Application examples

8

Literature Number: SLVAE04

In applications where this performance is not required, sensitivity can be reduced by placing an R-C filter in

front of SENSE input. To avoid a unacceptable change of the threshold voltage of circuits with a fixed

threshold voltage (TL7705, TL7709, TL7712, TL7715) the value of the resistor should be a few 10

In application like this it may be better to use the TL7702A. With this circuit the threshold voltage can be

set to the desired value by a high impedance voltage monitor. A small filter capacitor C

input reduces the trigger sensitivity (figure 8).

In some applications it is necessary to keep the output of the reset circuit active even if no supply voltage

is applied. Under normal conditions the output transistor of the TL77xxA is turned off (inactive state), when

the supply voltage is lower than 3 V (V

the TL77xxB). In figure 9 a P-channel field effect

transistor is connected to the RESET output. This transistor conducts when the supply voltage drops below

3 V i.e. at a gate source voltage of -3 V or less. To ensure that the transistor is switched off when the

supply voltage has reached the nominal value, the gate has to be at least 6V more positive than the

source (or the required high level e.g. 2.4 V). In figure 9 therefore the voltage monitor is supplied by supply

voltage of 12 V. Since the requirements for this supply voltage in terms of stability are not high, this supply

voltage for example can be taken from the filter capacitor in front of the voltage regulator in the power

supply.

Figure 9. Modified Output Circuit

A further application for these integrated circuits is in battery-buffered memory systems. When the line

voltage fails, the content of the memory must not be corrupted by a random write operation of the

microcomputer. These uncontrolled write operations may take place at low supply voltage even if a reset

signal is applied to the processor. Generally, it is sufficient to switch the chip select line into the inactive

state (some memories require that the write line also be disabled). A switch, which consists of transistor Q

1

and diode D

present) the output of the TL7705B is turned off (high); the transistor Q

transistor Q

microprocessor, the transistor conducts and the CS input of the memory goes low and the memory is

enabled. Because of the small DC load of the resistor R