DESIGN IDEAS 1.2MHz, 2A, Monolithic Boost Regulator Delivers High Power in Small Spaces Introduction 1.3 TA = 25°C VOUT = 5V COUT = 22µF 1.1 L = 2.2µH IOUT(MAX) (A) Even as cell phones, computers and PDAs shrink, they require an increasing number of power supply voltages. The challenge, of course, is how to squeeze more voltage converter circuits into less space—without sacrificing power or efficiency. Boost converters, in particular, are becoming more prevalent, as main supply voltages are lowered to accommodate core logic circuits, while many components require a higher supply voltage. The LTC3426 boost converter meets the challenge with converter-shrinking features, including a low RDS(ON) monolithic switch, internal compensation and a 3mm × 3mm × 1mm ThinSOT package. The LTC3426 operates at high frequency and therefore works with small, low cost inductors and tiny ceramic capacitors. The LTC3426 incorporates a constant frequency current mode architecture, which is low noise and provides fast transient response. With VOUT 500mV/DIV IOUT 250mA/DIV 0.9 0.7 IL 500mA/DIV 0.5 0.3 1.8 2.2 2.6 3 VIN (V) 3.4 3.8 VIN = 1.8V VOUT = 3.3V COUT = 22µF L = 2.5µH 4.2 Figure 1. High current outputs are attainable with minimum 2A switch limit. 3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning ........... 23 a minimum peak current level of 2A, the LTC3426 delivers up to 900mA of output current. Figure 1 shows the converters output current capability at 5V as a function of VIN with peak inductor current at 2A. An input supply range of 1.6V to 4.3V makes the LTC3426 ideal for local supplies ranging from 2.5V to 5V. Efficiencies above 90% are made possible by its low 0.11Ω (typ.) RDS(ON) internal switch. There is no need for an external compensation network because the LTC3426 has a built-in loop compensation network. This reduces size, lowers overall cost and greatly simplifies the design process. Figure 2 shows the VOUT response to a 250mAto-500mA load step in a 1.8V to 3.3V application. Redundant 2-Wire Bus for High Reliability Systems ............. 25 VIN 2.5V DESIGN IDEAS 1.2MHz, 2A, Monolithic Boost Regulator Delivers High Power in Small Spaces........................... 22 Kevin Ohlson by Xiaoyong Zhang John Ziegler –48V Backplane Impedance Analyzer Takes the Guesswork Out of Sizing Clippers and Snubbers .. 27 Mitchell Lee Compact Power Supply Drives TFT-LCD and LED Backlight ......... 31 Dongyan Zhou Tiny, Low Noise Boost and Inverter Solutions ........................ 33 Eric Young 22 by Kevin Ohlson L1 2.5µH D1 SW VOUT VIN C1 10µF OFF ON LTC3426 SHDN GND FB R1 75k 1% R2 44.2k 1% C1: TDK C1608X5R0J106 C2: TAIYO YUDEN JMK316BJ266 D1: ON SEMICONDUCTOR MBRM120LT3 L1: SUMIDA CDRH5D28-2R5 2 Figure 3. Application circuit for 3.3V output delivers 800mA VOUT 3.3V 800mA C2 22µF 40µs/DIV Figure 2. Fast transient response to load step of 250mA to 500mA The Shutdown input can be driven with standard CMOS logic above either VIN or VOUT (up to 6V maximum). Quiescent current in shutdown is less than 1µA. A simple resistive pull-up to VIN configures the LTC3426 for continuous operation when VIN is present. 3.3V Output 800mA Converter Some applications require local 3.3V supplies which are utilized periodically yet are required to deliver high currents. The LTC3426 is an ideal solution which requires minimal board space and, when in shutdown, draws less than 1µA quiescent current. Figure 3 shows a circuit which delivers up to 800mA at 3.3V from a 2.5V input. This circuit also works with VIN down to 1.8V with 750mA output. The output voltage is easily programmed by changing the feedback ratio of R1 and R2 according to the formula: R1 VOUT = 1.22V • 1 + R2 Lithium-Ion 5V Boost Converter Some portable applications still require a 5V supply. Figure 4 shows a circuit which operates from a single Lithium-Ion battery and delivers at continued on page 32 Linear Technology Magazine • November 2004 DESIGN IDEAS at around 42V to protect the internal power devices. Proper layout is important to achieve the best performance. Paths that carry high switching current should be kept short and wide to minimize the parasitic inductance. In the boost regulator, the switching loop includes the internal power switch, the Schottky diode (internal or external), and the output capacitor. In the negative output regulator, the switching loop includes the internal power switch, the flying capacitor between the SW2 and D2 pins, and the internal Schottky diode. Connect the output capacitors of the AVDD and LED switchers directly to the PGND14 pin before returning to the ground plane. Connect the output capacitor of the VON switcher to the PGND23 pin before returning to the ground plane. Also connect the bottom feedback resistors to the AGND pin. Connect the PGND14, PGND23 and AGND pins to the top layer ground pad underneath the exposed copper ground on the backside of the IC. The exposed copper helps to reduce thermal resistance. Multiple vias into ground layers can be placed on the ground pad directly underneath the part to conduct the heat away from the part. LTC3426, continued from page 22 Component Selection current should be greater than 1A. A low forward voltage Schottky diode reduces power loss in the converter circuit. Layout Considerations least 750mA from a VIN as low as 3V. When fully charged to 4.2V, over 1A can be supplied. The photograph of a demonstration board in Figure 5 shows just how small the board area is for this application, 10mm × 12mm. Tiny ceramic bypass capacitors and surface mount inductors keep the design small. Figure 6 shows efficiency exceeding 90% and remaining greater than 85% over a load range from 10mA to 900mA with a fully charged battery. LTC3426 SHDN FB R1 95.3k 1% R2 30.9k 1% VOUT 5V 750mA AT 3V C2 22µF 80 VIN = 3V 75 70 65 60 50 Figure 4. Compact application circuit for VOUT at 5V further eases the burden of heavy capacitive loads by providing strong pull-up currents during rising edges to reduce the rise time. Thanks to these two features, the LTC4302 enables the implementation of much larger 2-wire bus systems than are possible with a simple unbuffered multiplexer. VIN = 4.2V 85 55 C1: TDK C1608X5R0J475M C2: TAIYO YUDEN JMK316BJ226ML D1: ON SEMICONDUCTOR MBR120VLSFT1 L1: SUMIDA CDRH4D28-2R2 2 LTC4302, continued from page 26 EFFICIENCY (%) VOUT GND 32 100 90 SW OFF ON The addition of the LTC3426 to Linear Technology’s high performance boost converter family allows the designer to deliver high current levels with minimal board space. An on chip switch and internal loop compensation reduces component count to provide an inexpensive solution for spot regulation applications. D1 VIN C1 10µF Conclusion 95 L1 2.2µH VIN 3V TO 4.2V The LTC3426 requires just a few external components to accomodate various VIN and VOUT combinations. Selecting the proper inductor is important to optimize converter performance and efficiency. An inductor with low DCR increases efficiency and reduces selfheating. Since the inductor conducts the DC output current plus half the peak-to-peak switching current, select an inductor with a minimum DC rating of 2A. To minimize VOUT ripple, use low ESR X5R ceramic capacitors. The average Schottky diode forward current is equal to the DC output current therefore the diode average Figure 5. Photograph of demo board of circuit in Figure 4—board area is 10mm × 12mm For further information on any of the devices mentioned in this issue of Linear Technology, use the reader service card or call the LTC literature service number: 1-800-4-LINEAR Ask for the pertinent data sheets and Application Notes. 1 10 100 LOAD CURRENT (mA) 1000 Figure 6. Up to 92% efficiency in Lithium-Ion battery to 5V output applications Impedance Analyzer, continued from page 30 assume that either the inductance is well damped, or it is shunted by large value capacitances. Notes 1. This subject is treated in some detail in the LTC1647 data sheet, Figures 9, 10, and 11 inclusive. 2. An hp 5210A Frequency Meter or any common counter gives adequate accuracy for most measurements. Linear Technology Magazine • November 2004