The sequencing requirements can vary depending on the particular FPGA being used, and many newer FPGAs specify no sequencing is required. While this is technically true for the FPGA, it is not the optimal way to design a power solution.

National offers several devices to address sequencing requirements. The LM3880 is designed to address sequential sequencing of multiple supply rails. This device is available in a small SOT-23 package and can sequence up to three supply rails.

Figure 1. Simplified Buck Converter Schematic

Many options are available to control the up-and-down, three-flag outputs sequencing timing. National also provides devices to support customized flag order and timing. Figure 1 illustrates a typical application circuit for the LM3880.

Voltage tracking is another method of sequencing power supplies applicable to FPGAs and many processors. The most common, and generally recommended, method to power up FPGAs and other processors is to have the CORE voltage track the I/O voltage during startup as shown in Figure 2.

Figure 2. Startup voltage tracking

This power up technique is known as simultaneous startup, and its primary advantage is that it avoids turning on any parasitic conduction paths that may exist between the CORE and IO supply rails. Turning on a parasitic conduction path may lead to unreliable startup or even damage to the FPGA or DSP.

Figure 3. Typical voltage tracking configuration

Some of National's devices that feature voltage tracking include the LM20k family of high performance synchronous DC/DC converters, as well as the LM3743 controller. Figure 3 illustrates a typical voltage tracking configuration for these devices.