HC5513 datasheet(9/20 Pages) RENESAS

9 / 20 page

HC5513

FN3963 Rev.12.00

Page 9 of 20

August 2003

Figure 15 shows the relationship between the saturation guard

voltage, the loop current and the loop resistance. Notice from

Figure 15 that for a loop resistance <1.2k

 (RSG = 21.4k) the

SLIC is operating in the constant current feed region and for

resistances >1.2k

 the SLIC is operating in the resistive feed

region. Operation in the resistive feed region allows long loop

and off-hook transmission by keeping the tip and ring voltages

off the rails. Operation in this region is transparent to the

customer.

The Saturation Guard circuit (Figure 13) monitors the tip to ring

voltage via the transconductance amplifier A1. A1 generates a

current that is proportional to the tip to ring voltage difference. I1

is internally set to sink all of A1’s current until the tip to ring

voltage exceeds 12.5V. When the tip to ring voltage exceeds

12.5V (with no RSG resistor) A1 supplies more current than I1

can sink. When this happens A2 amplifies its input current by a

factor of 12 and the current through R1 becomes the difference

between I2 and the output current from A2. As the current from

A2 increases, the voltage across R1 decreases and the output

voltage on RDC decreases. This results in a corresponding

decrease in the loop current. The RSG pin provides the ability to

increase the saturation guard reference voltage beyond 12.5V.

Equation 3 gives the relationship between the RSG resistor

value and the programmable saturation guard reference

voltage:

where:

VSGREF = Saturation Guard reference voltage.

RSG = Saturation Guard programming resistor.

When the Saturation guard reference voltage is exceeded, the

tip to ring voltage is calculated using Equation 4:

where:

VTR = Voltage differential between tip and ring.

RL = Loop resistance.

For on-hook transmission RL = , Equation 4 reduces to:

The value of RSG should be calculated to allow maximum loop

length operation. This requires that the saturation guard

reference voltage be set as high as possible without clipping

the incoming or outgoing VF signal. A voltage margin of -4V on

tip and -4V on ring, for a total of -8V margin, is recommended

as a general guideline. The value of RSG is calculated using

Equation 6:

where:

VBAT = Battery voltage.

VMARGIN = Recommended value of -8V to allow a maximum

overload level of 3.1VPEAK.

For on-hook transmission, RL = , Equation 6 reduces to:

SLIC in the Standby Mode

Overall system power is saved by configuring the SLIC in the

standby state when not in use. In the standby state the tip and

ring amplifiers are disabled and internal resistors are connected

between tip to ground and ring to VBAT. This connection enables

a loop current to flow when the phone goes off-hook. The loop

current detector then detects this current and the SLIC is

configured in the active mode for voice transmission. The loop

current in standby state is calculated as follows:

where:

IL = Loop current in the standby state.

RL = Loop resistance.

VBAT = Battery voltage.

(AC) Transmission Path

SLIC in the Active Mode

Figure 16 shows a simplified AC transmission model. Circuit

analysis yields the following design equations:

LOOP CURRENT (mA)

VBAT = -24V, RSG = 

VBAT = -48V, RSG = 21.4k

SATURATION GUARD

RESISTIVE FEED

REGION

CONSTANT CURRENT

FEED REGION

RRSG = 21.4k

100k



100k



1.5k



700



<400



<1.2k



RRSG = 

VOLTAGE, VTR = 38V

SATURATION GUARD

VOLTAGE, VTR = 13V

FIGURE 15. VTR vs IL AND RL

VSGREF

12.5 510



RSG

------------------

(EQ. 3)

VTR

16.66 5 10



RSG



RL RDC1 RDC2

 600



----------------------------------------------------------------------



(EQ. 4)

VTR

16.66 510



RSG

------------------

(EQ. 5)

RSG

510



VBAT VMARGIN

–



RDC1 RDC2



600RL

------------------------------------------







16.66V

–



--------------------------------------------------------------------------------------------------------------------------------------------------

(EQ. 6)

RSG

510



VBAT VMARGIN 16.66V

–

----------------------------------------------------------------------------

(EQ. 7)

VBAT 3V

–

RL 1800

--------------------------------



(EQ. 8)

VTR

VTX IM 2RF



(EQ. 9)

VTX

-----------

VRX

ZRX

-----------

1000

-------------

(EQ. 10)

VTR

EG IM ZL



–

(EQ. 11)

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