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LM3915 Datasheet(PDF) 10 Page - Texas Instruments

Part No. LM3915
Description  LM3915 Dot/Bar Display Driver
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
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LM3915 Datasheet(HTML) 10 Page - Texas Instruments

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LM3915
SNVS740C – JANUARY 2000 – REVISED MARCH 2013
www.ti.com
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 VLED. This condition is
sensed by comparator C2, referenced 600 mV below VLED. This forces the output of C2 low, which shuts off
output transistor Q2, extinguishing LED #10.
VLED 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 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 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 with a 1 mA
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 “overlap” 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 currents 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 bottom of the resistor string to a
single point very near pin 2 is the best solution.
Long wires from VLED 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 suggested 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 capacitor, or up to 0.1 μF in noisy environments.
10
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Product Folder Links: LM3915


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