A modern high-end FPGA — an Intel Agilex or AMD/Xilinx Versal — requires five or more separate power rails supplying its core logic, transceivers, PLL circuitry, I/O banks, and configuration memory. These rails must be activated in a specific order determined by the device manufacturer and must reach their target voltages within defined timing windows relative to one another. Violating this sequence — even briefly, even during the first power-up of a prototype — can forward-bias internal ESD structures, latch up the device, or permanently damage the oxide layers of core transistors. Complex digital systems like FPGAs require PMICs that manage this specific order and timing to prevent damage to sensitive internal logic.
Why Sequencing Is Mandatory: The Physics of Multi-Rail Startup
FPGAs contain multiple functional blocks — core logic, high-speed serial transceivers, PLLs, I/O banks — each with different voltage requirements and internal circuit structures. The core logic may operate at 0.8V, the transceiver power at 1.8V, and the I/O bank at 3.3V. During power-up, if a higher voltage rail reaches its target before the core supply, the voltage difference can forward-bias ESD clamp diodes between I/O pins and the core supply rail, injecting current into the core domain with no well-defined path to ground. This parasitic injection triggers latch-up — a destructive positive feedback condition in CMOS circuits where a parasitic thyristor structure turns on and conducts unlimited current until power is removed or the device is destroyed.
The device manufacturer's power sequencing requirement is not a conservative guideline — it is a derived specification based on the actual circuit structures inside the device and the failure modes associated with each sequencing violation. In FPGA transceivers, the analog PLL and clock distribution circuits require their voltage supply to be stable before the digital configuration logic begins loading bitstream data. Starting configuration before the PLL supply is settled results in clock jitter that corrupts the loaded configuration, potentially leaving the device in an undefined state.
Three Sequencing Strategies: Sequential, Ratiometric, and Simultaneous
Sequential timing is the most intuitive approach: Rail 1 is enabled and must reach regulation before Rail 2 is enabled; Rail 2 must reach regulation before Rail 3 is enabled. A power sequencer IC monitors each rail with a window comparator and enables the next rail's converter only when the preceding rail passes its voltage threshold within a specified timeout window. If any rail fails to reach regulation — due to a short circuit, current limiting, or component failure — the sequencer halts and asserts a fault signal, making sequential timing the easiest approach for fault isolation.
Ratiometric timing starts all rails simultaneously but controls their rise time so they reach regulation at the same moment, maintaining a fixed voltage ratio between rails throughout the ramp. This approach is preferred when the device specification requires the I/O and core supplies to track each other during ramp — preventing the I/O supply from exceeding the core supply at any point during startup. Simultaneous start uses the same enable signal for all rails and relies on each converter's natural soft-start characteristic to ramp at a rate determined by its soft-start capacitor, accepting that different rails will reach regulation at different times without enforcing a tracking ratio.
Implementation: Dedicated Sequencers and PMIC Integration
Power sequencing can be implemented with a dedicated power sequencer IC — a device containing multiple comparators, timers, and logic gates specifically designed for this function — or with a multi-output PMIC that integrates the sequencing logic alongside the voltage converters. Dedicated sequencers from vendors like Texas Instruments, Analog Devices, and Renesas provide the most flexibility: programmable enable/disable sequences, configurable timeout windows, fault masking, and I²C or PMBus interfaces for runtime monitoring. They are preferred in complex systems with more than four rails or where the sequence must be modified during system bring-up.
For simpler FPGA systems, many PMICs targeting FPGA applications integrate a fixed or semi-configurable sequencing engine alongside three to six DC-DC converters. A single PMIC can supply all rails from a 12V or 5V input, apply a manufacturer-validated sequencing scheme loaded from an OTP (one-time programmable) or I²C-configurable register set, and monitor all rails simultaneously. The integration reduces BOM count, board space, and the number of discrete components that must be individually validated — at the cost of less flexibility when changing sequence or adding a rail.
Published by Beamed Silicon Intelligence. Analysis reflects publicly available information as of publication date. Nothing herein constitutes investment advice.