθJC RθCA + () T A ++ = T J LED () I COL 28 ⁄ ()V F LED () ZθJC [] n35 ⁄ ()I COL DF 5V COL () V CC I CC + [] RθJC RθCA + [] T A + ⋅ + = T JIC () P COL RθJC RθCA + () T A + = T JIC () 5V COL V F LED () – () I COL 2 ⁄ () n35 ⁄ ()DF ⋅⋅ V CC I CC ⋅ + [] RθJC RθCA + [] ⋅ T A + = |
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