1.3 Soft Start and Monotonic Ramp Requirements

Implementing a soft start function is critical when powering FPGAs, as it ensures a gradual increase in supply voltage. This controlled ramp-up prevents excessive inrush current, which could otherwise damage the FPGA, power supply components, or PCB traces. Due to the substantial decoupling capacitance typically present in FPGA designs, an abrupt application of voltage can overstress regulators and cause undesirable voltage overshoots.

The soft start feature also enables the FPGA’s power-on reset (POR) circuitry to detect stable voltage levels, ensuring proper device initialization and reducing the risk of configuration errors. Additionally, it helps prevent unintended activation of brown-out detection mechanisms, which could result in repeated resets or operational instability. Soft start is particularly important for protecting sensitive analog subsystems within the FPGA, such as high-speed transceivers and phase-locked loops (PLLs), from voltage transients and overshoots, thereby supporting reliable performance.

A monotonic ramp during power-up is equally essential. A monotonic ramp ensures that the supply voltage increases smoothly and continuously, without any dips or oscillations (see Figure 1-2). This is crucial for preventing power-up oscillations and current surges, which can lead to unstable operation or potential damage to the FPGA. If the voltage fluctuates or oscillates during ramp-up, the FPGA’s internal logic circuits may misinterpret the power state, resulting in improper initialization or the device entering an undefined state.

To avoid these issues, it is important to follow the FPGA data sheet’s start-up guidelines. Integrators should incorporate soft start circuits, utilize DC-DC converters with controlled ramp-up features, and implement appropriate input/output filtering to minimize noise and prevent oscillations during both ramp-up and ramp-down phases.

When two voltage rails are ramped simultaneously to different target levels, there is a risk of sequence timing violations at the target device. The time delay between the two rails may result in a timing discrepancy that falls outside the specified guidelines, leading to an incorrect power-up sequence. In such cases, a ratiometric ramp approach is recommended. This method ensures that both power rails reach their target voltages simultaneously, starting with the rail that has the lower voltage (see Figure 1-3). This coordinated ramp-up helps maintain proper power sequencing, which is critical for system stability and reliable operation.