Electronic Design

Hot Swap Higher Voltage Supplies

Some applications, such as communications, fault-tolerant computer systems, and networking, require circuit boards to be inserted into a backplane without turning the power off. The bulk supply capacitors on the circuit board can draw huge transient currents from the backplane power bus as they charge. These transient currents can permanently damage the connector pins and produce glitches on the system supply, causing other boards in the system to reset.

Figure 1 shows a circuit that allows a printed-circuit board to be safely inserted into or removed from a live backplane. It’s based on U1, an LTC 1422 hot-swap controller, which is designed to ramp a board’s supply voltage on and off in a controlled manner. U1 is designed for 2.7-V to 12-V applications, but in some cases it may be necessary to control a higher supply voltage. The circuit in Figure 1 extends U1’s supply range to 18V to 48V.

As soon as power is applied, D1 clamps the input voltage seen by the VCC pin and the GND pin to 10 V. U1’s GATE pin is initially low and is connected to the source of p-channel MOSFET Q3. The gate of Q3 is connected to VIN, which is higher in potential than the source (VIN − 10 V), causing Q3 to remain off. When the voltage on the FB pin is less than 1.232 V, the —RESET pin (the output of the internal comparator) will be pulled low. Because the FB pin is tied to the GATE pin, which is low, the —RESET pin is low. This turns Q2 and Q4 on, pulling MOSFET Q1’s gate to ground.

Once the ON pin has been high for at least one timing cycle (t = 1.232 × C2/2 mA), the internal charge pump is turned on. Also, the voltage at the GATE pin begin to rise with a slope equal to 10 mA/C1. As the voltage on the GATE pin rises to 1.232 V above U1’s ground (VIN − 10 V), the voltage on the FB pin also rises, causing the —RESET pin to go high, turning Q2 and Q4 off. In addition, the source voltage of Q3 rises to VIN+10 V, which is greater than the gate voltage (VIN). Therefore, Q3 turns on and C1 ramps up the gate voltage of MOSFET Q1. As Q1’s gate voltage rises, the source voltage follows and turns on the MOSFET in a controlled fashion. Figure 2a shows the turn-on for the 18-V supply, while Figure 2b indicates the turn-on for the 48-V supply.

Placing a sense resistor, R1 (0.2 Ω), between the VCC pin and the SENSE pin allows the circuit breaker to be tripped whenever the voltage across the sense resistor is greater than 50 V for more than 10 ms. This corresponds to a load current of about 2.5 A (50 mV/0.02 Ω).

When the circuit breaker trips, the GATE pin is immediately pulled low, turning Q3 off and turning Q2 and Q4 on via the —RESET pin. This pulls the gate of the MOSFET Q1 low and turns Q1 off. Figure 3a shows the turn-off for the 18-V supply and Figure 3b shows the turn-off for the 48-V supply.

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