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MH88620BR

C.O. SLIC

Mitel Networks 

Preliminary Information

MH88620BR

Applications

As shown in the application diagram, Figure 7, the

ringing voltage, typically 80 VRMS 25Hz biased at

-VBAT, is applied to the subscriber line through an

external relay, K1, K1 is a DPST 2-form-C, EM

(electro-mechanical) relay which switches protection

diodes D3 and D4 out when Ringing is switched in,

K1 is enabled by applying a logic low level to the

relay driver control input, RNGC.

An additional relay driver is provided to control an In/

Out Test Bus relay, K2. K2 is a DPST 2-form-C, EM

relay which is enabled by applying a logic low level

to the relay driver control input.

Protection Circuitry

Primary protection, from lightning strikes and AC line

faults, is normally located in the MDF (main

distribution feeder) which is located external to the

PABX or CO switching system. The primary

protection circuitry is normally housed in a 5-pin

connector and consists of either carbon blocks, with

spark gaps (older technology), gas discharge limits

the high voltage to approximately 300 to 500 volts

before entering the switching system. Secondary

protection, in the switching system, is required to

further limit these high voltages/ currents. Secondary

protection is normally implemented on each line card

and is designed to protect the SLICs from permanent

damage. The basic secondary high voltage

protection circuitry for the MH88620BR, as illustrated

in Figure 7, consists of PTC1, PTC2 and clamping

diodes D1 to D4. During a fault condition, the diodes

clamp the overvolt Ground and -VBAT. PTC1 and

PTC2 current limit as their resistance increases with

power dissipation caused by the over-voltage/

over-current condition. The ground that D1 and D3

are connected to, must be an EDG (energy dumping

ground) which is connected to the chassis or system

ground. This is a seperate conductor from LPGND or

AGND on the line card PCB. D2 and D4 conduct the

energy into a -VBAT supply which is a seperate

conductor from the -VBAT feed supply to the SLICs.

A power MOSFET circuit as shown in Figure 8, can

be used to divert the energy normally dumped into

-VBAT, the EDG conductor. Usually one MOSFET

circuit can be used for 16 SLICs or per line card.

Depending on the additional level of protection

required, PRO1 and/or PRO2 protectors may be

used. These are used to protect the SLICs Ring

sense resistor and/or Ring generator, from being

damaged if a fault condition occurs during the

application of Ringing to the line. PRO2 can be

implemented using two back to back zener diodes,

or an equivalent transcient suppressor.

The clamping voltage should be >16 Vdc and

<26Vdc. PRO2 may not be required depending on

the value and power dissipation of PRO1.

MH88620BR

R

Internal

8500 Ω

24

Z2

23

Z1

MH88620BR

24

R

Internal

8500Ω

Z2

23

Z1

External

0.22µF

MH88620BR

24

Z2

R

Internal

8500Ω Z1

R

External

340Ω

A.

B.

C.

Notes

a) to accommodate the use of 2 x 25Ω PTCs, connect Z1 and Z2 together, Zin = 900Ω.

b) to accommodate the use of 2 x 8Ω PTCs, connect 340Ω between Z1 and Z2 = 900Ω.

c) to accommodate the use of 2 x 25Ω PTCs, connect 0.22µF between Z1 and Z2 = 900Ω + 2.2µF.

Figure 3 - Input Impedance (Zin) setting

2-153

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