Safely swap SCSI disk drives

By Ajmal Godil, Linear Technology Corp, Milpitas, CA -- EDN, 9/30/1999

When you insert an SCA2 (single-connector attach) drive into a live backplane, the power supply's bypass capacitors in the drive can draw huge transient currents from the backplane's power bus as they charge. The transient currents may cause permanent damage to the connector pins and glitches in the system supply, thereby causing other boards in the system to reset. A viable option for solving this problem is to slowly increase the supply voltage as the drive mates with the backplane. According to the SFF8046 specification, an SCA2-drive connector should have a 5V precharge pin, allowing precharging to occur before the voltage pins make contact (Reference 1, Figure 1). A simple method for providing the precharge voltage is to use a power resistor from the 5V line to the precharge pin. However, after the precharge cycle completes and before the 5V pin mates with the precharge pin, some drives draw as much as 1A of load current. The voltage drop across the power resistor prevents a full precharge and results in a glitch in the 5V backplane supply when the power pin finally mates.

Figure 2 shows a solution to the 5V-precharge problem, using an LTC1422 hot-swap controller and using an LT1490 as a comparator. The output capacitor, C₁, charges to 4.3V (5V–V_DIODE) through R₂ and D₁. IC_2A's output, Pin 1, is low, and Pin 7 is high. When you insert the drive and it mates with the 5V precharge-drive 1 pin, the drive pulls the voltage on C₁ to ground. This action forces IC_2A's Pin 1 to switch high, thereby turning on the LTC1422. The load current then starts ramping up. At approximately 1A, the voltage drop across R₁ is sufficient to trip IC_2B's output, Pin 2, low, thus keeping the LTC1422's On pin high. Q₁'s source voltage rises to 5V, and the cathode of D₃ (5V precharge-drive 1 pin) rises to 4.7V. Reducing or increasing the value of C₂ can shorten or lengthen the output-voltage turn-on time, respectively. When you fully insert the drive, the 5V line mates with the 5V precharge-drive 1 pin and reverse biases D₃, allowing the output voltage to jump from 4.7 to 5V. The load current through Q₁ drops to zero, thereby toggling IC_2B's Pin 7 high and Pin 1 to a low state. This action shuts off the gate driver in the LTC1422, and the chip powers down. The output on C₁ falls to 4.3V, and, as soon as Drive 2 mates with the 5V precharge-drive 2 pin, the LTC1422 powers up, providing a controlled ramp-up output voltage. The process repeats for drives 3, 4, and so on. (DI #2419).

REFERENCE

1.SFF Committee, SFF8046 specification for 80-pin SCA-2 connector for SCSI disk drives, Revision 2.7, Oct 3, 1996.