Building a simple power supply for an FPGA

Tag : DC Power Supply


Modular IC systems such that come from Linear Technology'sµModule family are a good solution for simple and compactlow-voltage, high-current power supply requirements. We recentlyprovided power management support to one designer, who asked forupwards of 40 amps at 1.5 volts to power four FPGAs in the smallestboard area possible and the highest efficiency to minimizecomponent overheating. Such a solution requires very low profile toallow efficient airflow and to prevent thermal shadow onsurrounding ICs; current-sharing capability to spread the heatevenly to eliminate hot spots and minimize or eliminate the needfor heat sinks; and a surface-mount package. The µModule andproducts of that kind ideally include a DC/DC controller, MOSFETs,inductor, capacitors, and compensation circuitry for a quick andeasy solution. They minimize the number of fans, or fans speed, aswell as the number of heat sinks. As a result, they're lower incost.

Figure 1 shows a test board for the (up to) 48-amp, 1.5-volt systemusing the LTM4601, which includes the PWM controller, inductor,input and output capacitors, ultralow R DS(ON), FETs, Schottky diodes and compensation circuitry. Each blacksquare is a complete DC/DC circuit and is housed in a 15-by-15mm-by-2.8mm (high) package. Each LTM4601 circuit can deliver 12amps from a 4.5- to 20-volt input. The pin-compatible LTM4601HVextends the input range to 28 volts. Figure 1: Each of the four µModules in this current-sharingDC/DC power system occupies just 15-by-15 mm of real estate. Eachchip weighs just 1.8 g and has an IC form-factor that's suited forany pick-and-place machine.

Another significant advantage of these modular designs is theirability to easily scale up as loads increase; simply add modules inparallel. The board design of such parallel systems involves littlemore than duplicating the layout of each µModule. Electricallayout issues are taken care of within the package; there are fewexternal components to worry about. The more advanced modular ICsalso include output voltage tracking and margining. The highswitching frequency (typically 850 kHz at full load), constanton-time, zero-latency controller has fast transient response toline and load changes. Should frequency harmonics be a concern, anexternal clock can control synchronization via an on-chipphase-lock loop. Figure 2: Simply add regulators in parallel to achieve higheroutput current.

Figure 2 shows the schematic, which comprises four LTM4601s inparallel. The synchronized regulators operate 90 degrees out ofphase, thereby reducing the amplitude of input and output ripplecurrents through cancellation (Fig. 3). The attenuated ripple inturn cuts the external capacitor RMS current rating and sizerequirements, further reducing solution cost and board space.

Synchronization and phase shifting is implemented via a LTC6902oscillator, which provides four clock outputs, each phase shifted90 degrees (for 2- or 3-phase relationships, the LTC6902 can beadjusted via a resistor). By operating such modules out of phase,you minimize peak input and output current by approximately 20percent, depending on the duty cycle (see LTM4601 data sheet). Thisreduction, in turn, reduces the requirement for input and outputcapacitance.

The clock signals serve as the input to the PLLIN (phase-lockedloop in) pins of the four LTM4601s. The LTM4601's phase-locked loopcomprises a phase detector and a voltage controlled oscillator,which combine to lock onto the rising edge of an external clockwith a frequency range of 850 kHz. The phase-locked loop turns onwhen a pulse of at least 400 ns at 2 volts is detected at the PLLINpin (although it is disabled during start-up). Figure 3 shows theswitching waveforms of four LTM4601 µModule regulators inparallel.

Figure 3: Each DC/DC module operates 90 degrees out of phase, thusreducing input and output ripple.

Only one resistor is required to set the output voltage. In aparallel setup, the value of the resistor depends on the number ofLTM4601s used. As such, the effective value of the top (internal)feedback resistor changes as you parallel LTM4601s.