Importance

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Abstract

A dual-supply line-interface circuit (100) uses a -48V power supply (V.sub.BAT1) to drive long subscriber loops (120) and uses a -28V power supply (V.sub.BAT2) to drive short subscriber loops. For intermediate-length loops, a dual-slope current-feed profile (FIG. 4) is employed to limit the line-circuit's power dissipation. The line-interface circuit operates in an apparent constant-current mode, generating about 40 mA of differential line current using the low power supply, up to a threshold line voltage of about 25V, which is equal to the low power supply voltage minus required overhead. For longer loops, the line-interface circuit switches to a second constant-current mode, generating about 22 mA of differential current using the high power supply, which maintains the loop current constant until it drops to the 48V resistive-feed value.

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First Claim