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LM3914N-1 Arkusz danych(PDF) 8 Page - National Semiconductor (TI) |
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LM3914N-1 Arkusz danych(HTML) 8 Page - National Semiconductor (TI) |
8 / 19 page Mode Pin Functional Description (Continued) LED No. 1 of the second device comes on. The connection for cascading in dot mode has already been described and is depicted below. As long as the input signal voltage is below the threshold of the second LM3914, LED No. 11 is off. Pin 9 of LM3914 No. 1 thus sees effectively an open circuit so the chip is in dot mode. As soon as the input voltage reaches the thresh- old of LED No. 11, pin 9 of LM3914 No. 1 is pulled an LED drop (1.5V or more) below V LED. This condition is sensed by comparator C2, referenced 600 mV below V LED. This forces the output of C2 low, which shuts off output transistor Q2, ex- tinguishing LED No. 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 No. 9 does not noticeably affect its intensity. An auxiliary current source at pin 1 keeps at least 100 µA flowing through LED No. 11 even if the input voltage rises high enough to extinguish the LED. This ensures that pin 9 of LM3914 No. 1 is held low enough to force LED No. 10 off when any higher LED is illuminated. While 100 µA does not normally produce significant LED illumination, it may be no- ticeable when using high-efficiency LEDs in a dark environ- ment. If this is bothersome, the simple cure is to shunt LED No. 11 with a 10k resistor. The 1V IR drop is more than the 900 mV worst case required to hold off LED No. 10 yet small enough that LED No. 11 does not conduct significantly. OTHER DEVICE CHARACTERISTICS The LM3914 is relatively low-powered itself, and since any number of LEDs can be powered from about 3V, it is a very efficient display driver. Typical standby supply current (all LEDs OFF) is 1.6 mA (2.5 mA max). However, any reference loading adds 4 times that current drain to the V + (pin 3) sup- ply input. For example, an LM3914 witha1mA reference pin load (1.3k), would supply almost 10 mA to every LED while drawing only 10 mA from its V + pin supply. At full-scale, the IC is typically drawing less than 10% of the current supplied to the display. The display driver does not have built-in hysteresis so that the display does not jump instantly from one LED to the next. Under rapidly changing signal conditions, this cuts down high frequency noise and often an annoying flicker. An “over- lap” is built in so that at no time between segments are all LEDs completely OFF in the dot mode. Generally 1 LED fades in while the other fades out over a mV or more of range (Note 3). The change may be much more rapid be- tween LED No. 10 of one device and LED No. 1 of a second device “chained” to the first. The LM3914 features individually current regulated LED driver transistors. Further internal circuitry detects when any driver transistor goes into saturation, and prevents other cir- cuitry from drawing excess current. This results in the ability of the LM3914 to drive and regulate LEDs powered from a pulsating DC power source, i.e., largely unfiltered. (Due to possible oscillations at low voltages a nominal bypass ca- pacitor consisting of a 2.2 µF solid tantalum connected from the pulsating LED supply to pin 2 of the LM3914 is recom- mended.) This ability to operate with low or fluctuating volt- ages also allows the display driver to interface with logic cir- cuitry, opto-coupled solid-state relays, and low-current incandescent lamps. Cascading LM3914s in Dot Mode DS007970-6 www.national.com 8 |
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