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 mA for the LEFT-and-RIGHT FRONT-and-REAR

OUTPUT buffers

The Electrical Specifications table lists a maximum input sig-

OUT pins This distortion level is achieved when the mini-

mum ac load impedance seen by the SELECT OUT pin is

325 kX (325V1 mA) For the LEFT-and-RIGHT FRONT-

and-REAR OUTPUTs the typical maximum output is 12

is 443 kX (155 V035 mA) Trying to use a lower imped-

ance results in a clipped output signal Therefore

the

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

For applications that require dc coupling and the INPUTs

from that detailed in the above discussion The emitter fol-

lowers may be potentially operating at high currents be-

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

4mA

e

1638X

To allow for variations and part tolerances 20 kX is a good

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

Latch-up may occur if the total emitter current exceeds

1 mA This current is a combination of the emitter follower’s

035 mA current source and 065 mA drawn by the external

load Therefore to prevent this possibility the minimum dc

load impedance should be

065 mA

e

9kX

TLH10789 – 20

FIGURE 2 Input Bias Network

To allow for variations and part tolerances 10 kX is a good

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 MX

The SELECT IN pins have an input impedance that varies

with the BASS and TREBLE control settings The input im-

pedance is 96 kX at dc and 27 kX at 1 kHz when the con-

trols are set at 0 dB Minimum input impedance of 28 kX at

dc and 24 kX at 1 kHz occurs when maximum boost is

selected At 10 kHz the minimum input impedance with the

tone controls flat is 8 kX and with the tone controls at

maximum boost is 3 kX

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

THD) would then swing from 075V to 725V 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-

nal emitter follower Therefore if the selected input has a

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

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