High Efficiency, Low Input Voltage, Synchronous Buck Controller Drives up to 15A Load Current

Introduction

The LTC3822 is a synchronous step-down DC/DC converter that drives external N-channel power MOSFETs to maximize average current drive for the lowest cost. Its No RSENSE constant frequency architecture minimizes the number of external components, and a programmable frequency of up to 750kHz allows the use of small surface-mount inductors and capacitors. This DC/DC controller is optimized for 3.3VIN and Lithium-Ion applications allowing VOUT as low as 0.6V while maintaining 1% precision. The all N-channel MOSFET drive simplifies component selection as well as drastically increasing the current capabilities of a typical circuit. Even with 3.3V gate drive, the LTC3822 is capable of controlling more than 15A load current while maintaining high efficiency.

Compact, 1.8V, 8A Application

Figure 1 shows a 1.8V, 8A application that operates over input voltages between 2.75V and 4.5V, perfect for 3.3V or Li-Ion inputs. This application occupies much less space than would be expected for its current capabilities, as shown in Figure 2.

Figure 1. Typical application delivering 1.8V at 8A.

Figure 2. Sample footprint for application circuit in Figure 1.

During startup, the internal soft-start circuitry smoothly ramps the output voltage from 0V to its final value in 800μs (Figure 3). This is done without the need for an external capacitor. The LTC3822 incorporates No RSENSE technology to sense the inductor current from the drain to source voltage (VDS) of the top-side power MOSFET. The maximum load current that the controller is capable of driving is determined by the RDS(ON) of this MOSFET. Since the LTC3822 incorporates all N-channel MOSFET drive, lower RDS(ON) (and cheaper) devices are available for the top-side MOSFET, when compared to traditional complementary MOSFET drive.

Figure 3. Internal soft-start ramps the output voltage smoothly without requiring an external capacitor.

Increasing the Current to 20A

Figures 4 and 5 show two ways to raise the current capability of the regulator by lowering the RDS(ON) of the MOSFETs. In Figure 4, MOSFETs with a much lower RDS(ON) than those of Figure 1 are used. Because they are in individual SO-8 packages, their thermal capabilities are also higher. This application is designed for a 15A continuous current load. Figure 5 instead utilizes a secondary 5V supply to provide a higher gate drive voltage to the MOSFETs. Higher gate drive voltages lower RDS(ON) while simultaneously allowing the use of cheaper logic-level MOSFETs. The maximum load current can also be tailored using the current limit programming pin, IPRG. This three-state pin sets the peak current sense voltage across the top-side MOSFET. Combining all three high current approaches (utilizing low RDS(ON) MOSFETs, powering the gate drive from a secondary 5V supply, and setting current limit to its highest value) enables applications in excess of 20A.

Figure 4. High current application delivering 1.8V at 15A.

Figure 5. High efficiency application deriving gate drive voltage from a secondary 5V supply.

OPTI-LOOP Compensation

The LTC3822 incorporates OPTI-LOOP® compensation to enable the user to choose optimal component values to compensate the loop over a wide range of operating conditions with the minimum number of output capacitors. Figure 6 shows the transient response for the circuit in Figure 1 with a load step of 1A to 3A. The output overshoots by approximately 100mV on a 1.8V output and then settles in about 50μs.

Figure 6. Transient performance of the converter in Figure 1.

Conclusion

The LTC3822 delivers currents as high as 20A for single-output applications using a minimum number of components in a tiny complete solution footprint.

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Joseph Duncan