Summary of Missing datasheet details can cause problems
The article describes how overlooked datasheet details in an LT1640 hot-swap controller implementation caused premature MOSFET turn-on during hot insertion, producing an 800 µs conduction that let inrush current sag the system. Reverse engineering and bench testing with -48 V and contact-bounce emulation revealed the failure mode, underscoring the need to understand reference circuits and anticipate deviations when copying datasheet designs.
Parts used in the LT1640 hot-swap reverse-engineering project:
- LT1640 hot-swap controller IC
- Series-pass MOSFET <li-48V power supply
- Resistive-only load
- Grabber clip for contact-bounce emulation
- Input-filter capacitors (load side)
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
For more detail: Missing datasheet details can cause problems
- What problem was discovered when testing the LT1640 circuit?
The MOSFET turned on prematurely during power-up, conducting for 800 µs and allowing harmful inrush current. - How was the failure mode reproduced on the bench?
By connecting to a -48 V power supply with a resistive load and using a grabber clip to emulate metallic contact bounce. - Why is copying a datasheet reference circuit risky?
Because hidden details in the datasheet can be overlooked, and slight deviations can cause unexpected failures in the implemented circuit. - Can premature MOSFET conduction affect the entire system?
Yes; the inrush-current power-suckout can cause a momentary voltage sag that may reset the entire system. - What component primarily limits dV/dt and inrush current in the design?
The series-pass MOSFET in conjunction with the hot-swap controller is intended to slow MOSFET turn-on and limit inrush. - Was the unit sent in likely experiencing the same failure observed on the bench?
Yes; the observed premature turn-on explains how a fan tray could cause a shelf-wide reset during hot insertion.
