TCM29C23_12 Datasheet, PDF (15/20 Pages) Texas Instruments – VARIABLE-FREQUENCY PCM OR DSP INTERFACE

TCM29C23, TCM129C23
VARIABLE-FREQUENCY PCM OR DSP INTERFACE
SCTS029B − AUGUST 1989 − REVISED NOVEMBER 1996
PRINCIPLES OF OPERATION
system reliability and design considerations
TCM29C23, TCM129C23 system reliability and design considerations are described in the following
paragraphs.
latch-up
Latch-up is possible in all CMOS devices. It is caused by the firing of a parasitic SCR that is present due to the
inherent nature of CMOS. When a latch-up occurs, the device draws excessive amounts of current and will
continue to draw heavy current until power is removed. Latch-up can result in permanent damage to the device
if supply current to the device is not limited.
Even though the TCM29C23 and TCM129C23 are heavily protected against latch-up, it is still possible to cause
latch-up under certain conditions in which excess current is forced into or out of one or more terminals. Latch-up
can occur when the positive supply voltage drops momentarily below ground, when the negative supply voltage
rises momentarily above ground, or possibly if a signal is applied to a terminal after power has been applied
but before the ground is connected. This can happen if the device is hot-inserted into a card with the power
applied, or if the device is mounted on a card that has an edge connector and the card is hot-inserted into a
system with the power on.
To help ensure that latch-up does not occur, it is considered good design practice to connect a reverse-biased
Schottky diode (with a forward voltage drop of less than or equal to 0.4 V − 1N5711 or equivalent) between the
power supply and GND (see Figure 7). If it is possible that a TCM29C23- or TCM129C23-equipped card that
has an edge connector could be hot-inserted into a powered-up system, it is also important to ensure that the
ground edge connector traces are longer than the power and signal traces so that the card ground is always
the first to make contact.
device power-up sequence
Latch-up also can occur if a signal source is connected without the device being properly grounded. A signal
applied to one terminal could then find a ground through another signal terminal on the device. To ensure proper
operation of the device and as a safeguard against this sort of latch-up, it is recommended that the following
power-up sequence always be used:
1. Ensure that no signals are applied to the device before the power-up sequence is complete.
2. Connect GND.
3. Apply V_BB (most negative voltage).
4. Apply V_CC (most positive voltage).
5. Force a power-down condition in the device.
6. Connect clocks.
7. Release the power down condition.
8. Apply FS synchronization pulses.
9. Apply the signal inputs.
When powering down the device, this procedure should be followed in the reverse order.
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