ISL5585_04 Datasheet, PDF(9/24 Page) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband (VOB)

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ISL5585_04 Datasheet, PDF (9/24 Pages) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband (VOB)

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ISL5585

Design Equations

Switch Hook Detect

The switch hook detect threshold is set by a single external

resistor, RSH. Equation 1 is used to calculate the value of RSH.

RSH = 600 â ISH

(EQ. 1)

The term ISH is the desired DC loop current threshold. The

loop current threshold programming range is from 5mA to

15mA (40kâ¦ < RSH<120kâ¦).

Ground Key Detect

The ground key detector senses a DC current imbalance

between the Tip and Ring terminals when the ring terminal is

connected to ground. The ground key detect threshold is not

externally programmable and is internally fixed to 12mA

regardless of the switch hook threshold.

Ring Trip Detect

The ring trip detect threshold is set by a single external

resistor, RRT. IRT should be set between the peak ringing

current and the peak off hook current while still ringing.

RRT = 1800 â IRT

(EQ. 2)

In addition, the ring trip current must be set below the

transient current limit, including tolerances. The capacitor

CRT, in parallel with RRT, will set the ring trip response time.

Loop Current Limit

The loop current limit of the device is programmed by the

external resistor RIL. The value of RIL can be calculated

using Equation 3.

RIL

1----7---6----0-

ILIM

(EQ. 3)

The term ILIM is the desired loop current limit. The loop

current limit programming range is from 15mA to 45mA

(39kâ¦ < RIL<117kâ¦).

Transient Current Limit

The drive current capability of the output tip and ring

amplifiers is programmed by an external resistor RTL. This

output current limit is separate from the DC loop current

limit function. The current limit circuit works in both the

source and sink direction, with an internally fixed offset to

prevent the current limit functions from turning on

simultaneously. The current limit function is provided by

sensing line current and reducing the voltage drive to the

load when the externally set threshold is exceeded, hence

forcing a constant source or sink current.

Transient Source Current Programming

The source current is externally programmed as shown in

Equation 4.

RTL

-1---6---5----0--

ISRC

(EQ. 4)

For example a source current limit setting of 95mA is

programmed with a 18.7kâ¦ (RTL) resistor connected from the

TL pin of the device to ground. This setting determines the

maximum amount of current which flows from Tip to Ring

during an off hook event until the DC loop current limit

responds. In addition this setting also determines the amount

of current which will flow from Tip or Ring when external

battery faults occur.

Transient Sink Current Programming

The sink current limit is internally offset 20% higher than the

externally programmed source current limit setting.

ISNK = 1.20 Ã ISRC

(EQ. 5)

If the source current limit is set to 95mA, the sink current limit

will be 114mA. This setting will determine the maximum current

that flows into Tip or Ring when external ground faults occur.

Understanding Transient Current Limit

Each tip and ring amplifier is designed to limit source current

and sink current. Figure 1 shows the functionality of the

circuit for the case of limiting the source current. A similar

diagram applies to the sink current limit with current polarity

changed accordingly.

During normal operation, the error current (IERR) is zero and

the output voltage is determined by the signal current (ISIG)

multiplied by the 200K feedback resistor. With the current

polarity as shown for ISIG, the output voltage moves

positivewith respect to half battery. Assuming the amplifier

output is driving a load at a more negative potential, the

amplifier output will source current.

IO/K

200K

IERR

IREF = 1.21/TL

TIP or RING

ISIG

VB/2

FIGURE 1. CURRENT LIMIT FUNCTIONAL DIAGRAM

During excessive output source current flow, the scaled

output current (IO/K) exceeds the reference current (IREF)

forcing an error current (IERR). With the polarity as shown

the error current subtracts from the signal current, which

reduces the amplifier output voltage. By reducing the output

voltage the source current to the load is decreased and the

output current is limited.

Setting the Proper Transient Current Limit

Since this feature programs the maximum output current of

the device, the setting must be high enough to allow for

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