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NE587 Datasheet(PDF) 7 Page - NXP Semiconductors |
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NE587 Datasheet(HTML) 7 Page - NXP Semiconductors |
7 / 9 page Philips Semiconductors Linear Products Product specification NE587 LED decoder/driver August 31, 1994 536 However, a major portion of this power dissipation (PD MAX) is because the current source output is operating with 3.25V across it. In practice, the outputs operate satisfactorily down to 0.5V, and so the extra voltage may be dropped external to the integrated circuit. Suppose the worst-case VCC/VS supply is 4.75 to 5.25V, and that the maximum VE for the LED display is 2.25V. Only 2.75V is required to keep the display active, and hence 2.0V may be dropped externally with a resistor from VCC to VS. The value of this resistor is calculated by: R S + 2.0 7 x I SEG [ 10 W ( 1 2 W rating) assuming worst case ISEG of 30mA. Hence now PD MAX = VCC × ICC + (VS - VV - RX × 7 × ISEG) × 7 × ISEG × KDC = 5.25 × 50 + 1.25 × 7 × 30mW = 525mW and PD av = 5.0 × 30 + 1.25 × 5 × 25 = 306 mW. If a diode (or 2) is used to reduce voltage to the display, then the voltage appearing across the display driver will be independent of the number of “ON” segments and will be equal to VS - VF - nVd, VD ≈ 0.8V Where n is the number of diodes used, power dissipation can be calculated in a similar manner. In a multiplexed display system, the voltage drop across the digit driver must also be considered in computing device power dissipation. It may even be an advantage to use a digit driver which drops an appreciable voltage, rather than the saturating PNP transistors shown in Figure 9. For example a Darlington PNP or NPN emitter-follower may be preferable. Figure 8 shows the NE591 as the digit driver in a multiplexed display system. The NE591 output drops about 1.8V which means that the power dissipation is evenly distributed between the two integrated circuits. Where VS and VCC are two different supplies, the VS supply may be optimized for minimum system power dissipation and/or cost. Clearly, good regulation in the VS supply is totally unnecessary, and so this supply can be made much cheaper than the regulated 5V supply used in the rest of the system. In fact, a simple unsmoothed full-wave rectified sine wave works extremely well if a slight loss in brightness can be tolerated. A transformer voltage of about 3-4.5VRMS works well in most LED display systems. Waveforms are shown below: VS ISEG The duty cycle for this system depends upon VS, VF and the output characteristics of the display driver. With VS = 4.9V peak VF = 2.0V The duty cycle is approximately 60%. |
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