Conventional PCI and PCI-X computer slots have four supply voltages available: +12, +5, +3.3, and 12 V. PCI and PCI-X cards rely on +5 V and +3.3 V for most of the power and are limited to 500 mA of load current on the +12-V supply. This standard was defined when the majority of digital ICs operated from +5- or +3.3-V power.
But today's high-speed chips operate from +2.5 V, +1.8 V, or even lower supply voltages. Therefore, many PCI and PCI-X cards contain step-down converters to generate these lower-voltage supplies. The newly standardized PCI Express computer bus has only two supply voltages, +12 V and +3.3 V, and the standard allows for up to 5.5 A from the +12-V supply to drive dc-dc converters.
PCI hot-plug power ICs, such as the TPS2340, TPS2341, and TPS2342, contain power FETs for +12 V. But these power FETs are limited to 500 mA. Using the simple circuit shown in the figure, you can boost the drive from these power ICs and maintain current limiting.
In this circuit, the power FET for +12 V in the hot-plug power IC isn't used. Instead, the gate drive to that power FET is used to drive an external power FET (Q1). A current-sensing resistor (RSENSE), combined with a low-cost PNP transistor array (U1), senses load current and drives an unused current-sense channel of the power IC.
Because the base-emitter voltages of the two PNP transistors in U1 are close in value, the voltage dropped across RSENSE is replicated across R1. This produces current in the left transistor of U1 approximately equal to:
IE = (ILOAD × RSENSE)/R1
The current produces a voltage drop across the current-sense inputs of U2 (5VSx and 5VISx) approximately equal to:
V5VSx 5VISx = (R3 × ILOAD × RSENSE)/R1
With the values shown, this circuit will limit +12-V load current to approximately 5.1 A, which allows for 3 A of inrush current in an eight-lane PCI Express slot.
As an added benefit, this circuit senses the +12-V load voltage and will shut down the slot if load voltage drops out of tolerance while the slot is operating. The exact slot shutdown voltage is determined by R4, R5, and the 5VISx threshold of U2 and is approximately:
VSD = 4.65 × [1 + (R5/R4)]
With the values shown, this circuit will shut down if the +12-V load voltage drops below approximately 10 V while the slot is powered.
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