Application examples
Figure 6. Voltage Supervision of a multiple power supply
In larger systems, where several supply voltages are required, it is necessary to supervise all supply
voltages which may cause dangerous conditions in case of power failure. In the circuit diagram of figure 6,
two TL7712A's are used to monitor the positive and the negative 12 volt supplies. Their outputs are fed to
the RESIN input of the TL7705A, which monitors the 5 volt supply. The output of this device provides a
reset signal, which becomes active whenever any one of the three supply voltages fails. A reset signal can
be generated manually via a switch which is connected to the voltage monitor of the positive 12V supply.
When designing a supply voltage monitor the designer has to take care, that when only one of the supply
voltages becomes marginal or fails, a defined reset signal is generated (at least if the main supply voltage
5 V is still available). Therefore the circuit which monitors the 12V supply voltage is supplied by the 5V
supply. The reset signal of the circuit which monitors the negative supply is fed via a resistor divider to the
base of the transistor BC546, which controls the RESIN input of the TL7705A. The voltage divider is
designed so that a reset is generated even if the negative supply fails totally.
The capacitor which determines the delay of the two circuits which monitors the both 12v supplies, can be
chosen short (in the example shown here it is 0.01mF). The output of these circuits has only to trigger the
third monitor TL7705A. The final duration of the reset signal will be determined by the capacitor Cof the
t
last mentioned circuit.
These supply voltage supervisor circuits were designed to detect supply voltage drops as short as
>300 ns. In figure 7 the minimum pulse width tdmin at the SENSE input is shown versus the amplitude of the
6
Literature Number: SLVAE04
TI [ TEXAS INSTRUMENTS ]
