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MC33215

Telephone Line Interface and Speakerphone Circuit

Motorola => Freescale 

Freescale SMeCm3i3c2o15nductor, Inc.

Table 2. Logic Table for Handsfree Mode

Logic Inputs

Amplifiers

SPS

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1

MUT

1

1

0

PRS

1

0

X

Mode

Handsfree Normal

Handsfree Privacy

Handsfree DTMF

HMIC BMIC DTMF

Rx

Off

On

Off

On

Off

Off

On

On

Off

Off

On

Off

RXatt

On

On

On

EAR

Off

Off

Off

By applying a logic 0 to Pin MUT, the DTMF mode is

entered where the DTMF amplifier is enabled and where the

Rx amplifier is muted. A DTMF signal can be sent to the line

via the MFI input for which the gain ADTMF is given as:

+ + ń ) ADTMF

Vline

VMFI

3.75

RSLP 11

x

Zline x

Zline

Zset

Zset

In the typical application, the gain equals 35 dB. The

DTMF gain can be controlled by a resistor divider at the input

MFI as shown in the typical application. The signal has to be

capacitively coupled to the input via CMFI which creates a

high pass filter with the input impedance. The line length

AGC has no effect on the DTMF gains.

The signal applied to the MFI input is made audible at the

earpiece output for confidence tone. The signal is internally

applied to the GRX pin where it is amplified via the EAR

amplifier which is used as a current to voltage amplifier. The

gain is therefore proportional to the feedback resistor RRXO.

For RRXO = 180 kΩ the gain equals 6.0 dB. The confidence

tone is also audible at the loudspeaker output when the

loudspeaker amplifier is activated, see speakerphone

operation.

By applying a logic 0 to Pin PRS, the MC33215 enters

privacy mode. In this mode, both handset and handsfree

microphone amplifiers are muted while the DTMF amplifier is

enabled. Through the MFI input, a signal, for example music

on hold, can be sent to the line. In the same way, the MFI

input can also be used to couple in signals from, for instance,

an answering machine.

HANDSFREE OPERATION

Handsfree operation, including DTMF and Privacy modes,

can be performed by making Pin SPS high according Table 2.

The handset amplifiers will be switched off while the base

amplifiers will be activated. The MC33215 performs all the

necessary functions, such as signal monitoring and

switch–over, under supervision of the duplex controller.

With the MC33215 also a group listening–in application

can be built. For more information on this subject please refer

to application note AN1574.

Base Microphone Amplifier

The base microphone can be capacitively connected to

the circuit via the differential input BM1 and BM2. The setup

is identical to the one for the handset microphone amplifier.

The total transmit voltage gain ABM from the base

microphone inputs to the line is:

+ + ń ) ABM

Vline

VBM

37.5

RSLP 11

x

Zline x

Zline

Zset

Zset

With the typical application and Zline = 600 Ω the transmit

gain calculates as 55 dB.

The electret base microphone can be supplied directly

from VHF but it is advised to use an additional RC filter to

obtain a stable supply point, as shown in the typical

application. The microphone can also be supplied by VMC.

The transmit gain is set by adjusting the sensitivity of the

base microphone by adjusting the resistors RBM1 and RBM2.

It is not advised to adjust the gain by including series

resistors towards the Pins BM1 and BM2.

A high pass filter is introduced by the coupling capacitors

CBM1 and CBM2 in combination with the input impedance. A

low pass filter can be created by putting capacitors in parallel

with the resistors RBM1 and RBM2.

Loudspeaker Amplifier

The loudspeaker amplifier of the MC33215 has three major

benefits over most of the existing speakerphone loudspeaker

amplifiers: it can be supplied and used in a telephone line

powered application but also stand alone, it has an all NPN

bootstrap output stage which provides maximum output

swing under any supply condition, and it includes a

peak–to–peak limiter to limit the distortion at the output.

The loudspeaker amplifier is powered at Pin VLS. In

telephone line powered applications, this pin should be

connected to VCC where most of the line current is available,

see the VCC supply paragraph. In an application where an

external power supply is used, VLS and thus the loudspeaker

amplifier can be powered separately from the rest of the

circuit. The amplifier is grounded to PGD, which is the circuits

power ground shared by both the loudspeaker amplifier and

the current splitter of the VCC supply. Half the supply voltage

of VLS is at BVO, filtered with a capacitor to VLS. This

voltage is used as the reference for the output amplifier.

The receive signal present at RLS can be capacitively

coupled to LSI via the resistor RLSI. The overall gain from

RLS to LSO follows as:

+ + ALS

VLSO

VRLS

–

RLSF

RLSI

x

4.0

In the typical application this leads to a loudspeaker gain

ALS of 26 dB. The above formula follows from the fact that the

overall amplifier architecture from RLS to LSO can be looked

at as an inverting voltage amplifier with an internal current

gain from LSI to LSF of 4. The input LSI is a signal current

summing node which allows other signals to be applied here.

12

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