TCM38C17IDL

QCombo

™ FOUR-CHANNEL (QUAD) PCM COMBO

SLWS040C – JUNE 1996 – REVISED OCTOBER 1999

13

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

system reliability and design considerations

The TCM38C17IDL 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 QCombo is 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

supply voltage drops momentarily below 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 5). It is possible that a QCombo-equipped card with an edge connector can

not be hot inserted into a powered-up system. In this case, 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.

VDD

GND

Figure 5. Latch-Up Protection Diode Connection

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 power.

4.

Force a power down-condition in the device.

5.

Connect the master clock.

6.

Release the power-down condition.

7.

Apply FS synchronization pulses.

8.

Apply the analog signal inputs.

When powering down the device, this procedure should be followed in the reverse order.