DESIGN IDEAS Versatile LTC3830 and LTC3832 Deliver High Efficiency for Step-Down, Step-Up and Inverting Power Conversions by Wei Chen and Charlie Zhao Introduction The LTC3830 and the LTC3832 are pin-to-pin compatible upgrades to the LTC1430—a popular IC for low voltage step-down applications due to its simplicity and high efficiency. The LTC3830 and the LTC3832 remove the LTC1430’s frequency foldback at startup, thus eliminating inrush current and resulting output overshoot. Other improvements over the LTC1430 include tighter gm distribu- + D3 MBR0520LT1 DZ MMSZ5242B 5V + PVCC1 PVCC2 + 2.2µF SENSE + SENSE – NC Doug La Porte Tiny and Efficient Boost Converter Generates 5V at 3A from 3.3V Bus ................................................... 28 VOUT 2.5V 12A RA 12.4k 1% RB 12.7k 1% NC FB Q1, Q2: SILICONIX Si7440DP CIN: SANYO POSCAP 10TPB220M COUT: PANASONIC EEFUD0D181R LO: SUMIDA CDEP105-1R3-MC-S (800) 554-5565 (619) 661-6835 (714) 373-7334 (847) 956-0667 Figure 1. Schematic diagram of 2.5V/12A synchronous step-down power supply How to Use the LTC6900 Low Power SOT-23 Oscillator as a VCO ......... 23 Save Space and Expense by Extracting Two Lowpass Filters Out of a Single LTC1563 ............. 25 COUT 180µF 470pF GND COMP RC 18.2k CC 1500pF D1 B320A Q2 + PGND SHDN Wei Chen and Charlie Zhao Nello Sevastopoulos 1k G2 FREQSET RUN DESIGN IDEAS Versatile LTC3830 and LTC3832 Deliver High Efficiency for Step-Down, Step-Up and Inverting Power Conversions ................................. 21 Q1 LO 1.3µH 13A LTC3830 IFB 130k C1 68pF 0.1µF IMAX SS 0.01µF 12k G1 VCC 1k 10µF 0.1µF 10µF 10Ω VIN 3.3V–8V CIN 220µF tion of the error amplifier and tighter current limiting. The LTC3832 is identical to the LTC3830, except that it incorporates a 0.6V reference for the output feedback, a larger gm and a default frequency of 300kHz (instead of the 200kHz for the LTC3830), making it good match for very low output applications. The higher frequency of the LTC3832 also allows the use of smaller inductors and capacitors, making for a smaller overall solution. Dongyan Zhou Small, Portable Altimeter Operates from a Single Cell ....................... 29 VIN 3.3V + Simple Isolated Telecom Flyback Circuit Provides Regulation Without Optocoupler ................................ 30 10Ω MBR0520 CIN 330µF Todd Owen 10µF 0.1µF 0.1µF 10µF John Shannon PVCC1 Space Saving Dual Output ±5V High Current Power Supply Requires Only One 1.25MHz Switcher and One Magnetic Component ................... 31 VCC 2.2µF 0.47µF NC Keith Szolusha Efficient DC/DC Converter Provides Two 15A Outputs from a 3.3V Backplane ................................... 32 David Chen Design Low Noise Differential Circuits Using the LT1567 Dual Amplifier Building Block ............................ 34 Philip Karantzalis Linear Technology Magazine • May 2002 PVCC2 IMAX SS SHDN C1 68pF NC RC NC 15k CC 3300pF 5mΩ MBR0520 LIN 1.3µH B320A 5.6k Q2 10µF + VOUT COUT 5V 330µF 5A ×2 IFB G1 FREQSET SHDN PGND COMP GND SENSE+ G2 SENSE– FB LTC3830 Q1 37.4k 1% 12.7k 1% Q1, Q2: SILICONIX Si7440DP (800) 554-5565 CIN, COUT: SANYO POSCAP 6TPB330M (619) 661-6835 LIN: SUMIDA CDEP105-1R3-MC-S (847) 956-0667 Figure 2a. Schematic diagram of 3.3V to 5V synchronous boost converter 21 DESIGN IDEAS VIN CF 1µF LIN Q2 5.6k LTC3830 0.1µF + RF 10k IMAX VOUT COUT IFB G1 Q1 G2 Figure 2b. How to use the DC resistance of the boost inductor to control current limiting This article shows several designs using the LTC3830 for step down, step up and inverting applications. The LTC3832 can be used in place of the LTC3830 in any of these designs. All that is required are some minor adjustments to the feedback resistor divider and the compensation RC component values. Figure 1 shows the schematic diagram of a 12A step down design based on LTC3830. The input is 3.3V to 8V and the output is 2.5V. To obtain different output voltages, vary the ratio of RA/RB. With only two tiny PowerPak SO8 MOSFETs and 300kHz switching frequency, this design achieves close to 90% efficiency with 5V input and 2.5V output. The overall footprint of this design is less than 1"×1.2", with all of the components placed on the same side of the board. For higher output currents, simply parallel more MOSFETs and use an inductor with a higher current rating. 12A High Efficiency Step Down Power Supply Converts 3.3V–8V Input to a 2.5V Output LTC3830/3832 are voltage mode synchronous buck controllers with two powerful MOSFET drivers for both the main MOSFET and a synchronous MOSFET. The RDS(ON) of the main MOSFET is used to establish the current limit, thus eliminating the sense resistor and its associated power loss. The current limit and switching frequency can be programmed easily through external resistors. 5A Step Up Power Supply Converts 3.3V to 5V Although intended for synchronous buck applications, LTC3830 and LTC3832 can also be used in other circuit topologies. Figure 2a shows a synchronous boost design using LTC3830 converting 3.3V to 5V. Com- VIN 3.3V + MBR0520 100Ω 10µF 1µF DZ 8.2V PVCC2 VCC 1µF SS 0.01µF NC SHDN CC 1.5nF 0.1µF 3.6k Q1 0.1µF IMAX 1k FREQSET G2 SHDN LO 1.3µH Q2 5A Inverter Converts 3.3V to –5V The LTC3830 and LTC 3832 can also be used in inverting applications. Figure 3 shows a synchronous buckboost power supply which converts 3.3V into –5V. The total VCC supply voltage in this design is the sum of the absolute values of input and output voltages, which is about 8.3V; and the PVCC1 voltage is the VCC voltage plus 5V, which is 13.3V. Since these voltage stresses are very close to the maximum voltage ratings for the LTC3830 and the LTC3832 (VCC(MAX) = 9V and PVCC1(MAX) = 14V), Zener diodes should be placed on VCC and PVCC1 pins to provide overvoltage protection. Conclusion G1 IFB COMP C1 68pF PVCC1 CIN 330µF pared to a conventional boost converter, this design uses a low RDS(ON) N-channel MOSFET to implement the synchronous rectification, therefore improving efficiency by 5% to 10%. The maximum output current is 8A with only two PowerPak SO8 MOSFETs. A current sense resistor is used for more accurate current limiting than can be achieved by sensing RDS(ON) of the MOSFET. One may also use the DCR of the inductor to implement the current limit function, as shown in Figure 2b. RF and CF filters out the AC voltage components of the inductor voltage to obtain the DC voltage drop on the DC resistance of the inductor. This scheme eliminates the sense resistor and its associated power loss, but the response to overcurrent conditions is slower than a topology that uses a sense resistor. The delay time is determined by the product of RF • CF. + 10µF 13V COUT 330µF 37.4k 1% FB 12.7k 1% RC 15k SENSE+ PGND NC GND SENSE– NC LTC3830 VOUT –5V 5A Q1, Q2: SILICONIX Si7440DP (800) 554-5565 CIN, COUT: SANYO POSCAP 6TPB330M (619) 661-6835 LO: SUMIDA CDEP105-1R3-MC-S (847) 956-0667 The LTC3830 and LTC3832 are versatile voltage mode controllers that can be used in variety of applications including step up, step down and voltage inversion. Their integrated high current MOSFET drivers and programmable frequencies allow users to minimize power loss and total solution size. Figure 3. Schematic diagram of 3.3V to –5V inverting converter 22 Linear Technology Magazine • May 2002