Circuit monitors quad supply for PCI systems

Roger Zemke, Linear Technology Corp, Milpitas, CA -- EDN, 8/19/1999

Many modern systems operate with multiple power supplies that must meet tight tolerances to accommodate high-end µPs and peripherals. For instance, PCI applications may use as many as four supplies: 5V, 3.3V, and ±12V. The circuit in Figure 1 can monitor all these voltages for an undervoltage condition and can also monitor the three positive voltages for an overvoltage condition. The LTC1536 has a built-in ability to monitor the 3.3 and 5V levels and one adjustable level for undervoltage conditions. The adjustable input of the IC monitors the 12V supply. The LTC1536 also acts as the logic input for the open-drain outputs of the LTC1444 quad comparator.

Comparators A, B, and C of the LTC1444, along with resistors R₁ through R₆, monitor the 3.3, 5, and 12V positive supplies, respectively, for an overvoltage condition. The nominal overvoltage trip points are 3.61, 5.28, and 12.6V. Comparator D, aided by resistors R₇ and R₈, monitors the –12V supply for an undervoltage condition. The nominal trip point for the –12V supply is 10.8V. The open-drain outputs of all the comparators, tied together, connect to the adjustable input of the LTC1536. The resistor divider comprising R₉ and R₁₀ provides a pull-up function for the open-drain outputs. The divider also sets up the 12V undervoltage trip point, which is nominally 11.3V. A Schottky diode tied to the LTC1536's adjustable input can provide another logic input, such as a Power-Good signal. If any undervoltage or overvoltage fault conditions occur, the LTC1536 issues a reset until 200 msec after the fault condition goes away. (DI #2403).