LMC1992CCN National Semiconductor, LMC1992CCN Datasheet - Page 7

• Manufacturer Part Number: LMC1992CCN
• Description: IC CTRLR DGTL STEREO 4CHAN 28DIP
• Manufacturer: National Semiconductor
• Type: Tone and Volume Controlr

Specifications of LMC1992CCN

• Applications: Audio Systems
• Mounting Type: Through Hole
• Package / Case: *
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LMC1992CCN

chance of clipping can be greatly reduced and much lower
distortion levels can be achieved by using load impedances
that are an order of magnitude higher than shown here
Applications Information
MINIMUM LOAD IMPEDANCE
The LMC1992 employs emitter-follower buffers at pins 8
and 22 (SELECT OUT) 13 and 14 (LEFT FRONT and
REAR OUTPUTs) and 16 and 17 (RIGHT FRONT-and-
REAR OUTPUTs) that buffer output signals Typical bias
current of 1 mA is used for the SELECT OUTPUT buffers
and 350
OUTPUT buffers
The Electrical Specifications table lists a maximum input sig-
nal of 2 3 V
OUT pins This distortion level is achieved when the mini-
mum ac load impedance seen by the SELECT OUT pin is
3 25 k
and-REAR OUTPUTs the typical maximum output is 1 2
V
is 4 43 k
ance results in a clipped output signal Therefore the
For applications that require dc coupling and the INPUTs
biased to V
from that detailed in the above discussion The emitter fol-
lowers may be potentially operating at high currents be-
cause there is a dc voltage V
OUT pins dc resistance to ground will result in increased
current flow Latch-up may occur if the total emitter current
exceeds 5 mA This current is a combination of the emitter
follower’s 1 mA current source and 4 mA drawn by the ex-
ternal load Therefore to prevent this possibility the mini-
mum dc load impedance should be
V
V
To allow for variations and part tolerances 2 0 k
choice for this minimum dc load impedance
When dc coupling is used at the LEFT-and-RIGHT FRONT-
and-REAR OUTPUTs the output emitter followers will be
operating at a nominal dc voltage of V
Latch-up may occur if the total emitter current exceeds
1 mA This current is a combination of the emitter follower’s
0 35 mA current source and 0 65 mA drawn by the external
load Therefore to prevent this possibility the minimum dc
load impedance should be
V
V
rms
peak
peak
a
a
e
e
(1 55 V
e
e
8V
8V
(3 25V 1 mA) For the LEFT-and-RIGHT FRONT-
3 25V
3 25V
A for the LEFT-and-RIGHT FRONT-and-REAR
(1 55 V 0 35 mA) Trying to use a lower imped-
V
a
rms
peak
V
peak
peak
2 the minimum load impedance will differ
(3 25 V
) Therefore the minimum load impedance
a
a
(V
0 65 mA
(V
4 mA
a
a
peak
2
2
) for 1% THD at the SELECT
b
b
a
2(0 7V))
0 7V)
2
b
e
0 7V at the SELECT
e
1638
a
9 k
2
b
is a good
2(0 7V)
7
To allow for variations and part tolerances 10 k
choice for this minimum dc load impedance
INPUT IMPEDANCE
For ac coupled input signals the input impedance value is
determined by bias resistor R1 as shown in Figure 2 A
directly coupled input signal will see an emitter follower’s
nominal input impedance of 2 M
The SELECT IN pins have an input impedance that varies
with the BASS and TREBLE control settings The input im-
pedance is 96 k
trols are set at 0 dB Minimum input impedance of 28 k
dc and 24 k
selected At 10 kHz the minimum input impedance with the
tone controls flat is 8 k
maximum boost is 3 k
STEREO SIGNAL INPUTS
When operating with a single supply voltage the stereo sig-
nal inputs must be dc biased to one-half of the supply volt-
age as shown in Figure 2 As an example with a supply
voltage of 8V all signal sources should have a dc bias of
4V The maximum input signal level of 6 5 V
THD) would then swing from 0 75V to 7 25V Input-to-input
crosstalk can be minimized by using a separate dc bias cir-
cuit for each stereo input pair
EXTERNAL SIGNAL PROCESSING
The signal present at the selected input will be available at
the SELECT OUT pins 8 (left) and 22 (right) The dc bias
voltage at those pins will be one base-emitter voltage ap-
proximately 0 7 V
nal emitter follower Therefore if the selected input has a
bias of 4 0 V
about 3 3 V
The LMC1992’s SELECT OUT emitter followers allow addi-
tional signal sources using emitter follower outputs (such as
multiple LMC1992s) to be ‘‘wired-ORed’’ together When
this feature is in use the input channel of the LMC1992 not
in use should be set to ‘‘open’’ input codes 01000XX0000 or
01000XX011X
dc
dc
FIGURE 2 Input Bias Network
at 1 kHz occurs when maximum boost is
the dc component at pins 8 and 22 will be
dc
at dc and 27 k
below the source because of the inter-
and with the tone controls at
at 1 kHz when the con-
TL H 10789– 20
p-p
is a good
(for 1%
at