XR-2279 (Exar)
Dot and Bar-Graph Display Generators

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XR·2279
Dot and Bar-Graph Display Generators
GENERAL DESCRIPTION
FUNCTIONAL BLOCK DIAGRAM
The XR-2279 is a 12 point logarithmic dot or bar-graph
generator/LED driver_ The device compares an input
signal with an adjustable reference and graphically dis-
plays the results_ Since LED driving current is provided
by on board adjustable current sources, no current lim-
iting resistors are required. A special feature of the XR-
2279 is the four highest outputs sink one half the cur-
rent of the lower eight; this allows equivalent bright-
ness with red and green mixed displays.
Dot or bar mode selection is provided. The twelve out-
put levels are in 3 dB steps from - 27 dB to + 6 dB. The
reference point is externally adjustable.
FEATURES
Direct LED Interface
Constant Three dB/Step Logarithmic Scale
External Dot/Bar Mode Select for Dot/Bar-Graph
Formats
Adjustable Output Current Levels
Current Source Outputs
APPLICATIONS
Bar Graph Display Generator
Moving-dot Display Generator
Logarithmic Level Indicator
Sequential Level Indicator
ABSOLUTE MAXIMUM RATINGS
Power Supply
Power Dissipation
Derate Above 25°C
Operating Temperature Range
Storage Temperature Range
15V
625mW
5 mW/oC
O°C to +75°C
- 65°C to + 150°C
ORDERING INFORMATION
Part Number
XR-2279CP
Package
Plastic
Operating Temperature
O°C to + 70°C
SYSTEM DESCRIPTION
The XR-2279 is a 12 point logarithmic level detector cir-
cuit designed for direct interfacing to light emitting di-
ode (LED) moving-dot or bar-graph displays. The circuit
is comprised of an input buffer amplifier and 12 com-
parators which are biased from an internal voltage ref-
erence. Each comparator provides a high impedance
current source output which are all very closely
matched and simultaneously adjustable with a single
external resistor. A control signal applied to the mode
select Pin 18 determines whether the display is driven
in a moving-dot or bar-graph format.
The circuit provides 12 discrete outputs for an input
level range of - 27 dB to + 6 dB, referenced to an inter-
nally set zero dB level, typically 0.2 VRMS. Each step
represents 3 dB, and the reference level is adjustable.
The upper four outputs, 9-12 (see Equivalent Schematic
Diagram), are internally set to provide one-half the cur-
rent out as outputs 1-8. This is for driving red LEOs (as
over-range indicators) which require less current than
other colors.
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XR-2279 (Exar)
Dot and Bar-Graph Display Generators

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XR·2279
ELECTRICAL CHARACTERISTICS
Test Conditions: VCC = 12 Volts, TA = 25°C, unless otherwise specified. (See Test Circuit of Figure 2.)
XR-2279
PARAMETERS
Supply Voltage
Supply Current
Output Current
Outputs 1 through 8
Outputs 9 through 12
Output Current Matching
Outputs 1 through 8
Outputs 9 through 12
Maximum Drive Current
Outputs 1 through 8
Outputs 9 through 12
Input Voltage for 0 dB Output
Input Current
Outputs
Output 1 (Pin 7)
Output 2 (Pin 16)
Output 3 (Pin 15)
Output 4 (Pin 14)
Output 5 (Pin 13)
Output 6 (Pin 12)
Output 7 (Pin 11)
Output 8 (Pin 10)
Output 9 (Pin 5)
Output 10 (Pin 6)
Output 11 (Pin 7)
Output 12 (Pin 8)
MIN
10
12
6
-2.0
-1.0
0.10
-25.5
-22.5
-19.5
-16.5
-13.5
-10.5
-7.0
-4.0
+2.0
+4.5
TYP
12
5
15
8
0.20
50
-27
-24.0
-21.0
-18.0
-15.0
-12.0
-9.0
-6.0
-3.0
0
+3.0
+6.0
MAX
14
10
18
10
+2.0
+1.0
22
11
0.25
-22.5
-19.5
-16.5
-13.5
-10.5
-7.5
-5.0
-2.0
+4.0
+7.5
UNITS
VDC
mA
mA
mA
mA
mA
mA
mA
VRMS
nA
CONDITIONS
VF(LED) = 2V
VIN = OV
R2 = 27KO
Measured at Pins 10 through 17
Measured at Pins 5 through 8
R2 = 27KO
Measured at Pins 10 through 17
Measured at Pins 5 through 8
R2 varied - see Figure 4
Measured at Pins 10 through 17
Measured at Pins 5 through 8
o dB Output Threshold
Measured at Pin 3
dB See Note 1
dB
dB
dB
dB
dB
dB
dB
dB
dB See Note 1
dB
dB
Note 1:
Determine exact value of input voltage to produce zero dB output. This is done by increasing the ac input signal
amplitude until Output 10 (Pin 6) begins conduction.
Reduce input voltage level to - 27 dB referenced to the input level of Note 1. Adjust R1 until Output 1 (Pin 17)
begins conduction.
EQUIVALENT SCHEMATIC DIAGRAM
OUTPUT
CURRENT AD.I.
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XR-2279 (Exar)
Dot and Bar-Graph Display Generators

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',,<>--------,--------,.-------r-----,
RL _ _ _ _ _ _ _ _ _ _ _ _ _ _ RL
r--+--_O'_'''--,''<>-I''''
RI :. 500U
RZ '" 27K!l
R3 '" 330KIl
A4'" 10KIl
RS'" lOKI!
C 1 '" 2.2/-1F
Cz '" lOJ-IF
RL'" 200n
Figure 2. Generalized Test Circuit
PRINCIPLES OF OPERATION
As shown in the equivalent circuit schematic, the cir-
cuit is comprised of 12 voltage comparators with cur-
rent source outputs. One input in each of the compara-
tors is connected to a common voltage line. The input
voltage, YIN, is applied to this signal line through a
buffer amplifier. The remaining input of each of the
comparators is biased from an internal resistor ladder
connected to a voltage reference on the IC Chip. Thus,
each of the 12 ladder taps corresponds to the particu·
lar output thresholds, listed as outputs one through
twelve, in the electrical characteristics.
As the input voltage applied to the device is increased,
each of the 12 comparators in the chip changes state
sequentially at the time the input signal levels reach
their respective threshold levels. The output currents of
the last four outputs (Pins 5 through 8) are set at one-
half the current output of the first eight outputs. This is
done to minimize power dissipation since the last four
outputs normally drive red LEDs to indicate "over·
range" condition. The red LEDs are normally more effi-
cient than other colors and require approximately one-
half as much current for the same brightness.
Figure 3 shows the typical output current waveforms
for operating in the moving-dot mode. The mode of op-
eration is selected by the logic state at Pin 18. If this pin
is grounded, the output display is in the moving-dot for-
mat where only one of the current outputs is active at
anyone time, depending on the input signal level (see
Figure 3(b)).
If Pin 18 is left an open circuit, then the IC operates as
a bar-graph display generator. In this mode of opera-
tion, the external LEOs are connected in series, in
groups of four to minimize power dissipation.
The outputs of the comparators, (4), (8), and (12), can·
tinue conducting in this manner as long as the voltage
level is above the respective threshold points.
XR·2279
V'2
Iwn
@i"~
~g
CO
Q. ...
:1 0
0u::"J
a:
"I-
V2
V,
INPUT SIGNAL LEVEL, VIN
(0) LINEARLY RISING INPUT SIGNAL
112 ON
OFF 1----------.....1
12 ON
OFF
1, ON
OFF I-~~--------------
(b) OUTPUT CURRENTS IN MOVING·DOT MODE
Figure 3. Typical Output Waveforms in Moving-Dot Display
Mode
EXTERNAL ADJUSTMENTS
Output Brightness Adjustment
The output current level for each of the 12 outputs is
controlled by an external current setting resistor, R2,
(R2 ~ 20 kO) connected from Pin 2 to Ground. Figure 4
shows the available output drive current, ILEO, as a
function of R2.
.
~
,. "-
f'....
r O~tr"U::U9h 8
"i r-....12 / O~'r'...=...u. ah 12 ............
; r---~
--- --- -I--
.. .25 30
35
50
Figure 4. Output Drive Current as a Function of Current
Setting ReSistor, R2
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XR-2279 (Exar)
Dot and Bar-Graph Display Generators

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XR·2279
Response Adjustment
Transient response of the circuit is adjusted by an exter-
nal resistor, R5, and capacitor, C2, connected from Pin
4 to ground. Typical component values for audio fre-
quency applications, from 20 Hz to 20 kHz, are: R5 =
10 kll and C2 = 10 /-IF. The internal impedance at Pin 4
is approximately 100 ohms. C2 functions as a holding
capacitor of the internal peak rectifier circuit, with R5
controlling its decay time.
Scale Adjustment
The output thresholds for the XR-2279 are measured
relative to an internal zero dB reference level. Thus, for
a given input signal dynamic range, each circuit must
be calibrated with respect to the zero dB reference lev-
el setting. This calibration is performed by adjusting the
potentiometer, R1, shown in Figure 2, with an audio fre-
quency ac signal applied to the circuit in two steps, as
follows:
Step 1: Determine exact value of input voltage to pro-
duce zero dB output. This is done by increasing the ac
input signal amplitude until Output 10 (Pin 7) begins
conduction.
Step 2: Reduce input voltage level to - 27 dB refer-
enced to the input level of Step 1. Adjust R1 until Output
1 (Pin 7) begins conduction.
Yee
·3330.
10.
aND
-R1- !Soon. CALIBRATION POTENTIOMETER.
Figure 5. Circuit Connection lor Moving-Dot Display
Generation
MOVING-DOT DISPLAY
Figure 5 shows the basic connection of the XR-2279 as
a moving-dot display generator and driver. In this mode
of operation pin 18 is connected to ground. Increasing
the voltage at the input will cause each one of the 12
LEOs to turn on, one at a time, at the appropriate input
level, and thus generate a moving dot of light. Output
waveforms for this mode are shown in Figure 3(b).
Yee
·3330.
.,'c, >-"W'o~~
~~I-"-fF---''''-'--'
Figure 6. Circuit Connection for Bar-Graph Display
Generation
will turn on and stay on as the input signal amplitude is
increased as long as the input voltage stays above the
threshold level corresponding to that particular output.
AUDIO LEVEL INDICATOR
Figure 7 shows a complete audio level indicator system
made up of the XR-2279 Display Generator and an ad-
justable gain amplifier. For a given dynamic range of
the input audio voltage, VA, the potentiometer R6 is
used to set the gain of the input amplifier which is ad-
justed to give the desired zero dB output level from the
display generator IC. The potentiometer R1 is then ad-
justed to set the lowest output level; i.e., the - 27 dB
level. The display output format can be either the
moving-dot or the bar-graph type, by choosing the LED
interconnections and the logic signal applied to Pin 18.
BAR-GRAPH DISPLAY
Figure 6 shows the basic circuit connection for the XR-
2279 as a bar-graph display generator and driver. Note
that in this mode of operation the 12 LEOs are connect-
ed in series in three groups of four LEOs, and the mode-
select terminal (Pin 18) is left an open circuit. Each LED
'~o------.--....,..---.-
"1I,,,CALJBIIATIONPOl'!NlIDMeTlR.
Figure 7. Typical Audio Level Indicator System
Using the XR-2279.
1-254




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