What is Power Sequencing? Power Sequencing is the process of controlling the order and timing in which multiple supply voltages (power rails) are applied to (power-up) and removed from (power-down) an integrated circuit (IC) or a printed circuit board (PCB). Most modern, complex digital chips (like FPGAs, CPUs, and DSPs) require several different voltages to operate their various internal blocks (e.g., 1.0V for the core logic, 2.5V for the auxiliary functions, and 3.3V for the I/O pins). Power sequencing ensures these rails are enabled with defined delays and rates to meet the component manufacturer's strict requirements. There are three common types of sequencing: * Sequential: Rails turn on one after the other (e.g., V_Core --> V_Aux --> V_IO). * Ratiometric: Rails start at the same time but are scaled to reach their final voltage simultaneously. * Simultaneous/Coincident: Rails turn on and ramp up at the exact same rate. Why Power Sequencing is Important in PCB and FPGA Design Power sequencing is a mandatory safety and reliability measure for complex digital devices. Improper sequencing can lead to anything from a non-functional board to permanent component destruction. 1. Preventing Damage (Latch-Up) The most critical reason is to prevent latch-up and long-term damage. * The Problem: Almost all ICs have internal protection structures (like ESD diodes) between their power pins. If an I/O pin's voltage rail (V_IO) is applied before the internal core power rail (V_Core), current can flow backward through these protection diodes, powering up the core logic incorrectly. * The Result: This can forward-bias a parasitic SCR (Silicon-Controlled Rectifier) structure, leading to a low-impedance path between the power rails and ground. This state, known as latch-up, causes excessive current draw and often permanently destroys the chip. * The Solution: The manufacturer specifies an order (e.g., V_Core must be stable before V_IO) that prevents this illegal current flow. 2. Controlling Inrush Current When a power rail turns on, all the decoupling capacitors attached to that rail begin charging instantly. * The Problem: If all power rails start simultaneously, the total inrush current drawn from the main power supply (like a wall adapter) can create a massive peak current, potentially tripping power supply protection or causing significant voltage dips (brownouts) across the board. * The Solution: Staggering the power-up sequence spreads the charging events over time, significantly reducing the peak current demand.
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