23

gate of the transistor Q. As long as the

RESET output is Low, the transistor Q will remain non conducting, and no

the transistor Q conducts and energizes the relay REL.

The output circuit is protected with a clamp diode, in order to eliminate safely the energy stored in the coil of the

relay REL when the transistor is switched off.

5.2 Applications in Systems

An additional potential application of the TLC770X is in battery backed memory systems (Figure 21.).

Figure 21.

Voltage monitoring in a battery backed memory system

follows.

The microcomputer controls the input CONTROL of the TLC7705, by means of the signal

CSH1. Whenever the

microcontroller makes access to the memory, the output

CSH1 becomes Low, and it selects the memory ( CS = Low)

via the path CONTROL-RESET of the TLC7705 (see Figure 5.).

Should there be a failure of the supply voltage, the memory must be isolated from the microcontroller, so than there

can be no uncontrolled write and read accesses to the memory. Therefore, whenever the supply voltage falls below

the permissible nominal value, the memory is put into an inactive state (

CS = High) by means of the output RESET.

To ensure that in such a case there will be no loss of data in the memory, the latter, together with the monitoring

circuit, are supplied by the battery should there be a failure of the supply voltage. In this situation the very low

current consumption of the TLC7705 is an advantage, since this results in a very long life for the battery. The

CONTROL input is switched off in this state, so that undefined levels at the microcontroller output

CSH1 will not

result in an increase of the current consumption (see Section 3).

5.3 Application Hints

When using the TLC770X, the following precautions must be taken in order to ensure that a RESET circuit operates

absolutely correctly.