SCTS005B - OCTOBER 1984 - REVISED DECEMBER 1990
The TCM1520A is a monolithic ring-detection integrated circuit designed for use in isolated or nonisolated telephone applications. The device uses a modified form of the Texas Instruments BIDFET
During standby, the input impedance is approximately 1 M or greater, which prevents any interference with parallel off-hook telephones transmitting DTMF or voice frequencies. The device achieves this high input impedance with an on-chip series zener diode that does not conduct until the voltage across AC INPUT (pins 1 and 8) exceeds 8 V. When the voltage across pins 1 and
8 exceeds 18 V, the internal switch is closed, which bypasses the 6.8-V zener diode series resistor. This allows more efficient power transfer to the load when the device is in the operating mode. In the operating mode, the impedance of the device varies from 30 k to 7 k over the ring signal of 40 V at 16 Hz to 150 V at 68 Hz and is reasonably independent of the output load.
In typical telephone applications, the TCM1520A is activated through the telephone line by a ring voltage of 40V at 16 Hz to 150 V at 68 Hz. The TCM1520A generates a signal suitable to drive an optocoupler or TTL, NMOS, or CMOS logic. The 5-V OUTPUT can be used as a supply source for optocouplers or low-power logic. This output is noninverting and is at a high level during ringing.
The TCM1520A incorporates lightning and transient protection that is designed to withstand lightning strikes of 1.5-kV amplitude and 200-us duration and also features built-in circuitry to avoid tapping or false triggering due to transients.
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