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TND302 Hot-Swapping Boards Facilitated by Low Cost Analog Switches
Prepared by: Fred Zlotnick ON Semiconductor http://onsemi.com
TECHNICAL NOTE
Many devices need to be capable of "hot-swapping". Telecomm and networking designers often need for their boards to be inserted or removed from an active operating system. When hot-swapping, often times it is one or two I/O lines that are causing a problem, or as many as 32 or more lines. Many devices have stringent requirements on current usage, sequencing of power supplies, test program modes, etc. One issue arises from the ESD protection that most CMOS families employ. Most CMOS devices have a protection diode that goes from its input or output to supply voltage. An example might be the use of an FPGA that is tied to a standard "HC04" gate, and the gate is on a card to be hot-swapped. Figure 1 shows the input configuration of many CMOS devices. When inserting a board, normally ground (GND) and VCC are established first. As the board is plugged in, VCC is momentarily at ground potential. Since there is almost assuredly a capacitor across the VCC terminal to GND, there will be a delay until the capacitor reaches close to VCC. When the board is inserted, the input is now shorted to GND via its ESD diode, causing either a glitch on the line, or possible latch-up of the driver.
Standard CMOS Input VCC
almost zero delay into the path. It provides a low cost addition to the circuit, and protects the devices from seeing glitches. The device is available in a single form as the MC74VHC1G66DFT2, and a quad version as the MC74LVX4066DT. Both devices insert only a few Ohms of series resistance (<20) and provide a delay of <1 ns, when enabled. The solution enables hot-swapping where the designer can choose a delay time. The device is bilateral so the signal can flow in either direction, just like the wire that connects it. Other than the small voltage drop and slight delay previously mentioned, the device is transparent, except at the time of start up. Example: an FPGA or DSP or MCU needs to be connected to a module and one line needs to function as a hot swap. Inserting a single 5-pin part, with its control gate connected to a VCC pin that represents known Power Up, or is simply a pass through to the module supply voltage. The
MC74VHC1G66 Hot-Swap Module R Pull-Up GND C FPGA, DSP, MCU, etc.
Figure 2. Connection to an FPGA, DSP or MCU
Figure 1. Input Configuration for a CMOS Device
There are many solutions to this problem, however one of the simplest and most elegant solutions provides a great deal of flexibility and control in the circuit. The ON Semicon- ductor MC74VHC1G66DFT2 takes up only 4.5 mm2 of board space, consumes very nearly zero power, and inserts
R/C time constant creates a delay, from a few ms, to perhaps 500 ms. When the control pin reaches threshold, the device turns on and becomes a transparent low resistance, near zero delay connection to the sensitive device. If the user has need for more than 1 line of isolation, 2 devices may be used, hooking all the control pins in parallel. A quad device with similar characteristics, is available as an MC74LVX4066 and multiple quad devices may be used for 8- or 16-pins of isolation. The VCC pin should be connected to the same power supply as the device it is protecting. This solution is simple, low cost and allows for almost any contingency in the future.
(c) Semiconductor Components Industries, LLC, 2001
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July, 2001 - Rev. 0
Publication Order Number: TND302/D
TND302
Notes
ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. "Typical" parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer.
PUBLICATION ORDERING INFORMATION
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TND302/D

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