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LM3916N-1 bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI) |
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LM3916N-1 bảng dữ liệu(HTML) 8 Page - National Semiconductor (TI) |
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8 / 27 page  Mode Pin Functional Description (Continued) DOT OR BAR MODE SELECTION The voltage at pin 9 is sensed by comparator C1, nominally referenced to (V + −100 mV). The chip is in bar mode when pin 9 is above this level; otherwise it’s in dot mode. The com- parator is designed so that pin 9 can be left open circuit for dot mode. Taking into account comparator gain and variation in the 100 mV reference level, pin 9 should be no more than 20 mV below V + for bar mode and more than 200 mV below V+ (or open circuit) for dot mode. In most applications, pin 9 is ei- ther open (dot mode) or tied to V + (bar mode). In bar mode, pin 9 should be connected directly to pin 3. Large currents drawn from the power supply (LED current, for example) should not share this path so that large IR drops are avoided. DOT MODE CARRY In order for display to make sense when multiple drivers are cascaded in dot mode, special circuitry has been included to shut off LED #10 of the first device when LED #1 of the sec- ond device comes on. The connection for cascading in dot mode has already been described and is depicted in Figure 1. As long as the input signal voltage is below the threshold of the second driver, LED #11 is off. Pin 9 of driver #1 thus sees effectively an open circuit so the chip is in dot mode. As soon as the input voltage reaches the threshold of LED #11, pin 9 of driver #1 is pulled an LED drop (1.5V or more) below V LED. This condition is sensed by comparator C2, refer- enced 600 mV below V LED. This forces the output of C2 low, which shuts off output transistor Q2, extinguishing LED #10. V LED is sensed via the 20k resistor connected to pin 11. The very small current (less than 100 µA) that is diverted from LED #9 does not noticeably affect its intensity. An auxiliary current source at pin 1 keeps at least 100 µA flowing through LED #11 even if the input voltage rises high enough to extinguish the LED. This ensures that pin 9 of driver #1 is held low enough to force LED #10 off when any higher LED is illuminated. While 100 µA does not normally produce significant LED illumination, it may be noticeable when using high-efficiency LEDs in a dark environment. If this is bothersome, the simple cure is to shunt LED #11 (and LED #1) with a 10k resistor. The 1V 1R drop is more than the 900 mV worst case required to hold off LED #10 yet small enough that LED #11 does not conduct significantly. In some circuits a number of outputs on the higher device are not used. Examples include the high resolution VU meter and the expanded range VU meter circuits (see Typical Ap- plications). To provide the proper carry sense voltage in dot mode, the LEDs of the higher driver IC are tied to V LED through two series-connected diodes as shown in Figure 2. Shunting the diodes with a 1k resistor provides a path for driver leakage current. DS007971-8 FIGURE 1. Cascading LM3914/15/16 Series in Dot Mode DS007971-9 FIGURE 2. Cascading Drivers in Dot Mode with Pin 1 of Driver #2 Unused www.national.com 8 |
Số phần tương tự - LM3916N-1 |
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Mô tả tương tự - LM3916N-1 |
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