ISL5585ECM datasheet(10/23 Pages) RENESAS

10 / 23 page

ISL5585

FN6026 Rev 6.00

Page 10 of 23

October 21, 2004

To allow for proper ring trip operation, the transient current limit

setting should be set at least 25% higher than the peak ring trip

current setting. Setting the transient current 25% higher should

account for programming tolerances of both the ring trip

threshold and the transient current limit.

If loop current is larger than ring trip current (low REN

applications) then the transient current limit should be set at

least 35% higher than the loop current setting. The slightly

higher offset accounts for the slope of the loop current limit

function.

Attention to detail should be exercised when programming the

transient current limit setting. If ring trip detect does not occur

while ringing, then re-examine the transient current limit and

ring trip threshold settings.

DC Loop Feed

The feedback mechanism for monitoring the DC portion of the

loop current is the loop detector. A low pass filter is used in the

feedback to block voice band signals from interfering with the

loop current limit function. The pole of the low pass filter is set

by the external capacitor CDC. The value of the external

capacitor should be 4.7

F, 6.3V rated polarized or non-

polarized capacitor.

Most applications will operate the device from low battery while

off hook. The DC feed characteristic of the device will drive Tip

and Ring towards half battery to regulate the DC loop current.

For light loads or Long Loops, Tip will be near

-4V and Ring will be near VVBL + 5V. Figure 2 shows the DC

feed characteristic in terms of tip to ring voltage and loop

current.

The point on the y-axis labeled VTR(OC) is the open circuit Tip

to Ring voltage and is defined by the feed battery voltage in

Equation 6.

Figure 3 illustrates the actual loop current for a given set of

loop conditions. The loop conditions are determined by the low

battery voltage and the DC loop impedance. The DC loop

impedance is the sum of the protection resistance, copper

resistance (ohms/foot) and the telephone off hook DC

resistance.

The slope of the feed characteristic and the battery voltage

define the maximum loop current on the shortest possible loop

as the short circuit current ISC.

The term ILIM is the programmed current limit, 1760/RIL. The

line segment IA represents the constant current region of the

loop current limit function.

Process variations in the ISL5585 effect the ILIM and 11.11k

slope in Equation 8. All units are tested with: a 300

 load

across tip and ring, VBAT=-24V and ILIM set to 25mA. Equation

8 can be used to predict the ideal current at this setting

(25.76mA). All units are tested to be within ±8.5% of this ideal

value (23.57mA to 27.95mA).

The maximum loop impedance for a programmed loop current

is defined as RKNEE.

When RKNEE is exceeded, the device will transition from

constant current feed to constant voltage or resistive feed.

The line segment IB represents the resistive feed portion of the

load characteristic

Impedance Matching

The impedance of the device is programmed with the external

component RS. RS is the gain setting resistor for the Transmit

Amplifier that provides impedance matching. If complex

impedance matching is required, then a complex network can

be substituted for RS.

The feedback mechanism for monitoring the AC portion of the

loop current consists of two amplifiers, the Sense Amplifier

(SA) and the Transmit Amplifier (TA). The AC feedback signal

is used for impedance synthesis. A detailed model of the AC

feed back loop is shown in Figure 4.

FIGURE 2. DC FEED CHARACTERISTIC

m = (

VTR/IL) = 11.1k

ILOOP (mA)

ILIM

VTR(OC)

LONG LOOP

SHORT LOOP

VTR OC



VBL 9

–

(EQ. 6)

FIGURE 3. ILOOP vs. RLOOP LOAD CHARACTERISTIC

RLOOP )

RKNEE

ILIM

ISC

2RP

LONG LOOP

SHORT LOOP

m=11.11k

m=Vtr(oc)/Rloop

CONSTANT CURRENT

CONSTANT VOLTAGE

RESISTIVE FEED

ISC

ILIM

VTR OC

 2RPILIM

–

11.11k

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

(EQ. 7)

IA ILIM

VTR OC

 RLOOPILIM

–

11.11k

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

(EQ. 8)

RKNEE

VTR OC



ILIM

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

(EQ. 9)

VTR OC



RLOOP

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

(EQ. 10)

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