Application Hints

Three of the most commonly needed precautions for using

the LM3914 are shown in the first typical application draw-

ing (see page 9-108) showing a 0V – 5V bar graph meter

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 (as illustrated) 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

005 mFto2 2 mF 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 Va voltage at pin 3 is usually below sug-

gested limits (see Note 2 page 9-108) Expanded scale me-

ter 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 0001 mF capacitor or up to 01 mF 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 75X resistor in series with the LED supply will

cut device heating in half The negative end of the resistor

should be bypassed with a 22 mF 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 mA or so Alternate-

ly 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 (j230X) of a stable low resistance divider greatly

reduces voltage changes due to IC resistor value changes

with temperature Voltage V1 should be trimmed to 11V first

by use of R2 Then the voltage V2 across the IC divider

string can be adjusted to 200 mV using R5 without affecting

V1 LED current will be approximately 10 mA

NON-INTERACTING ADJUSTMENTS FOR EXPANDED

SCALE METER (45V to 5V Bar

or Dot Mode)

This arrangement allows independent adjustment of LED

brightness regardless of meter span and zero adjustments

First V1 is adjusted to 5V using R2 Then the span (voltage

across R4) can be adjusted to exactly 05V using R6 without

affecting the previous adjustment

R9 programs LED currents within a range of 22 mA to

20 mA after the above settings are made

Greatly Expanded Scale (Bar Mode Only)

TLH7970 – 15

ADJUSTING LINEARITY OF SEVERAL STACKED

DIVIDERS

Three internal voltage dividers are shown connected in se-

ries to provide a 30-step display If the resulting analog me-

ter is to be accurate and linear the voltage on each divider

must be adjusted preferably without affecting any other ad-

justments To do this adjust R2 first so that the voltage

across R5 is exactly 1V Then the voltages across R3 and

R4 can be independently adjusted by shunting each with

selected resistors of 6 kX or higher resistance This is pos-

sible because the reference of LM3914 No 3 is acting as a

constant current source

The references associated with LM3914s No 1 and No 2

should have their Ref Adj pins (pin 8) wired to ground and

their Ref Outputs loaded by a 620X resistor to ground This

makes available similar 20 mA current outputs to all the

LEDs in the system

If an independent LED brightness control is desired (as in

the previous application) a unity gain buffer such as the

LM310 should be placed between pin 7 and R1 similar to

the previous application

13

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