MC33215 Просмотр технического описания (PDF) - Motorola => Freescale

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MC33215

Telephone Line Interface and Speakerphone Circuit

Motorola => Freescale 

Freescale SMeCm3i3c2o15nductor, Inc.

from Pin REG by the internal circuit (the 10 µA term in the

formulas). This built–in feature drops the line voltage and

therefore enables parallel operation.

The voltage over the line driver has to be limited to 12 V to

protect the device. A zener of 11 V at VLN is therefore the

maximum advised.

VDD Supply

The internal circuitry for the line driver and handset

interface is powered via VDD. This pin may also be used to

power peripherals like a dialer or microcontroller. The voltage

at VDD is not internally regulated and is a direct result of the

line voltage setting and the current consumption at VDD

+ ǒ ) Ǔ internally (IVDD) and externally (IPER). It follows that:

VDD VLN – IVDD IPER x Rset

For correct operation, it must be ensured that VDD is

biased at 1.8 V higher than SLP. This translates to a

maximum allowable voltage drop across ZVDD of

Vzener – 1.8 V. In the typical application, this results in a

maximum allowable current consumption by the peripherals

of 2.0 mA.

VMC Supply

At VMC, a stabilized voltage of 1.75 V is available for

powering the handset microphone. Due to this stabilized

supply, microphones with a low supply rejection can be used

which reduces system costs. In order to support the parallel

operation of the telephone set, the voltage at VMC will be

maintained even at very low line currents down to 4.0 mA.

Under normal supply conditions of line currents of 20 mA

and above, the supply VMC is able to deliver a guaranteed

minimum of 1.0 mA. However, for lower line currents, the

supply capability of VMC will decrease.

Figure 5. VMC Under Different Microphone Loads

1.8

1.7

1.6

Iline = 4.0 mA

Iline = 20 mA

2.7 k VMC–VHF

1.5

1.4

Iline = 4.0 mA

1.3

1.2

1.1

1.0

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

IVMC (mA)

If, during parallel operation, a high current is required from

VMC, a 2.7 k resistor between VMC and VHF can be applied.

In Figure 5, the VMC voltage under different microphone

currents, is shown.

VHF Supply

VHF is a stabilized supply which powers the internal

duplex controller part of the MC33215, and which is also

meant to power the base microphone or other peripherals.

The base microphone however, can also be connected to

VMC, which is preferred in case of microphones with a poor

supply rejection. Another possibility is to create an additional

filter at VHF, like is shown in the typical application. The

supply capability of VHF is guaranteed as 2.0 mA for line

currents of 20 mA and greater.

Since in parallel operation not enough line current is

available to power a loudspeaker and thus having a

speakerphone working, the current internally supplied to VHF

is cut around 10 mA of line current to save current for the

handset operated part. A small hysteresis is built in to avoid

system oscillations.

When the current to VHF is cut, the voltage at VHF will

drop rapidly due to the internal consumption of 1.4 mA and

the consumption of the peripherals. When VHF drops below

2.0 V, the device internally switches to the handset mode,

neglecting the state of the speakerphone select Pin SPS.

In case an application contains a battery pack or if it is

mains supplied, speakerphone operation becomes possible

under all line current conditions. In order to avoid switch–over

to handset operation below the 10 mA, VHF has to be

supplied by this additional power source and preferably kept

above 2.4 V.

VCC Supply

At VCC the major part of the line current is available for

powering the loudspeaker amplifier and peripheral circuitry.

This supply pin should be looked at as a current source since

the voltage on VCC is not stabilized and depends on the

instantaneous line voltage and the current to and consumed

from VCC.

The maximum portion of the line current which is available

+ ǒ ǒ ǓǓ at VCC is given by the following relation:

IVCC

10

11

x

Iline – IVDD

– IVMC – IVHF

This formula is valid when the voltage drop from VLN to

VCC is sufficient for the current splitter to conduct all this

current to VCC. When the drop is not sufficient, the current

source saturates and the surplus of current is conducted to

the power ground PGD to avoid distortion in the line driver. In

fact, when no current is drawn from VCC, the voltage at VCC

will increase until the current splitter is in balance. In Figure 6

this behavior is depicted.

MOTOROLA ANALOG IC DEVICEFDoArTAMore Information On This Product,

9

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