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LM3915 Datasheet(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM3915
Description  Dot/Bar Display Driver
Download  25 Pages
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3915 Datasheet(HTML) 8 Page - National Semiconductor (TI)

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Functional Description
The simplified LM3915 block diagram is included to give the
general idea of the circuit’s operation. A high input imped-
ance buffer operates with signals from ground to 12V, and is
protected against reverse and overvoltage signals. The sig-
nal is then applied to a series of 10 comparators; each of
which is biased to a different comparison level by the resistor
string.
In the example illustrated, the resistor string is connected to
the internal 1.25V reference voltage. In this case, for each
3 dB that the input signal increases, a comparator will switch
on another indicating LED. This resistor divider can be con-
nected between any 2 voltages, providing that they are at
least 1.5V below V
+ and no lower than V.
INTERNAL VOLTAGE REFERENCE
The reference is designed to be adjustable and develops a
nominal 1.25V between the REF OUT (pin 7) and REF ADJ
(pin 8) terminals. The reference voltage is impressed across
program resistor R1 and, since the voltage is constant, a
constant current I
1 then flows through the output set resistor
R2 giving an output voltage of:
00510405
Since the 120 µA current (max) from the adjust terminal
represents an error term, the reference was designed to
minimize changes of this current with V
+ and load changes.
For correct operation, reference load current should be be-
tween 80 µA and 5 mA. Load capacitance should be less
than 0.05 µF.
CURRENT PROGRAMMING
A feature not completely illustrated by the block diagram is
the LED brightness control. The current drawn out of the
reference voltage pin (pin 7) determines LED current. Ap-
proximately 10 times this current will be drawn through each
lighted LED, and this current will be relatively constant de-
spite supply voltage and temperature changes. Current
drawn by the internal 10-resistor divider, as well as by the
external current and voltage-setting divider should be in-
cluded in calculating LED drive current. The ability to modu-
late LED brightness with time, or in proportion to input volt-
age and other signals can lead to a number of novel displays
or ways of indicating input overvoltages, alarms, etc.
The LM3915 outputs are current-limited NPN transistors as
shown below. An internal feedback loop regulates the tran-
sistor drive. Output current is held at about 10 times the
reference load current, independent of output voltage and
processing variables, as long as the transistor is not satu-
rated.
LM3915 Output Circuit
00510406
Outputs may be run in saturation with no adverse effects,
making it possible to directly drive logic. The effective satu-
ration resistance of the output transistors, equal to R
E plus
the transistors’ collector resistance, is about 50
Ω. It’s also
possible to drive LEDs from rectified AC with no filtering. To
avoid oscillations, the LED supply should be bypassed with a
2.2 µF tantalum or 10 µF aluminum electrolytic capacitor.
MODE PIN USE
Pin 9, the Mode Select input, permits chaining of multiple
LM3915s, and controls bar or dot mode operation. The
following tabulation shows the basic ways of using this input.
Other more complex uses will be illustrated in the applica-
tions.
Bar Graph Display: Wire Mode Select (pin 9) directly to pin
3(V
+ pin).
Dot Display, Single LM3915 Driver: Leave the Mode Select
pin open circuit.
Dot Display, 20 or More LEDs: Connect pin 9 of the first
driver in the series (i.e., the one with the lowest input voltage
comparison points) to pin 1 of the next higher LM3915 driver.
Continue connecting pin 9 of lower input drivers to pin 1 of
higher input drivers for 30 or more LED displays. The last
LM3915 driver in the chain will have pin 9 left open. All
previous drivers should have a 20k resistor in parallel with
LED #9 (pin 11 to V
LED).
Mode Pin Functional Description
This pin actually performs two functions. Refer to the simpli-
fied block diagram below.
Block Diagram of Mode Pin Function
00510407
*High for bar
www.national.com
8


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