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contribute to lower system cost, consuming less power to remove heat). Figure 1 shows a test board for such a circuit. The design regulates 1.5V output while delivering 40A (up to 48A) of load current. Each “black square” is a complete DC/DC circuit and is houd in a 15mm × 15mm × 2.8mm surface mount package. With a few input and output capacitors and resistors, the design using the DC/DC μModule ® regulator systems is as simple as it’s shown in the photo.
Figure 1. Four DC/DC μModule Regulator Systems Current Share to Regulate 1.5V at 48A with Only 2.8mm Profi le and 15mm × 15mm of Board Area. Each μModule Regulator Weighs Only 1.7g and Has an IC Form-Factor That Can Easily Be Ud with Any Pick-and-Place Machine During Board Asmbly
I n a recent discussion with a system designer , the re-quirement for his power supply was to regulate 1.5V and deliver up to 40A of current to a load that consisted of four FPGAs. This is up to 60W of powe
r that must be delivered in a small area with the lowest height profi le possible to allow a steady fl ow of air for cooling. The power supply had to be surface mountable and operate at high enough effi ciency to minimize heat dissipation. He also demanded the simplest possible solution so his time could be dedicated to the more complex tasks. Aside from preci electrical performance, this solution had to remove the heat generated during DC to DC conversion quickly so that the circuit and the ICs in the vicinity do not overheat. Such a solution requires an innovative design to meet the criteria:
1. Very low profi le to allow effi cient air fl ow and to prevent thermal shadow on surrounding ICs
2. High effi ciency to minimize heat dissipation
3. Current sharing capability to spread the heat evenly to eliminate hot spots and minimize or eliminate the need for heat sinks
4. Complete DC/DC circuit in a surface mount package that includes the DC/DC controller , MOSFETs, inductor , capacitors and compensation circuitry for a quick and easy solution
Innovation in DC/DC Design
The innovation is a modular but surface mount approach that us effi cient DC/DC conversion, preci current shar-ing and low thermal impedance packaging to deliver the output power while requiring minimal cooling. Becau of the low profi le and power sharing among four devices, a system using this solution depends on fewer fans or a slower fan speed as well as few or no heat sinks. (The
Powering Complex FPGA-Bad Systems Using Highly Integrated DC/DC µModule Regulator Systems
Part 1 of 2 Circuit and Electrical Performance Alan Chern and Afshin Odabaee
DC/DC μModule Regulators:
Complete Systems in an LGA Package
The L TM4601 μModule DC/DC regulator is a high perfor-mance power module shrunk down to an IC form factor . It is a completely integrated solution—including the PWM controller , inductor , input and output capacitors, ultralow R DS(ON) FETs, Schottky diodes and compensation circuitry. Only external bulk input and output capacitors and one resistor are needed to t the output from 0.6V to 5V . The
IN
NOT NEEDED FOR LOW INDUCTANCE PLANE CONNECTION 60.4k+ R
SET
Figure 2. Simply Parallel Multiple DC/DC μModule Regulator Systems to Achieve Higher Output Current. Board Layout Is as Easy as Copying and Pasting Each μModule Regulator’s Layout With Very Few External Components Required
supply can produce 12A (more, if paralleled) from a wide input range of 4.5V to 20V, making it extremely versatile. The pin-compatible L TM4601HV extends the input range to 28V.
Another signifi cant advantage of the L TM4601 over power-module- or IC-bad systems is its ability to easily scale up as loads increa. I f load requirements are greater than one μModule regulator can produce, simply add more modules in parallel. The design of a parallel system involves little more than copying and pasting the layout of each 15mm × 15mm μModule regulator. Electrical layout issues are taken care of within the μModule package—there are no external inductors, switches or other components to worry about.个人理想
Output features include output voltage tracking and mar-gining. The high switching frequency, typically 850kHz at full load, constant on-time, zero latency controller delivers fast transient respon to line and load changes while maintaining stability. Should frequency harmonics be a concern, an external clock can control synchronization via an on-chip pha-locked loop.
48A from Four Parallel DC/DC μModule Regulators Figure 2 shows a regulator comprising four parallel L TM4601s, which can produce a 48A (4 × 12A) output. The regulators are synchronized but operate 90° out-of-pha with respect to each other, thereby reducing the amplitude of input and output ripple currents through cancellation (Figure 3).
Synchronization and pha shifting is implemented via the L TC6902 oscillator, which provides four clock outputs, each 90° pha shifted (for 2- or 3-pha relationships, the L TC6902 can be adjusted via a resistor). By operating the μModule regulators out-of-pha, peak input and output current is reduced by approximately 20% depending on the duty cycle (e the L TM4601 data sheet). The attenu-ated ripple, in turn, decreas the external capacitor RMS current rating and size requirements, further reducing solution cost and board space. The clock signals rve as input to the PLLI N (pha-locked loop in) pins of the four L TM4601s. The pha-locked loop of the L TM4601 is comprid of a pha detector and a voltage controlled oscillator, which combine to lock onto the rising edge of an external clock with a frequency range of 850kHz. The pha-locked loop is turned on when a pul of at least 400ns and 2V amplitude at the PLLIN pin is detected, though it is disabled during start-up. Figure 3 shows the switching waveforms of four L TM4601 μModule regulators in parallel.
Only one resistor is required to t the output voltage. In a parallel t-up, the value of the resistor depends on the number of L TM4601s ud. This is becau the effective value of the top (internal) feedback resistor changes as you parallel L TM4601s. The L TM4601’s reference volt-age is 0.6V and its internal top feedback resistor value is 60.4kΩ, so the relationship between V OUT, the output voltage tting resistor (R FB), and the number of modules (n) placed in parallel is:
挚友的反义词V OUT=0.6V
occupation60.4k
n
+R FB
R FB
Figure 3. By Operating Each DC/DC μModule Regulator 90° Out-of-Pha, the Input and Output Ripples Are Reduced, Which Also Reduces the Requirement for Input and Output Capacitors. Photo Shows Individual μModule Regulator Switching Waveforms for Figure 2
1μs/DIV
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Figure 4 illustrates the system’s high effi ciency over the vast output current range up to 48A. The system performs impressively with no dipping in the effi ciency curve for a broad range of output voltages. Start-Up, Soft-Start and Current Sharing
The soft-start feature of the L TM4601 prevents large inrush currents at start-up by slowly ramping the output voltage to its nominal value. The relation of start-up time to V OUT and the soft-start capacitor (C SS ) is:
t SOFTSTART =0.8•0.6V-V OUT(MARGIN)()
•C SS 1.5μA
where
V OUT(MARGIN)=%V
OUT 100•V OUT
For example, a 0.1μF soft-start capacitor yields a nominal
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8ms ramp (e Figure 5) with no margining.
Current sharing among parallel regulators is well balanced through start-up to full load. Figure 6 shows an evenly distributed output current curve for a 2-parallel L TM4601 system, as each ris to a nominal 10A each, 20A total.
Conclusion
The DC/DC μModule regulators are lf-contained and complete systems in an IC form factor . The low profi le, high effi ciency and current sharing capability allow practi-cal high power solutions for the new generation of digital systems. Thermal performance is impressive at 48A of output current with balanced current sharing and smooth uniform start-up. The ea and simplicity of this design mi
nimizes development time while saving board space. In part two of this discussion, the focus will be on thermal performance and layout of this circuit.
2ms/DIV
V IN = 12V V OUT = 1.5V LOAD = 40A
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0V V
OUT 1V/DIV
I LOAD 20A/DIV
V IN 5V/DIV
5ms/DIV
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I OUT(IC1)5A/DIV I OUT(IC2)
5A/DIV
V IN = 12V V OUT = 1.5V LOAD = 20A
Figure 5. Controlled Soft-Start Is Important in Proper Start-Up of the FPGA or the System as a Whole; Soft-Start Current and Voltage Ramp for Four DC/DC μModule Regulators in Parallel
成都软件培训学校Figure 6. Each DC/DC μModule Regulator Starts and Ends By Sharing the Load Current Evenly and Balanced, a Crucial Feature to Prevent One Regulator from Overheating; T wo Parallel L TM4601s, as Each Ris to a Nominal 10A Each, 20A Total
Figure 4. Effi ciency of the Four DC/DC μModule Regulators in Parallel Remains High Over a Wide Range of Output Voltages (12V Input)
LOAD CURRENT (A)
0E F F I C I E N C Y (%)
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