L7581

Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. These are absolute stress ratings only. Functional operation of the device is not implied at these or any other conditions in excess of those given in the operational sections of the data sheet. Exposure to absolute maximum ratings for extended periods can adversely affect device reliability.

The L7581 Ringing Access Switch is a monolithic solid-state device that provides the switching func-tionality of a 2 form C switch.

The L7581 is designed to provide power ringing access to tip and ring in central office, digital loop carrier, private branch exchange, digitally added main line, and hybrid fiber coax/fiber-in-the-loop ana-log line card applications. The L7581 has three states: the idle talk state (line break switches closed, ringing access switches open), the power ringing state (line break switches open, ringing access switches closed), and an all OFF state.

The L7581 offers break-before-make or make-before-break switching, with simple logic level input control. Because of the solid-state construction, voltage tran-sients generated when switching into an inductive ringing load during ring cadence or ring trip are mini-mized, possibly eliminating the need for external zero cross switching circuitry. State control is via logic level inputs, so no additional driver circuitry is required.

The line break switch is a linear switch that has exceptionally low ON-resistance and an excellent ON-resistance matching characteristic. The ringing access switch has a breakdown voltage rating >480 V which is sufficiently high, with proper protec-tion, to prevent breakdown in the presence of a tran-sient fault condition (i.e., passing the transient on to the ringing generator).

Incorporated into the L7581Axx is a diode bridge/SCR clamping circuit, current-limiting circuitry, and a thermal shutdown mechanism to provide protection to the SLIC device and subsequent circuitry during fault conditions (see Figure 1). Positive and negative lightning is reduced by the current-limiting circuitry and steered to ground via diodes and the integrated SCR. Power cross is also reduced by the current-limiting and thermal shutdown circuits.

The L7581Bxx version provides only an integrated diode bridge along with current limiting and thermal shutdown, as shown in Figure 2. This will cause pos-itive faults to be directed to ground and negative faults to battery. In either polarity, faults are reduced by the current-limit and/or thermal shutdown mecha-nisms.