QUAD POWER FAULT MONITOR
Application Information
Setting the Fault Tolerance Window
The fault tolerance window is set by applying a voltage less than the +2.50Vreference to the Lower
Threshold input (Pin 1). The voltage is obtained by a resistor divider from the reference (Pin 3) to ground. If
±5% tolerance is desired, then 95% of the reference (+2.375V) is applied to Pin 1. If ±40% is wanted, then
60% of the reference (+1.50V) is applied. In the example on the back page, the tolerance is ±5%. The
nominal overvoltage and undervoltage thresholds are centered about the reference at +2.625V and +2.375V
(+2.500V ±0.125V).
Scaling the Monitored Supply Voltages
Each positive voltage to be monitored is divided down to +2.50V with a resistor network and connected to
one of the Sense inputs. Unused Sense inputs should be connected to the reference. This will not increase
the bias current. A variation of the monitored voltages out of the programmed tolerance range will cause the
appropriate overvoltage or undervoltage fault output to switch LOW. The effective tolerance on any input
may be broadened with an additional resistor to the voltage reference. The example on the back page
shows a ±10% tolerance on the +5Vsupply although the SG1548 is programmed for a ±5% tolerance. The
procedure for calculating the resistor value is found in the SG1548 Application Note.
Monitoring a Negative Voltage
A negative voltage can be converted to a positive one and simultaneously scaled to +2.50V by using the
internal operational amplifier as an inverter. Only an input resistor and feedback resistor are required.
Setting the Fault Delay
A single capacitor at the Delay pin sets the time an out-of tolerance fault must persist before a fault is
actually declared. This feature allows switching noise on the supplies to be rejected. The delay time is given
by: Delay = 25ms/µF.
AC Line Monitoring
The AC line voltage can be monitored for single-cycle dropouts with the few components shown in the
example. A half-wave rectifier charges the capacitor on positive line cycles. After the positive peak and
during the negative line cycle the capacitor discharges from a fixed voltage controlled by the internal Zener
diode. If a positive cycle is missing, the capacitor discharges to below the +2.5V trip point of the comparator,
causing the output transistor to turn on.
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