The Best of Both Worlds - Combining “Good” Characteristics of Linear Regulators with High Efficiency of Switching Regulators

By Ralf Muenster, Micrel, Inc

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With process technologies migrating to sub-100nm levels, supply voltages on many microprocessors, ASICs and FPGAs are now down tothe 1V level, leading to decreasing margins for voltage fluctuations. The challenge that arises with these new low voltage requirements is that the tolerance on the supply voltages is remaining a fixed percentage of the supply voltage, instead of being an absolute value in volts. This means that as the core voltage goes down, the amount of tolerance that the processor can handle becomes smaller and smaller. For example, many high end ICs require supply voltages regulated to tighter ±5%, which translates to only ±50mV at the 1V level.

Switching regulators seem the logical choice to power today’s low voltage cores due to their good efficiency converting voltages downward from available 5V and 3.3V rails. However, by its very nature, a switch-mode converter creates a significant amount of ripple voltage. A synchronous single-phase DC-to-DC converter that is fully loaded can have anywhere from 10mV to 100mV of voltage ripple on the output. Relative to a 1V output voltage, that is 1% to 10% of the regulator output voltage. On the other hand, linear regulators provide clean low noise outputs with good ripple rejection from the input.

Output ripple is not the only problem when supplying today’s advanced process cores. With the lower geometries, the operating frequencies of these uPs, ASICs and FPGAs have risen to GHz levels at the very same time. Fast switching speeds can create dramatic dynamic load changes depending on what software code is executed. In turn, these fast load changes can result in huge fluctuations on the supply voltage depending upon the response time of the voltage regulator that is used. The response to load changes is sometimes referred to as transient performance.

Switch-mode converters in general have slower response times to load changes compared to linear regulators. The load transient response of a linear regulator can be made to be much higher than that of a DC-to-DC converter because it is a linear system. With the proper loop design, the output of the linear regulator can have gain-bandwidth loops as fast as 10MHz. A typical DC-to-DC converter is limited by the switching frequency, which is generally around 1MHz. Moreover, in order to remain stable under all conditions, the DC-to-DC converter typically needs to roll off its gain and operate with a gain-bandwidth product of one-tenth to one-fifth of the switching frequency. This means that the transient voltage regulation of a switching regulator will be 10 to 100 times worse when compared to a linear regulator. Figure 1 illustrates the transient response of a linear regulator as compared to a switching regulator and assuming the same output capacitance.

 

 

Linear regulators still have one essential drawback; their efficiency for a 5V to 1V or 3.3V to 1V conversion is very poor. This is due to the simple fact that they have to dissipate the energy of the voltage drop in the device. For example, the efficiency for the 3.3V to 1V conversion will be a mere 30% and 6.6W of power would have to be dissipated as heat to provide an output of 2A. This is too much for a conventional IC package and a large heat sink would be needed, making the solution very large in size.

There is therefore a need for new type of voltage regulator, a high efficiency linear regulator that provides a low noise output, fast transient performance, good PSRR, small solution size while at the same time providing good efficiency.

Micrel recently introduced a new SuperLNR™ family that combines the “good” characteristics of both linear regulators and DC-to-DC converters to produce a new generation of easy-to-use Low Noise Regulators. Micrel’s MIC38300, the first device in this family, is a 3A peak and 2.2A continous output current voltage regulator housed in a tiny 4mm x 6mm x 0.9mm MLF® package that produces less than 5mV of output noise and 70dB of PSRR. Industry leading, ultra-fast dynamic performance also allows the MIC38300 to achieve the same transient performance as high speed LDOs and maintain less than 30mV of output voltage deviation even during fast load transients. The MIC38300 features an input voltage range of 3.0V to 5.5V and adjustable output voltages as low as 1V. The solution requires no external inductor and fits into a total solution footprint of less than 50mm².

 

 

Figure 2 shows the MIC38300 solution compared to a 3A LDO and a 3A synchronous switching regulator solution.

MIC38300 has a junction operating range from –40ºC to +125ºC.

The device is targeted at applications that need an easy upgrade from LDOs as power dissipation becomes an issue or where low noise performance, small size and fast transient performance are paramount. Target markets include point-of-load and digital IC power regulators for networking, servers, wireless base stations, industrial and RF applications

 

Note: MLF is a registered trademark of Amkor Technology. SuperLNR is a trademark of Micrel, Inc.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
MIC38300HYHL		3A SuperLNR™ Low Noise High Efficiency Regulator

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