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Tip of the Week: Simple power supply for FPGAs (Part 1)

Posted: 14 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FPGA power supply? MOSFETs? power management? uModule family?

By Alan Chern, Afshin Odabaee
Linear Technology

Modular IC systems such that come from Linear Technology's ?Module family are a good solution for simple and compact low-voltage, high-current power supply requirements. We recently provided power management support to one designer, who asked for upwards of 40A at 1.5V to power four FPGAs in the smallest board area possible and the highest efficiency to minimize component overheating. Such a solution requires very low profile to allow efficient airflow and to prevent thermal shadow on surrounding ICs; current-sharing capability to spread the heat evenly to eliminate hot spots and minimize or eliminate the need for heat sinks; and a surface-mount package. The ?Module and products of that kind ideally include a DC/DC controller, MOSFETs, inductor, capacitors, and compensation circuitry for a quick and easy solution. They minimize the number of fans, or fans speed, as well as the number of heat sinks. As a result, they're lower in cost.

Figure 1 shows a test board for the (up to) 48A, 1.5V system using the LTM4601, which includes the PWM controller, inductor, input and output capacitors, ultralow Rds(on), FETs, Schottky diodes and compensation circuitry. Each black square is a complete DC/DC circuit and is housed in a 15mm x 15mm x 2.8mm (high) package. Each LTM4601 circuit can deliver 12A from a 4.5V to 20V input. The pin-compatible LTM4601HV extends the input range to 28V.

Figure 1: Each of the four ?Modules in this current-sharing DC/DC power system occupies just 15mm x 15mm of real estate. Each chip weighs just 1.8g and has an IC form-factor that's suited for any pick-and-place machine.

Another significant advantage of these modular designs is their ability to easily scale up as loads increase; simply add modules in parallel. The board design of such parallel systems involves little more than duplicating the layout of each ?Module. Electrical layout issues are taken care of within the package; there are few external components to worry about. The more advanced modular ICs also include output voltage tracking and margining. The high switching frequency (typically 850kHz at full load), constant on-time, zero-latency controller has fast transient response to line and load changes. Should frequency harmonics be a concern, an external clock can control synchronization via an on-chip phase-lock loop.

Figure 2: Simply add regulators in parallel to achieve higher output current.

Figure 2 shows the schematic, which comprises four LTM4601s in parallel. The synchronized regulators operate 90 out of phase, thus reducing the amplitude of input and output ripple currents through cancellation (Figure 3). The attenuated ripple in turn cuts the external capacitor RMS current rating and size requirements, further reducing solution cost and board space.

Synchronization and phase shifting is implemented via a LTC6902 oscillator, which provides four clock outputs, each phase shifted 90 (for 2- or 3-phase relationships, the LTC6902 can be adjusted via a resistor). By operating such modules out of phase, you minimize peak input and output current by approximately 20 percent, depending on the duty cycle (see LTM4601 data sheet). This reduction, in turn, reduces the requirement for input and output capacitance.

The clock signals serve as the input to the PLLIN (phase-locked loop in) pins of the four LTM4601s. The LTM4601's phase-locked loop comprises a phase detector and a voltage controlled oscillator, which combine to lock onto the rising edge of an external clock with a frequency range of 850 kHz. The phase-locked loop turns on when a pulse of at least 400ns at 2V is detected at the PLLIN pin (although it is disabled during start-up). Figure 3 shows the switching waveforms of four LTM4601 ?Module regulators in parallel.

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

Only one resistor is required to set the output voltage. In a parallel setup, the value of the resistor depends on the number of LTM4601s used. As such, the effective value of the top (internal) feedback resistor changes as you parallel LTM4601s. The LTM4601's reference voltage is 0.6V and its internal top feedback resistor value is 60.4k. The output voltage (volts) is:

Vout = 0.6 [(60.4k/n) + RFB]/RFB

where n is the number of paralleled modules.

Figure 4 illustrates the system's high efficiency over the output current range, with no dip in the efficiency over a broad range of output voltages.

Figure 4: Efficiency of the four DC/DC ?Modules in parallel remains high over a wide range of output voltages.

Start-up, soft-start, current sharing
The LTM4601's soft-start feature prevents large inrush currents at startup by slowly ramping the output voltage to its nominal value. The relation of startup time to Vout and the soft-start capacitor (Css) is:

Vout, margin = (%Vout/100) Vout

tsoftstart = 0.8 (0.6 - Vout, margin) (Css/1.5 ?A)

For example, a soft-start capacitor with a value of 0.1?f yields a nominal 8ms ramp (Figure 5) with no margining.

Figure 5: Soft-start current and voltage ramp for four DC/DC ?Modules in parallel.

Figure 6: Each ?Module (output shown for two LTM4601s) evenly shares the load current throughout the operating cycle.

Current-sharing among parallel regulators is well balanced through start-up to full load. As seen in Figure 6 for a 2-parallel LTM4601 system (10A each, 20A total), the current is evenly distributed throughout the operating cycle.

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