Current-mode controller benefits defined
Background
As microprocessors and digital signal processors demand progressively higher current at lower operating voltages, it becomes more critical to minimize power supply conduction losses by making the resistance of the current sense element as low as possible. However, a low resistance current sensing element produces a lower ramp voltage which is not generally conducive for stable operation when using a current mode controller. A low ramp voltage causes a current mode controlled switching power supply to have significant jitter and can become unstable in most applications. Accordingly, a voltage mode controller is normally used for these applications even though it has deficiencies and potential reliability issues.
A current mode controlled switching power supply has several advantages over a voltage mode switching power supply, these are:
1. Higher reliability with fast, cycle-by-cycle current sensing for output short circuit and overload protection. A voltage mode controlled power supply is slower to react to an over current condition which can result in a failure in some applications.
2. Simple and reliable feedback loop compensation allowing the power supply to be stable with all ceramic output capacitors making for a smaller solution size.
3. Easy and accurate current sharing in high current multiphase designs.
However, for high current outputs of typically greater than 20A a low DCR inductor will not produce enough of a voltage ramp signal for a current mode controller to be stable under all operating conditions and so a voltage mode controller has had to be used. This is about to change.
Linear Technology has recently released the LTC3866, a current mode controller that has the ability to sense a very low ramp voltage and maintain excellent stability. The LTC3866 breaks through the minimum 1mΏ required DC resistance inductor and still maintains stability.
Introduction
The LTC3866 is a peak current-mode synchronous step-down DC/DC controller that allows the use of very low DC resistance power inductor using a novel DCR sensing architecture that enhances the signal-to-noise ratio of the cur-rent sense signal. A power inductor DC resistance of as low as 0.17 milliohms can be used to maximize converter efficiency and increase power density. Furthermore, this new DCR sensing technique dramatically reduces the switching jitter normally associated with low DCR resistance applications. DCR temperature compensation maintains a constant and accurate current limit threshold over a broad temperature range.
This device operates from a 4.5V to 38V input voltage range that encompasses a wide range of applications. Strong onboard N-channel MOSFET gate drivers allow the use of high power external MOSFETs, DrMOS devices or power blocks for an output current of up to 40A, with output voltages ranging from 0.6V to 3.5V when using the onboard remote sense Diff Amp and 0.6V to 5V when not using remote sense. The LTC3866 can be paralleled by tying the ITH pins together of multiple devices for even higher power multiphase applications. A low current sense threshold from 10mV to 30mV can be selected. The fixed operating frequency is adjustable from 250kHz to 770kHz or can be synchronized to an external clock. Additional features include an internal bias voltage regulator, soft start or tracking, overvoltage protection, soft short-circuit recovery, current limit foldback, thermal shutdown and external VCC control. The LTC3866 is available in thermally enhanced 4mm x 4mm QFN-24 and TSSOP-24E packages.
Typical application
The LTC3866 can work with very low DCR inductors due to its ability to operate with only a small peak-to-peak sense voltage. Figure 1 below shows a LTC3866 schematic circuit design that operates from a nominal 12V input and produces a 1.5V output at up to 30A. An inductor with DCR = 0.32mΩ is used to maximize efficiency at greater than 90 percent as shown in Figure 2.
Figure 1: Typical LTC3866 applications schematic for 12VIN to 1.5VOUT at 30A
Click on image to enlarge
Figure 2: Efficiency curve of figure 1 schematic, showing efficiencies >90 percent are possible
Features
The LTC3866 comprises two positive sense pins, SNSD+ and SNSA+, to acquire the ramp signal and process it internally to provide a 14dB signal-to-noise ratio improvement in response to low voltage sense signals. The current limit threshold is still a function of the inductor peak current and its DCR value, and can be accurately set from 10mV to 30mV in a 5mV steps. The part-to-part current limit error is only about 1mV over the full temperature range assuring good accuracy.
In addition, since the LTC3866 uses constant frequency peak current mode control architecture, it guarantees cycle-by-cycle peak current limit and current sharing between different power supplies. It is especially well-suited to low voltage, high current supplies because of a unique architecture that enhances the signal-to-noise ratio of the current sense circuit. The improved signal to noise ratio minimizes jitter due to switching noise, which could corrupt the signal. The worst case switching jitter is reduced by 60 percent when compared to a standard current mode controller.
Low output ripple application
Because the LTC3866 only requires a ramp signal about a quarter of the sense signal of the next best current mode converters, output ripple can be drastically reduced by increasing the inductance and capacitance of the output filter. Figure 3 shows a high efficiency converter with the benefit of low output ripple voltage. The much lower output voltage ripple of less than 10 mV, as shown in figure 4 is critical for extremely noise sensitive applications such as test/measurement systems and audio devices.
Figure 3: High efficiency, 1.5V/25A step-down converter with very low output ripple
Figure 4: Low output ripple curve of LTC3866 figure 3 schematic
Figure 5: 1.5V/80A power with two LTC3866’s paralleled, each using power blocks
Conclusion
The LTC3866 allows the use of an ultralow DCR current sensing element to increase the efficiency in high current applications. Its current mode control provides the benefits over the alternative voltage mode controller of high re-liability with fast, cycle-by-cycle current sensing, simple feedback loop compensation and the ability to use all ceramic capacitors for the smallest solution size. The LTC3866 is suitable for low voltage, high current step-down converter applications needing high efficiency and high reliability. Tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features. The LTC3866 applies to point-of-load computer and telecom systems, industrial and medical instruments, and DC power distribution systems. Finally, a power supply designer can have a controller that incorporates the best of both current and voltage mode control schemes.
About the author:
Bruce Haug is senior product marketing engineer at Linear Technology
See related links:
UMC copper process targets power management ICs
Distributed power management cuts wiring weight
Power management IC market set to decline
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