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 1m? required DC resistance inductor and still maintains stability.
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.
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 = 0.32mO 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