Charlie Zhao, Applications Engineer and Jian Li Senior Applications Engineer at Linear Technology discuss the merits of a 2-phase synchronous buck controller featuring light load stage shedding mode, active voltage positioning, low RSENSE and remote VOUT sensing and how this is advancing power supplies for electronics of the modern world for greater power efficiency

Today’s computer, datacom, and telecom systems demand power supplies that are efficient, respond quickly to load transients and accurately regulate the voltage at the load. For example, load current can be measured by using the inductor DCR, thus eliminating the need for a dedicated sense resistor.

Inductor DCR sensing increases efficiency – especially at heavy load – while reducing component cost and required board space. The LTC3856 from Linear Technology is a single-output 2-phase synchronous buck controller that improves the accuracy of inductor DCR sensing by compensating for changes in DCR due to temperature.

DCR temperature compensation is just one of many performance enhancing features offered in this device. It also includes on-chip gate drivers, remote output voltage sensing, Stage Shedding mode for improved light load efficiency and adaptive voltage positioning for fast transient response.

The controller can convert a wide input voltage range, 4.5V to 38V, to outputs from 0.6V to 5V. Despite the many features, the chip is small, available in 32-pin 5mm by 5mm QFN and 38-pin TSSOP packages.

Figure 1 shows a typical LTC3856 application in a 4.5V to 14V input, 1.5V/50A output converter. The controller’s two channels operate out-of-phase, which reduces the input RMS current ripple and thus the required input capacitance. Up to six LTC3856s can be paralleled for up to 12-phase operation.

This controller has a phase-locked loop (PLL) and can be synchronised to an input frequency between 250kHz and 770kHz. Due to its peak current-mode control architecture, the device provides fast cycle-by-cycle dynamic current sharing plus tight DC current sharing, as shown in Figure 2.

Stage shedding mode

At light loads, this controller can be programmed to operate in one of three modes: Burst Mode operation, forced continuous mode or Stage Shedding mode. With Stage Shedding mode, the LTC3856 can shut down one channel to reduce switching related loss which is the dominant loss at light loads. Stage Shedding mode is selected by simply tying the MODE pin to INTVCC.

Due to strong gate drivers and shorter dead-time, this controller can achieve 4 percent ~ 5 percent higher efficiency than the LTC3729, a comparable single-output, 2-phase controller, over the whole load range. With Stage Shedding mode, significant efficiency improvement is further achieved at light load. At 5 percent load, the efficiency is improved by 13 percent.

Current mode control allows the LTC3856 to transition smoothly from 2-phase to 1-phase operation and vice versa.

User-selectable active voltage positioning (AVP) is another unique design feature of the LTC3856. AVP improves overall transient response and reduces required output capacitance by modifying the regulated output voltage depending on its current loading. With proper design, AVP can reduce load transient-induced peak-to-peak voltage spikes by 50 percent.

Although not used here, inductor DCR sensing offers a lossless method of sensing the load current. The problem is that the DCR of the inductor typically has a positive temperature coefficient, causing the effective current limit of the converter to change with inductor temperature.

This controller can sense the inductor temperature with an NTC thermistor, thereby adjusting the current limit based on the temperature. The result is a constant current limit over a broad temperature range. This improves inductor DCR sensing reliability in high current applications.

For high output current, low voltage applications, board or wire interconnect resistance can cause a severe load regulation problem. To solve this problem, this device includes a low offset, unity-gain, high bandwidth differential amplifier for true remote sensing.

Common mode noise and ground loop disturbances can be rejected, and load regulation is greatly improved, especially when there are long trace runs between the load and the converter output.

The LTC3856 is a feature-rich single output, 2-phase synchronous step-down DC/DC controller. It achieves high efficiency in both heavy load and light load conditions, with temperature compensated DCR sensing and Stage Shedding mode or Burst Mode operation. AVP improves transient response even when the output capacitance is reduced.

Remote sensing, a tight ±0.75 percent reference voltage accuracy over temperature voltage tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features.

This controller is ideal for high current applications and can meet the high standards of today’s power supplies for telecom and datacom, industrial and computer applications.

Linear Technology