ISL5585ECM datasheet(9/23 Pages) RENESAS

9 / 23 page

ISL5585

FN6026 Rev 6.00

Page 9 of 23

October 21, 2004

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.

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

loop current threshold programming range is from 5mA to

15mA (40k

 < RSH<120k).

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.

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.

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

limit programming range is from 15mA to 45mA

(39k

 < RIL<117k).

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.

For example a source current limit setting of 95mA is

programmed with a 18.7k

 (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.

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.

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 detection

of ring trip or programmed off hook loop current, whichever is

greater.

RSH

600 ISH



(EQ. 1)

RRT

1800 IRT



(EQ. 2)

RIL

1760

ILIM

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

(EQ. 3)

RTL

1650

ISRC

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

(EQ. 4)

ISNK

1.20 ISRC



(EQ. 5)

FIGURE 1. CURRENT LIMIT FUNCTIONAL DIAGRAM

TIP or RING

IREF = 1.21/TL

IO/K

IERR

ISIG

200K

VB/2

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