It is often said that "the devil is in the details." All too often those details are hidden deep within a datasheet where you can easily overlook them. When a datasheet reference circuit is copied into a product, the designer must still be fully aware of how the circuit functions and anticipate unexpected problems that might arise from slight deviations.
Take a recent case of an LT1640 hot-swap controller IC, often used in a hot-plug telecom fan tray. I was asked to reverse-engineer this so our technicians would know how to power it on the bench without a using a chassis. Nothing complicated about it, just the usual slow turn-on of a pass MOSFET in series with the load, thereby slowing the dV/dt and limiting the inrush current to the load input-filter capacitors.
After drawing some schematics, I connected it to my -48V power supply and a resistive-only load, hit it a few times with a grabber clip at -48V to emulate true metallic contact bounce, and saw the nasty little surprise shown in Figure 1.
Figure 1. What caused this negative-going glitch in a hot-swap circuit?
(Click on image to enlarge)
For a circuit whose main purpose is to prevent sudden surges upon power-up, this one failed miserably. Now what?
Well, maybe that's why the customer sent this unit in for repair. An inrush-current power-suckout on hot-insertion can cause a momentary voltage sag that results in an entire system reset. I could easily imagine how a technician plugged in this fan tray and the entire shelf came crashing down. There must be a problem with the fan tray, right?
Unfortunately, because we engineers are such experts in fault-fixing, our esteemed-and-mighty management does not require our customers to include such mundane details as actually describing the failure mode of whatever they send in for repair. So, we were forced to guess.
A close-up of the premature MOSFET turn-on is shown in Figure 2. On power-up the series-pass MOSFET is conducting for 800 µs, plenty of time to wreak havoc on the rest of the system.
Figure 2. A MOSFET conducts for 800 µs (low-going part of blue trace). That's enough to cause a system reset. (Click on image to enlarge)
It so happened that this card included an identical and totally isolated slow-start circuit for the usual redundant second -48V supply. It too failed in exactly the same way.
Figure 3 shows the recommended slow-start circuit copied from the Linear Technology LT1640 Hot Swap Controller datasheet.
Figure 3. The hot-swap circuit published by Linear Technology in its LT1640 datasheet.