Texas Instruments TPS3801, Отчет о приложении

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TPS3801/09 – Smallest SVS for Monitoring DSPs and Processors
Bernd Kornmeier
ABSTRACT
This application report describes the TPS3801 and TPS3809 supply voltage supervisor
(SVS) families of devices. The report gives a general introduction to reset generators
followed by an overview of the technical parameters and the special features of the
TPS380x. Each feature is discussed separately. Measurements make it easy to
understand SVS principles of operation. Typical applications that increase system
reliability are included. Layout considerations and design issues help in the system
integration of the TPS3801 and TPS3809.
1 Introduction
After power-on, a digital system must be forced into a definite initial state. Digital
signal processors (DSPs), microcontrollers, and microprocessors have a reset
input for this purpose. In the simple application shown in Figure 1, an RC network
is connected to the RESET input to deliver the necessary reset pulse. After
switching on the power supply, this circuit keeps the logic level at the RESET input
low for a time determined by the capacitor and resistor values. This delay time,
required by the system to complete the initialization, allows the power supply to
become stable.
However, this circuit has some disadvantages. In cases of brief reductions of the
supply voltage, it does not work correctly. It does not recognize spikes, because
the voltage at the RESET input does not decrease as fast as the supply voltage
due to the capacitor. Malfunction is thus possible. Only if V
DD
decreases by more
than 0.7 V, the capacitor can be discharged quickly through the diode. Because
of tolerances of discrete components (especially capacitors), the delay time can
not be calculated very precisely.
The problem is that this solution does not deliver a proper low to high transition
at reset input. It is a charge curve of a capacitor, which is for a certain period of
time undefined between low and high.
VCC
RESET
GND
VDD
VRES
Figure 1. A Simple Reset Solution