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LA8638NV

Low-Voltage Compander IC for Cordless Telephones

SANYO -> Panasonic 

LA8638NV

7. Protective Diodes Preventing Static Breakdown

The control pins and data output pins have had their upper protective diodes removed so as to permit direct

connection to a microcomputer.

No protective diodes:

Lower protective diodes only:

VCC (pin 15), GND (pins 1 and 12)

Pins 16 to 20, 22

Both upper and lower protective diodes: All other pins

8. Preemphasis and Deemphasis

This chip provides preemphasis in the microphone amplifier and deemphasis in the BTL amplifier's input stage. The

amount depends on the CR time constants for the filters on the corresponding pins—the primary high pass filter on

the microphone amplifier's positive (pin 28) or negative (pin 27) input for preemphasis and the primary low pass filter

between pins 10 and 11 for deemphasis.

9. Full-Wave Rectifier Smoothing Capacitors

The external capacitors on pins 8 and 25 are for the full-wave rectifiers for the expander and compressor. They not

only smooth the output but also determine the time constant for the transient characteristics. This time constant is the

product of the capacitance and 15 kΩ, the input resistance of the full-wave rectifier. Although there is a tendency to

lower the time constant for the expander to reduce noise at the ends of words, the designer must keep in mind that

such cuts reduce the amount of smoothing and thus raise the risk of distortion.

10. Compressor's Summing Amplifier

Achieving a DC gain of 1 and an AC gain of infinity from the compressor's summing amplifier requires suppressing

AC feedback with the capacitor on pin 3. The cutoff frequency is determined by the product of its capacitance and the

internal resistance of 22.5 kΩ.

11. Standby Function

The chip's standby function does not produce a total shutdown of all circuits. It disables the audio signal processing

block, but leaves the waveform shaper block for the receiving signal operating. For this reason, it is not possible to

connect the battery directly to the power supply pin (pin 15). There must be an intervening transistor switch for an

intermittent power supply.

12. Control Modes

Pin 17

SUB-CNT1

OPEN/HIGH

OPEN/HIGH

LOW

LOW

Pin 18

SUB-CNT2

OPEN/HIGH

LOW

OPEN/HIGH

LOW

Mode

Standby

Receiver muted

Normal receiver output levels

Low receiver output levels

Pin Number

Pin Name

OPEN/HIGH

Pin 19

BTL-CNT

BTL amplifier disabled

Pin 20

TX-MUTE

Transmitter muted

Pin 22

TX-LVL-CNT Normal transmitter output levels

Note: The standby mode overrides all other mode settings.

LOW

BTL amplifier enabled

Transmitter enabled

High transmitter output levels

No. 6627-7/16

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