SI3232 datasheet(21/128 Pages) SILABS

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Si3232

Preliminary Rev. 0.96

Figure 7. DC Linefeed Overhead Voltages

(Forward State)

4.3.1. Calculating Overhead Voltages

The two programmable overhead voltages, VOV and

VCM, represent one portion of the total voltage between

VBAT and ground as illustrated in Figure 7. In normal

operating conditions, these overhead voltages are

sufficiently low to maintain the desired TIP-RING

voltage, VOC. There are, however, certain conditions

under which the user must exercise care in providing a

battery supply with enough amplitude to supply the

required TIP-RING voltage as well as enough margin to

accommodate these overhead voltages. The VCM

voltage is programmed for a given operating condition.

Therefore, the open-circuit voltage, VOC, varies

according to the required overhead voltage, VOV, and

the supplied battery voltage, VBAT. The user should pay

special attention to the maximum VOV and VCM that

might be required for each operating state.

In the off-hook active state, sufficient VOC must be

maintained to correctly power the phone from the

battery supply that has been provided. Since the battery

supply depends on the state of the input supply (i.e.,

charging, discharging, or battery backup mode), the

user must decide how much loop current is required and

determine the maximum loop impedance that can be

driven based on the battery supply provided.

The minimum battery supply required can be calculated

according to the following equation.

VCM and VOV are provided in Table 8.

The default VCM value of 3 V provides sufficient

overhead for a 3.1 dBm signal into a 600

Ω loop

impedance.

A VOV value of 4 V provides sufficient headroom to

source a maximum ILOOP of 45 mA along with a

3.1 dBm audio signal and an ABIAS setting of 16 mA.

For a typical operating condition, VBAT = –56 V and

ILIM =22mA:

VOC,MAX =56V – (3 V + 4V)=49 V

These conditions apply when the dc-sensing inputs,

STIPDCa/b and SRINGDCa/b, are placed on the SLIC

side of any protection resistance placed in series with

the TIP and RING leads. If line-side sensing is desired,

both VOV and VCM must be increased by a voltage

equal to RPROT x ILIM where RPROT is the value of each

protection resistor. Other safety precautions may apply.

See "4.7.3. Linefeed Overhead Voltage Considerations

During Ringing" on page 40 for details on calculating the

overhead voltage during the ringing state.

The Si3232 uses both voltage and current information to

control TIP and RING. Sense resistor RDC (see

Figure 6) measures dc line voltages on TIP and RING;

capacitor CAC couples the ac line voltages on the TIP

and RING leads to be measured. The Si3232 uses the

Si3200 linefeed interface IC to drive TIP and RING and

to isolate the high-voltage line from the low-voltage

Si3232.

The Si3232 measures voltage at various nodes to

monitor the linefeed current. RDC and RBAT provide

these measuring points. The sense circuitry is

calibrated on-chip to ensure measurement accuracy.

See "4.4. Linefeed Calibration" on page 25 for details.

Constant I Region

Constant V Region

LOOP

BATH

TIP

RING

BATL

Secondary V

BAT

Selected

Loop Closure Threshold

VBAT VOC VCM VOV

≥

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