LM3914DWF National Semiconductor Corporation, LM3914DWF Datasheet - Page 14

• Manufacturer Part Number: LM3914DWF
• Description: LM3914 - Dot/bar Display Driver, Package: Mdip, Pin Nb=18
• Manufacturer: National Semiconductor Corporation

www.national.com
Typical Applications
*
Programs LEDs to 10 mA
Application Hints
Three of the most commonly needed precautions for using
the LM3914 are shown in the first typical application drawing
showing a 0V–5V bar graph meter. The most difficult prob-
lem occurs when large LED currents are being drawn, espe-
cially 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
(as illustrated) to a single point very near pin 2 is the best so-
lution.
Long wires from V
cillations. 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 inac-
cessible, 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 prob-
lem. In cases where proper wiring and bypassing fail to stop
oscillations, V
limits. Expanded scale meter applications may have one or
both ends of the internal voltage divider terminated at rela-
The exact wiring arrangement of this schematic shows the need for Mode Select (pin 9) to sense the V
+
voltage at pin 3 is usually below suggested
LED
to LED anode common can cause os-
(Continued)
20-Segment Meter with Mode Switch
14
tively 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.
Power dissipation, especially in bar mode should be given
consideration. For example, with a 5V supply and all LEDs
programmed to 20 mA the driver will dissipate over 600 mW.
In this case a 7.5 resistor in series with the LED supply will
cut device heating in half. The negative end of the resistor
should be bypassed with a 2.2 µF solid tantalum capacitor to
pin 2 of the LM3914.
Turning OFF of most of the internal current sources is ac-
complished by pulling positive on the reference with a cur-
rent source or resistance supplying 100 µA or so. Alternately,
the input signal can be gated OFF with a transistor switch.
Other special features and applications characteristics will
be illustrated in the following applications schematics. Notes
have been added in many cases, attempting to cover any
special procedures or unusual characteristics of these appli-
cations. A special section called “Application Tips for the
LM3914 Adjustable Reference” has been included with
these schematics.
+
voltage exactly as it appears on pin 3.
DS007970-14