TLC770X Datasheet, PDF (5/27 Pages) Texas Instruments – BiCMOS SUPPLY VOLTAGE SUPERVISORS

1. Introduction
This report describes the function and application of the supply voltage supervisor circuit TLC770X. Besides a
detailed functional description, the behavior of line and battery operated power supplies during power-up as well as
the behavior of digital circuits is presented. Finally application-examples of supply voltage supervision in general
and supply voltage supervision in systems are described.
1.1 RESET Circuits
In order to ensure the reliable operation of a digital system, it is important that the circuits of which it is composed
should have a clearly defined initial state. With microcomputers and microprocessors, this initial state is
implemented with a reset signal, which may for example be applied to the Reset input. This ensures that the system
is only switched into an active state when the supply voltage has reached its nominal value. In the same way, a
break down of the supply voltage affects the operation of integrated circuits and can result in their faulty operation.
For example, with CMOS circuits, a reduction of the operating voltage results in a slowing down of the circuit. This in
turn affects the internal timing of the circuitry, and consequently the operation of the complete system.
In order to avoid such disturbances, a circuit is needed which will generate a defined Reset signal. The simplest way
of implementing this is with the help of an RC network at the Reset input (Figure 1.)
Figure 1. RESET circuit
The voltage at the RESET input rises with a delay determined by the time constant t = R x C, until the threshold
value of the Reset input has been reached. At this point, the system is switched into an active state.
This method of generating the Reset pulse is however not very reliable, since the Reset time depends on the
boundary conditions of the power supply and of the complete system.
The Reset circuit must keep the resetting input of the system in an active state until the supply voltage has reached
its nominal value. After the full supply voltage has been reached, the Reset signal must be kept applied for a certain
time, which depends on the resetting time of the system.
The Reset time in the above circuit is however very dependent on the rise time of the supply voltage. For this
reason, it can not be guaranteed that the Reset pulse will be applied long enough to bring the system into a defined
initial state.
This circuit is not able to recognize minor disturbances of the supply voltage below its minimum permissible value.
The Reset input of the microcomputer is only able to recognize a Reset signal when the voltage across the capacitor
is below the threshold voltage V_t of the input stage of the microcomputer. The circuit therefore only operates
correctly when the supply voltage falls to at least the value V_t − V_f . With a TTL input stage, the value of V_t is about
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