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 .
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.
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.
On startup, after the undervoltage (UV) input is satisfied, capacitor C1 is slowly charged by a 45 µA current source from the LT1640. Any event that requires turning off the pass MOSFET Q1 causes the LT1640 GATE pin to discharge C1 and the MOSFET CGS with a 50 mA current sink. This is clearly explained in the data sheet electrical specifications.
The customer's unit included a small capacitor between the UV pin and VEE , as suggested in the datasheet.
Note that there is no discharge path for C1 other than the LT1640 gate current sink. When the fan tray is removed from the shelf, it loses VDD and the LT1640 can no longer sink current. With sufficient capacitance at VDD and C1 at 150 nF, this is not a problem. The LT1640 should discharge C1 in its last dying gasp (unfortunately, this aspect is not discussed in the datasheet). So, the original designer of the customer's product included only an EMI input filter with minimal capacitance.
To verify my failure mode assumption, I measured the Q1 gate-source voltage before and after power-up (Figure 4 ), note the oscilloscope ground is now moved to the MOSFET source because I really hate trying to think in terms of negative voltages.
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