In applications that require wholeboard power cycling or actual board “hot-swapping,” maintaining a highimpedance interface at power-off as well as the power-up and power-down cycles becomes a critical design problem. Ideally, system designers can create a high-impedance, low-capacitance interface to minimize any possible disruptions of system data resulting from inserting or removing a board.
At the heart of the circuit shown, which meets all of these design criteria, is the Fairchild Switch FST6800. It provides 10 bits of high-speed CMOS TTL-compatible bus switching. The low on-resistance (4 Ω) of the switch allows inputs to be connected to outputs without adding propagation delay or generating additional ground-bounce noise.
The FST6800 precharges the B-Port to a selectable bias voltage (VBIAS) to minimize live-insertion noise. The FST6800 is organized as a 10-bit flowthrough switch with an active low —OE control pin. The —OE control pin, when driven to the active low state, connects the A and B Ports together.
The bias voltage is set via a 3.6-V Zener diode stack and resistors. This circuit draws about 10 mA of current and has sufficient capacity to power several FST6800 VBIAS networks. In cases where the backplane VCC and board VCC are separated and the FST6800 must be connected to the board or local VCC supply, the diode stack on the VBIAS pin will prevent sourcing the local VCC supply to the backplane VCC.
The —OE voltage is set by either a pull-up resistor or on-board control logic. A simple pass gate shifts control between the two selections. Pass-gate control can include provisions for board-maintenance functions, low VCC, and board-level RESET.
When the —OE voltage is pulled up to a valid VIH, the NC7SZ125 singlegate-buffer will drive an “OK to Remove” LED on the board.
In applications where the board VCC and the backplane VCC aren’t hardwired together, the FST6800 should be connected to the backplane VCC. This ensures that the backplane VCC and the board VCC won’t be coupled together through the FST6800’s internal VBIAS circuits.