MH88610 데이터 시트보기 (PDF) - Mitel Networks

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MH88610

Subscriber Line Interface Circuit (SLIC)

Mitel Networks 

Preliminary Information

MH88610

Functional Description

The MH88610 performs a transformerless 2-wire to

4-wire conversion of the analog signal. The 2-wire

circuit is the balanced line going to the subscriber

loop, while the 4-wire circuit is the audio signal going

to an from devices such as the voice codec or

switching circuit. The SLIC also provides two status

signals. switch hook (SHK) and loop length (Loop).

The Loop signal is an analog voltage which is

proportional to the loop length and the SHK goes low

when the telephone set is off-hook.

Constant Current Feed

The MH88610 employs a complex feedback circuit to

supply a constant feed current to the line. This is

done by sensing the sum of the voltages across the

internal feed resistors and comparing it to an input

reference voltage (VRef) that determines the

constant feed current. This gives the loop current as:

ILoop = 25 - (VRef) mA (+2 mA)

1.25

Switch Hook Detection

When the DC current exceeds an internal threshold

level, the switch hook (SHK) will go low. If the loop

resistance is so high that VBat can no longer supply

the required amount of loop current as determined

by constant current supply circuit, the output of the

switch hook (SHK) will go high impedance (open

collector output) to indicate that the loop resistance

is too high and the line is on hook.

Ringing And Ring Trip Detection

In Figure 3 a ringing signal (e.g., 90Vrms=+/-48Vdc)

is applied to the line by disconnecting pin 15 (RV)

from pin 11(RF), and connecting the ringing voltage

at pin 11 (RF) by use of the relay K1.

The SLIC can detect an off-hook condition during

ringing but there is a large AC component which

must be filtered out to give a true off-hook condition

at SHK.

A 1.0µF capacitor connected from pin 19 (CAP) to

ground will provide adequate attenuation when

ringing is applied. Once an off-hook condition has

been detected an external circuit will deactivate the

relay (K1) to disconnect the ringing voltage from pin

11 and reconnect to pin 15. At that time the SLIC will

revert to constant current feed operation.

For applications requiring Dial Pulse Detection the

Capacitor connection to ground should be controlled

such that the capacitor is only connected during

ringing.

During off-hook conditions (closed loop), the

capacitor should be switched out. This can be

performed using a transistor, relay or system drive

output of a codec. Applying GNDA to the Ring Trip

Filter Control pin will switch in the filter, whilst

removing GNDA, (with the switch in a tristate

condition), will switch out the filter.

For applications using DTMF signalling, the

capacitor can be permanently connected to ground.

Current Limit

The Tip or Ring may accidentally short to ground. In

such a case, current will only flow through the feed

resistor. This high current will be sensed and

reduced by the current limit circuit to a lower value to

protect the internal circuitry.

Components List

R1

=Resistor 1/4w, + 5%, 30.0kΩ

R2

=Resistor 1/4w, + 5%, 750kΩ

R3

=Resistor 1/4w, + 5%, 3.9kΩ

R4

=Resistor 1/4w, + 5%, 4.7kΩ

R5

=Resistor 1/4w, + 5%, 43kΩ

R6

=Resistor 1/4w, + 5%, 470kΩ

RV1*, RV2* =Var. 160 VRMS, 250V,10J

D1*, D3*, D4* =Diode Rect., 200V, 1A IN4003

D2

=Diode Rect., 200V, 1A IN4003

D5

=Diode Sw., 75V, 02A, 0.5W IN4148

Q

=NPN, 2N2222 or equivalent

K1

=Relay E/M, 12V, 1 form C

C1*,C2*,C3*,C4*= Cap. Cer., 1.0µF, 50V, 20%

C5

=Cap. Elec., 1.0µF, 16V 10%

Note: Components marked * are optional for extra protection of

the hybrid.

2-99

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