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LM3915N Folha de dados(PDF) 9 Page - National Semiconductor (TI) |
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LM3915N Folha de dados(HTML) 9 Page - National Semiconductor (TI) |
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9 / 25 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 comparator 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 either 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 the display to make sense when multiple LM3915s are cascaded in dot mode, special circuitry has been included to shut off LED #10 of the first device when LED #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 LM3915, LED #11 is off. Pin 9 of LM3915 #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 LM3915 #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, 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 LM3915 #1 is held low enough to force LED #10 off when any higher LED is illuminated. While 100 µA does not nor- mally produce significant LED illumination, it may be notice- able when using high-efficiency LEDs in a dark environment. If this is bothersome, the simple cure is to shunt LED #11 with a 10k resistor. The 1V IR 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. OTHER DEVICE CHARACTERISTICS The LM3915 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. However, any reference loading adds 4 times that current drain to the V + (pin 3) supply input. For example, an LM3916 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 are all segments completely off in the dot mode. Generally 1 LED fades in while the other fades out over a mV or more of range. The change may be much more rapid between LED #10 of one device and LED #1 of a second device “chained” to the first. Application Hints The most difficult problem occurs when large LED currents are being drawn, especially in bar graph mode. These cur- rents flowing out of the ground pin cause voltage drops in external wiring, and thus errors and oscillations. Bringing the return wires from signal sources, reference ground and bot- tom of the resistor string to a single point very near pin 2 is the best solution. Long wires from V LED to LED anode common can cause oscillations. Depending on the severity of the problem 0.05 µF to 2.2 µF decoupling capacitors from LED anode common to pin 2 will damp the circuit. If LED anode line wiring is inaccessible, often similar decoupling from pin 1 to pin 2 will be sufficient. If LED turn ON seems slow (bar mode) or several LEDs light (dot mode), oscillation or excessive noise is usually the problem. In cases where proper wiring and bypassing fail to stop oscillations, V + voltage at pin 3 is usually below sug- gested limits. Expanded scale meter applications may have one or both ends of the internal voltage divider terminated at relatively high value resistors. These high-impedance ends should be bypassed to pin 2 with at least a 0.001 µF capaci- tor, or up to 0.1 µF in noisy environments. Cascading LM3915s in Dot Mode 00510408 www.national.com 9 |
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