ASIC and microprocessor chip power supplies

Today's high-performance ASICs and microprocessor chips can consume more than 150 watts of power. For supply voltages between 1 V and 1.5 V, the current required by these devices can easily exceed 100 A. The task of powering such devices can be made more tractable by using multiphase DC/DC converters. Scalable controllers are now available that allow designers to select the number of phases required for a particular DC/DC converter. Scalability also allows several controllers to be synchronized and used in parallel. Controller synchronization is supported by on-board PLL-based clock generators.

Multiphase Converter Topologies

As load currents exceed 20A to 30A, the advantages of designing with multiphase converters become more apparent. These advantages include lower input ripple current, significantly fewer input capacitors, effective multiplication of ripple frequencies to reduce output ripple voltage, and lower device temperatures by spreading energy losses over more devices, while also reducing the height of external components.

A multiphase converter is essentially multiple buck regulators operating in parallel, with their switching frequencies synchronized and phase-shifted by 360/n degrees, where n is the number of phases. Paralleling converters makes output regulation slightly more complicated, but this problem is easily solved using a current-mode control IC to regulate the current in each inductor and the output voltage.

Input ripple current

The key issue facing designers when selecting input capacitors is dealing with input ripple current. By utilizing a multiphase topology, the input ripple current can be greatly reduced, so the input current pulse amplitude through the input capacitor of each phase is smaller. The phase shift also improves the effective working factor in the current waveform, which also results in a lower RMS ripple current value. The ripple current levels shown in Table 1 show that a multiphase topology converter can reduce ripple current and reduce input capacitors.

High-K ceramic capacitors offer the best ripple handling performance and take up the least PCB area. Ceramic devices in the 1812 case have a ripple current rating of 2"3A per capacitor. For cost-sensitive designs, electrolytic capacitors are a good choice.

Reduce output ripple voltage

For processor core power supplies, the accuracy requirement is typically 2%. For a 1.2 V supply, this means that the output voltage is allowed to vary by ±25 mV. A technique that makes more efficient use of the output voltage window is called Active Voltage Positioning. Under light load conditions, the converter regulates the output voltage above the midpoint of the output voltage window, and under heavy loads, it regulates the output voltage below the midpoint of the output voltage window. For an output voltage window of ±25 mV, the output voltage is regulated at the high (low) end of the output voltage window under light load (heavy load), which allows the full use of the entire output voltage window as the load gradually increases (decreases).

Large load current step-downs require both very low ESR capacitors to minimize transients and large enough capacitance to absorb the stored energy released by the main inductor as the load is stepped down. Low ESR tantalum capacitors are available through the use of organic polymer compounds, which offer the lowest ESR values ​​and larger capacitance values. Ceramic capacitors have excellent high-frequency characteristics, but the total capacitance per device is only half to one-quarter that of tantalum and polymer capacitors, so ceramic capacitors are usually not the best choice for output capacitors.

Low-side MOSFET

A 12V"1.2V converter requires the low-side MOSFET to be on 90% of the time, at which point the conduction losses are much greater than the switching losses. For this reason, two or three MOSFETs are often used in parallel. Using several MOSFETs in parallel effectively reduces RDS(ON), and therefore reduces conduction losses.

High-side MOSFET

At a 10% duty cycle, the switching losses of the high-side MOSFET are greater than the conduction losses. Since the high-side MOSFET is on for very little time, the conduction losses are small, so low on-resistance is not as important as low switching losses. During switching (both on and off), the MOSFET must withstand voltage and on-current. The product of this voltage and current determines the peak power dissipation of the MOSFET, so the shorter the switching time, the lower the power dissipation. When selecting a high-side MOSFET, choose a MOSFET with low gate charge and gate-to-drain capacitance values, as these two parameters are more important than low on-resistance. Table 1 shows how the total MOSFET losses decrease as the number of phases increases.

Inductor Selection

The value of the inductor directly determines the peak-to-peak value of the ripple current. The allowable ripple current is usually calculated as a percentage of the maximum DC output current. In most applications, a ripple current of 20% to 40% of the maximum DC output current is ideal.

At low core voltages, the inductor current decreases more slowly than it increases. During load shedding, the output capacitors can overcharge, resulting in an overvoltage condition at the output. Using a smaller value inductor (allowing for more ripple current - closer to 40%), less stored energy is transferred to the output capacitors, thus minimizing the potential for overvoltage.

Thermal Design

Table 1 gives an estimate of the heat sink required for designs with different numbers of phases. In a forced convection cooling system that can provide 100" of 200 LFM, a single-phase design would require a fairly large heat sink to achieve a thermal resistance of 0.6 C/W. In a four-phase design, the thermal resistance can be increased to 2C/W, which is easily achievable even without a heat sink and 100" of 200 LFM of air flow.

Table 1 compares the key parameters of synchronous buck regulator designs based on the number of phases used in the design. The example in the figure is a 12V to 1.2V 100A buck regulator.

Design Examples

Figure 1 shows a four-phase DC/DC converter using the MAX5038. The MAX5038 master remote voltage sense input (VSP to VSN pins) provides a signal (DIFF) to both the master and slave EAN inputs, allowing parallel operation. The MAX5038 master also provides a clock (CLKOUT) to the MAX5038 slave. By leaving the PHASE pin floating, the slave locks the 90-degree phase shift to the CLKIN signal. The error amplifier also performs dynamic voltage positioning by setting the gain of the voltage error amplifier. Accurate load balancing is ensured by using precise gain setting resistors. The output of the voltage error amplifier (EAOUT) programs the load current for each phase. Compensation (not shown) is provided for each current loop at the CLP1 and CLP2 pins, providing a very stable output for most AC line and load conditions.

Figure 1. MAX5038 configured as a four-phase DC/DC converter

Multiphase synchronous DC/DC converters can efficiently power ASICs and processors that require 1"1.5 V·A and higher. This solves the basic issues related to capacitor ripple current, MOSFET power dissipation, transient response, and allowable output ripple voltage.