Circuit Description

2

Literature Number: SLVAE04

In larger computers several features are provided to prevent such errors: a power-fail interrupt signals

dangerous conditions in time, the content of the memory is protected by a battery back-up, and so on. In

small microcomputer systems this amount of effort is too expensive, and in most applications also not

required. It is usually sufficient if, after a serious voltage drop, the microcomputer is forced into a defined

initial condition. To implement this function, whilst preventing the problems mentioned above, the following

circuit features are required:

− Accurate detection of a serious voltage drop.

− Generation of a reset signal while as the supply voltage is not in the operational

range, to prevent undefined operations of the microcomputer.

− Maintenance of the reset signal for a certain time after the supply voltage has

returned to its nominal value, to ensure proper initialization of the circuit.

For these applications, Texas Instruments has developed a series of integrated circuits which, with a

minimum of external components and without additional adjustment, will fulfill the requirements described

above.

2. Circuit Description

The main part of this circuit is a reference voltage source, which consists of a very stable, temperature-

compensated band gap reference. An external capacitor (typ. 0.1

output V

is divided by a resistor divider and compared with the reference voltage by a comparator. To achieve high

accuracy, this divider is adjusted at wafer probe. When the input voltage is sensed to be lower than the

threshold voltage, the thyristor is triggered, which discharges the timing capacitor C

fire the thyristor via the RESIN input by a logic level (TTL level, active low). The thyristor is turned off again

when either the voltage at the SENSE input (or RESIN input) increases beyond the threshold, or - during

short supply voltage drops - the discharge current of the capacitor becomes lower than the hold current of

the thyristor.

Thereafter, the capacitor is recharged by a current source 100

follows:

t

C

d

t

=

⋅

⋅

1 3 10

4

.

The magnitude of charge current and therefore also the delay t

resistors in the integrated circuit. These tolerances, caused by the semiconductor manufacturing process,

are not negligible. Therefore the delay time may vary -50% to +100 %. However, for the applications

discussed here this will not be a restriction. The diagram in figure 3 shows the typical delay time t

the capacitance of the external capacitor C