LM3914DWF National Semiconductor Corporation, LM3914DWF Datasheet - Page 15

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

Application Hints
APPLICATION TIPS FOR THE LM3914 ADJUSTABLE
REFERENCE
GREATLY EXPANDED SCALE (BAR MODE ONLY)
Placing the LM3914 internal resistor divider in parallel with a
section ( 230 ) of a stable, low resistance divider greatly
reduces voltage changes due to IC resistor value changes
with temperature. Voltage V
by use of R2. Then the voltage V
can be adjusted to 200 mV, using R5 without affecting V
LED current will be approximately 10 mA.
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
across R4) can be adjusted to exactly 0.5V using R6 without
affecting the previous adjustment.
R9 programs LED currents within a range of 2.2 mA to 20 mA
after the above settings are made.
1
is adjusted to 5V, using R2. Then the span (voltage
1
should be trimmed to 1.1V first
(Continued)
2
across the IC divider string
1
.
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 meter
is to be accurate and linear the voltage on each divider must
be adjusted, preferably without affecting any other adjust-
ments. 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 k
cause 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 620
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.
Greatly Expanded Scale (Bar Mode Only)
or higher resistance. This is possible be-
resistor to ground. This
DS007970-15
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