Datasheet

LM3914

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SNVS761B –JANUARY 2000–REVISED MARCH 2013

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 0.001μF capacitor, or up to 0.1μF in noisy environments.

Power dissipation, especially in bar mode should be given consideration. For example, with a 5V supply and all

LEDs programmed to 20mA the driver will dissipate over 600mW. In this case a 7.5Ω resistor in series with the

LED supply will cut device heating in half. The negative end of the resistor should be bypassed with a 2.2μF solid

tantalum capacitor to pin 2 of the LM3914.

Turning OFF of most of the internal current sources is accomplished by pulling positive on the reference with a

current source or resistance supplying 100μA 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 applications. A special section called APPLICATION TIPS FOR THE LM3914 ADJUSTABLE REFERENCE

has been included with these schematics.

APPLICATION TIPS FOR THE LM3914 ADJUSTABLE REFERENCE

Greatly Expanded Scale (Bar Mode Only)

Placing the LM3914 internal resistor divider in parallel with a section (≃230Ω) of a stable, low resistance divider

greatly reduces voltage changes due to IC resistor value changes with temperature. Voltage V

should be

trimmed to 1.1V first by use of R2. Then the voltage V

across the IC divider string can be adjusted to 200mV,

using R5 without affecting V

. LED current will be approximately 10mA.

Non-Interacting Adjustments For Expanded Scale Meter (4.5V to 5V, Bar or Dot Mode)

This arrangement allows independent adjustment of LED brightness regardless of meter span and zero

adjustments.

First, V

is adjusted to 5V, using R2. Then the span (voltage across R4) can be adjusted to exactly 0.5V using

R6 without affecting the previous adjustment.

R9 programs LED currents within a range of 2.2mA to 20mA after the above settings are made.

Figure 25. Greatly Expanded Scale (Bar Mode Only)

Product Folder Links: LM3914