MH88511 datasheet(3/10 Pages) MITEL

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Preliminary Information

MH88510/11

Functional Description

The BORSH Functions

The MH88510/11 performs all of the Borsh functions

of Battery Feed, Overvoltage Protection, Ringing,

Supervision and Hybrid (2-2 Wire).

Battery Feed

The MH88510/11 powers the telephone set with

constant

loop

current

for

shortlines

and

automatically reverts to constant voltage for long

lines. Since the Tip drive is powered down from the

VDD supply, loop current flows through both the VBAT

supply and the VDD supply.

Overvoltage Protection

The MH88510/11 is protected from short term (20ms)

transients (+250V) between Tip and Ring, Tip and

ground, and Ring and Ground. However additional

protection circuitry may be needed depending on the

requirements which must be met. Normally, simple

external shunt protection as shown in Figures 5 and

6 is all that is required.

Ringing

The ringing insertion circuitry has the capability to

provide ringing voltage to the telephone set by

simply adding an external relay, ring generator and a

200

Ω limiting resistor. The internal relay driver

switches ringing voltage on to the line via the

external ring relay. The SLIC provides two internal

300W battery feed resistors through which the

ringing current will flow. A clamp diode is included

which suppresses voltage transients during relay

switching caused by the relay coil. In addition, the

circuit prevents connection of the ringing source

during off-hook conditions. See figure 5 for typical

application.

Hybrid

The 2-2 Wire hybrid circuit converts the incoming

balanced signal at Tip and Ring of the telephone line

into a ground referenced output signal at JUNC of

the SLIC, and converts the ground referenced input

signal at JUNC of the SLIC into a non-balanced

output signal at Tip and Ring of the telephone line.

Line Impedance

The MH88510/11’s Tip-Ring(Zin) impedance is fixed

at 600

Ω. For correct SLIC impedance, JUNC must

be appropriately terminated. See AC Electrical

Characteristics.

Supervision

The loop detection circuit determines whether a low

enough impedance is across Tip and Ring to be

recognised as an Off-Hook condition. When an off-

hook condition occurs, the SHK and LED outputs

toggle to a logic low level. These outputs also toggle

during incoming dial pulses.

Figure 3 - SLIC Crosspoint Switch Connection

MH88510/11

TIP

RING

TIP

RING

TIP

RING

TIP

RING

JUNC

Ω

CROSSPOINT

SWITCH

CROSSPOINT

SWITCH

SLIC 1

SLIC 2

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