DESIGN IDEAS L High Efficiency, Low Input Voltage, Synchronous Buck Controller Drives by Joseph Duncan up to 15A Load Current Introduction IPRG The LTC3822 is a synchronous stepdown 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. RUN ITH LTC3822 SW GND 59k GND VOUT 1.8V 500mV/DIV VIN = 4.2V RLOAD = 1Ω Figure 2. Sample footprint for application circuit in Figure 1 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 CMDSH-3 Si4866 0.22µH 0.22µF 47µF 2x VIN 2.75V TO 4.5V VOUT 1.8V 100µF 15A 2x CMDSH-3 BG 118k Figure 4. High current application delivering 1.8V at 15A. Linear Technology Magazine • June 2006 BG 118k VFB 59k VOUT 1.8V 100µF 8A Figure 1. Typical application delivering 1.8V at 8A. Si4866 680pF 0.47µH VFB BOOST 5.1k 0.22µF 680pF TG FREQ SW BOOST 5.1k 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. During startup, the inter nal soft-start circuitry smoothly VIN LTC3822 VIN 2.75V TO 4.5V FDS6898A TG FREQ ITH CMDSH-3 47µF RUN Compact, 1.8V, 8A Application IPRG VIN 200µs/DIV Figure 3. Internal soft-start ramps the output voltage smoothly without requiring an external capacitor. 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. 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 in41 L DESIGN IDEAS 5V SECONDARY SUPPLY CMDSH-3 IPRG VIN 47µF RUN ITH LTC3822 SW 0.22µF 0.47µH IL 2A/DIV VOUT 1.8V 100µF 10A BOOST 5.1k VOUT 100mV/DIV FDS6898A TG FREQ VIN 2.75V TO 4.5V ILOAD 2A/DIV 680pF GND BG VFB 59k VIN = 3.3V VOUT = 1.8V ILOAD = 1A TO 3A 118k 40µs/DIV Figure 6. Transient performance of the converter in Figure 1 Figure 5. High efficiency application deriving gate drive voltage from a secondary 5V supply. stead 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. The LTC3822 incorporates OPTILOOP® 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 tran- Conclusion LT6557, continued from page 35 matically tracks downward with the supply if below 4V. The selection of input bias point may depend on the application, but the values shown for the programming resistors in Figures 1 and 2 are representative of most designs. grammable biasing, these devices offer minimal parts-count AC-coupled amplifier solutions for very high-resolution applications. The LT6557, with its gain of two, is designed for RGB output ports such as in video routers and KVM switch products. The LT6558, with unity gain, is designed as an RGB input port buffer and/or ADC driver, such as in computer or home-theater display products. L The LT6557 and LT6558 are designed specifically with single-supply ACcoupled operation in mind. Each input includes an internal currentcontrolled bias voltage source like that shown in Figure 3. A single external resistor RBCV programs the input bias voltages as shown in Figure 4 for the LT6557. The LT6558 RBCV function is similar to Figure 4, but is optimized for producing higher biasing levels to account for the lower gain and auto- Conclusion The LT6557 and LT6558 triple video amplifiers are optimized specifically for operation on low voltage single supplies. With preset gain and pro- V+ VBCV = 48mV (TYPICAL) VSUPPLY = 3.3V VS = 5V 1.5 1.0 0.5 9.1k 2ns/DIV Figure 3. Fast pulse response of LT6558 on 3.3V single supply 42 VBCV RSET 2.5k IN V • 9.1k VBIAS(IN) = BCV RSET 2.5 2.0 I= OUTPUT 100mV/DIV The LTC3822 delivers currents as high as 20A for single-output applications using a minimum number of components in a tiny complete solution footprint. L INPUT VOLTAGE (V) Automatic Biasing Feature OPTI-LOOP Compensation sient 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 4. Simplified schematic of LT6557 input biasing circuit (LT6558 similar) 0 200 300 400 500 RBCV (Ω) 600 700 Figure 5. Relationship of LT6557 input bias voltage to programming resistor RBCV Linear Technology Magazine • June 2006