L DESIGN IDEAS I2C Quad Buck Regulator Packs Performance, Functionality, Versatility and Adaptability in a 3mm × 3mm QFN by Joe Panganiban I2C Programmable The LTC3562 is an I2C quadruple erating modes to satisfy the various Output Voltages Introduction step-down regulator composed of four extremely versatile monolithic buck converters. Two 600mA and two 400mA highly adjustable step-down regulators provide a total of 2A of available output current, all packed inside a 3mm × 3mm QFN package. All four regulators are 2.25MHz, constant-frequency, current mode switching buck converters whose output voltages and operating modes can be independently adjusted through I2C control. The 2.7V to 5.5V input voltage range makes it ideally suited for single Li-Ion battery-powered applications requiring multiple independent voltage supply rails. I2C Programmable Operating Modes All four LTC3562 step-down regulators have the unique ability to be programmed into four distinct op- noise/power demands of a variety of applications. These four modes are pulse skipping mode, Burst Mode operation, forced Burst Mode operation, and LDO mode. Pulse skipping mode allows the regulator to skip pulses at light load currents, providing very low output voltage ripple while maintaining high efficiency. Burst Mode operation and forced Burst Mode operation deliver bursts of current to the buck output and regulate the output voltage through hysteretic control, giving the highest efficiency at low load currents. In LDO mode, the bucks are converted to DC linear regulators and deliver continuous power from the switch pins through the inductor, providing the lowest possible output noise as well as the lowest no-load quiescent current. Another unique feature of the LTC3562 is its ability to adjust the output voltage of each regulator through I2C control. The chip contains two different flavors of output adjustable regulators. The Type A regulators (R600A, R400A) have programmable feedback servo voltages, while the Type B regulators (R600B, R400B) have directly programmable output voltages that do not need external programming resistors. The Type A regulators use external feedback resistors to set the output voltage based on a programmable feedback servo voltage. The feedback voltage values can be programmed from 800mV (full scale) down to 425mV in 25mV steps. This results in 16 possible feedback servo voltages, and thus 16 different output voltage settings for the same external programming resistors. Table 1. Feature comparison of the LTC3562’s four integrated regulators (two 600mA and two 400mA) R600A R400A R600B R400B Type A A B B Output Current 600mA 400mA 600mA 400mA I2C Programmable Operating Modes Pulse Skip Burst Forced Burst LDO Pulse Skip Burst Forced Burst LDO Pulse Skip Burst Forced Burst LDO Pulse Skip Burst Forced Burst LDO Feedback Servo Voltage I2C Programmable 425mV–800mV 25mV steps (16 settings) I2C Programmable 425mV–800mV 25mV steps (16 settings) 600mV (Fixed) 600mV (Fixed) I2C Programmable 600mV–3.775V 25mV steps (128 settings) No 34 Output Voltage Adjustable using External Resistors Adjustable using External Resistors I2C Programmable 600mV–3.775V 25mV steps (128 settings) RUN Pins Yes Yes No Linear Technology Magazine • January 2008 DESIGN IDEAS L 100k C5 10µF Li-Ion BATTERY 3.4V TO 4.2V SDA VIN VOUT 600B 3.3V 600mA VOUT 400B 1.2V 400mA L3 3.3µH C3 10µF C4 10µF DVCC R5 100k LTC3562 SW600B L1 3.3µH POR600A SW600A OUT600B L4 4.7µH SCL R1 634k FB600A RUN600A SW400B OUT400B VOUT 600A 1.8V 600mA C6 10pF C1 10µF POR SCL SDA VCC CORE VCC I/O MICROPROCESSOR R2 499k VOUT 400A 2.5V 400mA L2 4.7µH RUN400A SW400A R3 1070k FB400A PGND AGND C7 10pF C2 10µF R4 499k L1, L3: TOKO 1098AS-4R7M L2, L4: TOKO 1098AS-3R3M52 Figure 1. The LTC3562 configured in a quad step-down converter with pushbutton control and power sequencing. RUN pins and Default Settings I2C applications generally have a microprocessor in charge of the I2C communications between the various system blocks. A multi-channel buck converter such as the LTC3562 provides an excellent solution for efficiently stepping down the microprocessor’s core and I/O supply voltages from a higher input supply or battery. At the surface, using an I2C controllable voltage converter to generate the microprocessor’s power supplies seems to pose a bootstrap problem at system start-up. If the microprocessor initially has no power and thus there is no I2C control, what programs the LTC3562’s output to the proper voltage for the patiently waiting microprocessor? Linear Technology Magazine • January 2008 100 90 80 EFFICIENCY (%) The Type B regulators (R600B, R400B) do not require external programming resistors at all because they are integrated inside the chip. These internal feedback resistors not only save valuable board space, they are also I2C programmable. The values of the internal feedback resistors can be adjusted through I2C control to directly program the regulator output voltages from 0.6V to 3.775V in 25mV increments. That is 128 possible output voltage settings for each Type B regulator. FORCED Burst Mode OPERATION 600mA BUCKS 70 60 50 40 PULSE SKIP Burst Mode OPERATION 30 20 VIN = 3.8V VOUT = 2.5V 10 0 0.01 0.1 1 10 IOUT (mA) 100 1000 Figure 2. Efficiency of the 2.5V regulator The LTC3562 gets around this start-up issue by providing individual RUN pins for the two Type A regulators. These RUN pins bypass the I2C controls and enable the regulators if I2C is unavailable. When a RUN pin is used, the corresponding Type A regulator is enabled in a default setting, which is 800mV for the feedback voltage and pulse skipping mode for the operating mode. Once I2C becomes available to the system, these default settings can always be modified on the fly through I2C. Pushbutton Control and Power Sequencing Figure 1 shows an application circuit that uses the LTC3562 to power the core and I/O supplies of a system microprocessor. The RUN pin of R600A connects to a pushbutton circuit with a pull-up resistor used to power on the system. When the button is pushed, the RUN pin goes low which enables R600A to ramp up the power supply for the microprocessor’s core. The RUN pin of R400A is tied to R600A’s poweron-reset output signal (POR600A). Once R600A reaches regulation, POR600A goes high after a 230ms time delay, which would then enable R400A to power the I/O supply of the microprocessor. After both the core and I/O supplies are up, the microprocessor could then communicate back to the LTC3562 through I2C to program the part such that it keeps R600A enabled even after the pushbutton stimulus is removed. The microprocessor then can enable regulators R600B and R400B in any mode and program the output voltages to desired levels. Low Power Adaptability The ability to change the operating modes and output voltages at any time allows the LTC3562 to adapt to the constantly changing demands of many high performance systems. An example of this adaptability would be during lower power standby operation in handheld battery-powered systems. When going into standby mode, the regulators can be programmed into Burst Mode operation or forced Burst continued on page 37 35 DESIGN IDEAS L synchronization, user adjustable switching frequency and a SHARE pin for paralleling modules. The LTM8022 also employs Burst Mode operation, drawing only 50µA quiescent current at no load while maintaining only 30mV of output voltage ripple. Like the LTM8020, the quiescent current when shut down is less than 1µA. The schematic is very simple, with examples of 3.3V and 8V output designs shown in Figures 3 and 4, respectively. VIN 5.5V TO 36V VOUT RUN/SS BIAS AUX 2.2µF ADJ RT 49.9k operation and low quiescent current. The LTM8022 and LTM8023 share the same footprint and pin pattern, so even if you start a design with the LTM8022 but later find that you Max Load VOUT Range Size LTM8020EV 4V to 36V 200mA 1.25V to 5V 6.25 × 6.25 × 2.32mm LTM8022EV 3.6V to 36V 1A 0.8V to 10V 11.25 × 9 × 2.82mm LMT8023EV 3.6V to 36V 2A 0.8V to 10V 11.25 × 9 × 2.82mm the output is drawing full load. Its efficiency is shown in Figure 7. Conclusion The LTC3813 and LTC3814-5’s synchronous architecture and high voltage capability make them ideally suited for high voltage high power boost converters. They decrease comLinear Technology Magazine • January 2008 154k need more current, you can simply drop in the LTM8023. In most cases, the design will use identical passive components as the LTM8022, as seen in the 3.3V example in Figure 5. Conclusion VIN Range LTC3813 and LTC3814-5, continued from page 21 GND SYNC Figure 5. The LTM8023 produces 3.3V at 2A with the same footprint and components required for the LTM8022 producing 1A. Part Number Mode operation to maximize power efficiency at light loads. Under noload conditions, the regulators can also be programmed into LDO mode, which provides the lowest quiescent current (all four regulators in LDO mode only draw a combined 80µA for the entire chip). To save even more power, the LTC3562 can be programmed to reduce the regulators’ output voltages in Burst Mode operation or forced Burst Mode operation during light load conditions. Since power dissipation 22µF SHARE Table 1. Summary of LTM8000 series µModule regulators LTC3562, continued from page 35 VOUT 3.3V 2A LTM8023 …Or, Even More Power… The LTM8023 is the big brother of the LTM8022, capable of producing up to 2A of output current. The LTM8023 has the same input, output voltage range, and control features as the LTM8022. It also features Burst Mode VIN is directly proportional to the supply voltage multiplied by the load current, dropping the supply voltage effectively reduces the circuit’s total power dissipation. If the output load is resistive in nature, reducing the supply voltages has an even greater effect, since power dissipation in the load is proportional to the supply voltage squared. Conclusion The LTC3562 is a highly flexible I2C quad step-down converter composed of two 600mA and two 400mA buck plexity by eliminating the requirement for a large diode package and heat sink to dissipate its high power loss. Programmable frequency and current limit, wide output voltage range, and ability to drive logic-level or higher threshold MOSFETs provide maximum flexibility in using them for a variety of boost applications. Other The LTM8020, LTM8022 and LTM8023 µModule regulators make power supply development fast and easy. Their broad input and output voltage ranges, load capabilities and small size (see Table 1) make them readily fit into a wide variety of applications. L regulators in a 3mm × 3mm QFN package. The output voltages of the regulators can be switched on the fly using servo control or I2C control. Each regulator can also be switched on the fly into four possible high efficiency or low-noise operating modes. This is a perfect device for high performance applications that require constant control of the power supply. It can also be used to simplify design, build and test cycles, since output voltages can easily be changed without changing components. L features such as such as strong gate drivers to minimize transition losses, an accurate voltage reference, accurate cycle-by-cycle current limit, and an on-chip bias supply controller make the LTC3813 and LTC3814-5 the obvious choice for high performance, high power boost converters. L 37