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Issue A

LANSDALE Semiconductor, Inc.

ML33111

Expander

The expander is an noninverting amplifier with a fixed feed-

back resistor and a variable gain cell in its input path as shown

in Figure 16.

The input signal is sampled by the precision rectifier which,

in turn, supplies a DC signal (ICONTROL), representative of

the AC input signal, to the variable gain cell. The reference cur-

rent (IREF) is an internally generated precision current. The

effective impedance of the variable gain cell varies with the

ratio of the two currents, and decreases as ICONTROL increas-

es, thereby providing expansion. The output is related to the

input by the following equation (Vin and Vout are rms volts):

The input and output are internally biased at Vb (

≈ +1.5 V), and

must therefore be capacitor coupled to external circuitry. The input

impedance at Pin 14 is nominally 10.9 k

Ω (±20%), and the maxi-

mum functional input signal is listed in the Recommended

Operating Conditions table. Bias currents required by the op amp

and the variable gain cell are internally supplied. Due to clamp

diodes at the input (to VCC and ground), the input signal must be

maintained between the supply rails. If the input signal goes more

than 0.5 V above VCC or below ground, excessive currents will

flow, and distortion will show up at the output, and possibly in other

parts of the circuit.

The output of the rectifier is filtered by the capacitor at Pin 11,

which, in conjunction with an internal 20 k resistor, provides the

time constant for the attack and decay times. The attack and decay

times listed in the Electrical Characteristics were determined using

the test procedure defined in EIA-553. Figure 10 indicates how the

times vary with the capacitor value. If the attack and decay times

are decreased by using a smaller capacitor, performance at low fre-

quencies will degrade.

Op Amps

The two op amps (at Pins 6, 7, 9, and 10) are identical and

can be used for peripheral functions, such as a microphone

amplifier, buffer, filter, etc. They have an open loop gain of

≈100 dB, and a bandwidth of ≈ 300 kHz. The noninverting

inputs are internally biased at Vb (

≈ +1.5 V). The inverting

inputs (Pins 7, 9) require a bias current of

≈ 8.0 nA, which

flows into the pin. The outputs can typically supply a maxi-

mum of 3.7 mA load current (see Electrical Characteristics).

NOTE: If an op amp is unused, its output MUST be tied to

its input (Pin 6 to 7 and/or 9 to 10). Leaving an input open can

affect other portions of the IC.

Logic Inputs

The three inputs (Pins 4, 8, 12) provide for muting and

passthrough functions for the compressor and expander

according to the following truth table:

The logic section permits the compressor and expander to be

muted independently. The Passthrough control affects both sec-

tions simultaneously, but only if the Mute inputs are at a logic

level 0. If both the Passthrough and a Mute input are asserted,

the Mute will override the Passthrough. The logic controls do

not affect the two uncommitted op amps in any way.

Figure 17 depicts a typical logic input stage configuration,

and Figure 14 indicates the typical input current. The inputs’

threshold is

≈ +1.3 V, independent of VCC. An open input is

equivalent to a logic low, but good design practices dictate that

inputs should never be left open. The inputs must be kept with-

in the range of VCC and GND. If an input is taken more than

0.5 V above VCC or below GND excessive currents will flow,

and the device’s operation will be distorted.

Vout = 10 x (Vin)2

(3)

In terms of dB levels, the relationship is:

Vo(dB) = 2.0 x Vi(dB)

(4)

where 0 dB = 100 mVrms (See Figures 3 and 4).

CM

(Pin 4)

EM

(Pin 12)

PT

(Pin 8)

Function

0

0

0

Normal Operation

1

X

X

Compressor Mute

X

1

X

Expander Mute

0

0

1

Passthrough

Figure 17. Logic Input Stage

50 k

50 k

VCC

Pins

4, 8, 12

40 k

Output

15

20 k

∆ Gain

Rectifier

ICONTROL

Iref

11

Vb

15 k

VCC

14

Input

1.0

µF

Figure 16. Expander