Summary
Figure 15. Series Resistor at the Ct input of the TL770xB
To a small degree this resistor influences the delay time t. In most applications however this effect can be
d
neglected, because - when determining the length of the reset pulse - a large reserve is taken into
account.
When designing a supply voltage supervision circuit often an analysis of the noise on the supply lines is
required. The probes of the oscilloscope usually used for this kind of measurement mostly are not capable
of performing the measurements correctly. The main reason is the ground wire attached to the probe. This
wire often acts as an antenna which receives all the noise generated by the surrounding circuits. This
leads to a wrong display on the oscilloscope screen. More accurate results are found, when the signal to
be measured is taken via a 0.1mF capacitor (to block the DC voltage) directly soldered to the point of
measurement. The other end of the capacitor is connected to a coaxcable, whose shield is connected by
the shortest path to the next ground reference point. The coaxcable leads to the input of the oscilloscope,
where the cable has to be terminated correctly to avoid line reflections.
5. Summary
Monitoring of the supply voltage is absolutely mandatory to guarantee a correct initialization of the circuit
and to detect undefined operating condition e.g. an undervoltage. This report shows several application
examples for the supply voltage supervisor circuits of the series TL77xxA. Owing their very accurate
threshold voltage, they can be easily designed into systems, replacing expensive discrete circuits .
Supply voltage Supervisor TL77xx Series
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TI [ TEXAS INSTRUMENTS ]
