DESIGN IDEAS High Current Step-Down Conversion from Low Input Voltages by Dave Dwelley VIN 3.3V RIMAX 51k D1 MBR0530 drive to work efficiently. Two attractive solutions to generating 2.5V or less from a 3.3V supply are possible using the LTC1649 and the LTC1430A. The LTC1649 is a switching regulator controller designed to use 5V MOSFETs while running from an input supply as low as 2.7V. No 5V supply is required. The LTC1649 includes an Q1, Q2 IRF7801 TWO IN PARALLEL C2 1µF PVCC1 G1 PVCC2 IFB R3 22Ω C3 10µF 100 CIN 3300µF 90 LEXT 1.2µH VOUT 2.5V/15A R4 1k SHDN + C+ C1 CC 220pF 0.01µF CSS 0.1µF R2 12.7k CPOUT C4 10µF RIMAX 51k C2 1µF VCC 5V PVCC1 1649 TA01 (310) 322-3331 (800) 441-2447 C3 10µF CIN 3300µF LEXT 1.2µH VOUT 2.5V/15A IFB Q3 IRF7801 SENSE+ NC COMP SENSE– NC SS NC SHDN + RC 7.5k C1 220pF + R4 1k VCC G2 LTC1430A FB IMAX SHDN Q1, Q2 IRF7801 TWO IN PARALLEL G1 R3 22Ω PVCC2 GND FREQSET R1 12.4k + COUT 4400µF R2 12.7k PGND CC 0.01µF CSS 0.1µF IRF7801 = INTERNATIONAL RECTIFIER MBR0530 = MOTOROLA (310) 322-3331 (800) 441-2447 1649 TA01 Figure 3. 3.3V to 2.5V/15A converter using a 5V auxiliary supply and the LTC1430A 30 40 1 10 LOAD CURRENT (A) 100 1649 TA02 Figure 1. 3.3V to 2.5V/15A converter using the LTC1649 D1 MBR0530 60 Figure 2. Efficiency of Figure 1’s circuit C5 0.33µF + D2 MBR0530 IRF7801 = INTERNATIONAL RECTIFIER MBR0530 = MOTOROLA VIN 3.3V 70 0.1 C– GND COUT 4400µF 1µF SS 80 50 + VIN COMP RC 7.5k R1 12.4k Q3 IRF7801 VCC G2 LTC1649 IMAX FB SHDN + onboard charge pump to generate the 5V gate drive that the external power MOSFETs require. It also features an architecture designed to use all N-channel external MOSFETs and a high performance voltage mode feedback loop to ensure excellent transient response for use with high speed microprocessors and logic. EFFICIENCY (%) Many modern logic systems run with 3.3V as the sole power source. At the same time, some modern microprocessors and ASICs require supply voltages of 2.5V or less. Traditional step-down switching regulators can have difficulty running from the 3.3V supply, because affordable power MOSFETs generally require 5V gate A typical circuit is shown in Figure 1. The 3.3V supply voltage at VIN is converted to a regulated 5V output at CPOUT. This 5V supply powers the PVCC2 and VCC pins to provide gate drive to Q3. Q1 and Q2 require an additional charge-pump stage to drive their gates above the VIN supply voltage. D1 and C2 provide this boosted supply at PVCC1. The voltage feedback loop is closed through R1 and R2, with loop compensation provided by an RC network at the COMP pin. Softstart time is programmed by the value of CSS. Maximum output current is set by RIMAX at the IMAX pin and is sensed across the RDS(ON) of the Q1/Q2 pair, eliminating the need for a high current external resistor to monitor current. The circuit boasts efficiency approaching 95% at 5A (Figure 2). Some applications have a small 5V supply available, but need to draw the load current from the 3.3V supply. Such an application can use the circuit shown in Figure 3, with the continued on page 35 Linear Technology Magazine • February 1999 CONTINUATIONS range. This is true provided the filter magnitude response does not change with varying input signal levels, that is, the filter gain is linear. The gain linearity measured at the 100kHz theoretical center frequency of the filter is shown in Figure 7. The gain is perfectly linear for input amplitudes up to 1.25VRMS (3.5VP-P) so an 84dB dynamic range can be claimed. The input signal, however, can reach amplitudes up to 3VRMS (8.4VP-P, 92dB SNR) with some reduction in gain linearity. The LTC1735 and LTC1736 are the latest members of Linear Technology’s family of constant frequency, N-channel high efficiency controllers. With new protection features, improved circuit operation and strong MOSFET drivers, the LTC1735 is an ideal upgrade to the LTC1435/LTC1435A for higher current applications. With the integrated VID control, the LTC1736 is ideal for CPU power applications. The high performance of these controllers with wide input range, 1% reference and tight load regulation makes them ideal for next generation designs. LTC1562-2, continued from page 10 References level is 44µVRMS over a bandwidth of 800kHz or 98dB below the maximum unclipped output. 1. Hauser, Max. “Universal Continuous-Time Filter Challenges Discrete Designs.” Linear Technology VIII:1 (February 1998), pp. 1–5 and 32. 2. Sevastopoulos, Nello. “How to Design High Order Filters with Stopband Notches Using the LTC1562 Quad Operational Filter, Part 1.” Linear Technology VIII:2 (May 1998), pp. 28-31. 3. Sevastopoulos, Nello. “How to Design High Order Filters with Stopband Notches Using the LTC1562 Quad Operational Filter, Part 2.” in the Design Ideas section of this issue of Linear Technology. 4. LTC1562 Final Data Sheet. 5. For example: Schwartz, Mischa. Information Transmission, Modulation, and Noise, fourth edition, pp. 180–192. McGraw-Hill 1990. band gain can be higher than 0dB or if internal nodes are allowed to have gains higher than 0dB. Please contact the LTC Filter Design and Applications Group for further details. The low noise behavior of the filter makes it useful in applications where the input signal has a wide voltage LTC1735/LTC1736, continued from page 6 Conclusion Acknowledgments Philip Karantzalis and Nello Sevastopoulos of LTC’s Monolithic Filter Design and Applications Group contributed to the application examples. LT1505, continued from page 25 SW, VBAT and GND in Figure 2 will help in spreading the heat and will reduce the power dissipation in conductors and MOSFETs. By doing so, the required peak power from the wall adapter can be much lower than the peak power required by the load. The wall adapter has to supply the average power only. The LT1505 can also be used in other system topologies, such as the telecom application shown in Figure 5. The circuit in Figure 5 uses the battery to supply peak power demands. Conclusion The LT1505 is a complete, singlechip battery charger solution for today’s demanding charging requirements in high performance laptop applications. The device requires a small number of external components and provides all necessary functions for battery charging and power management. High efficiency and small size allow for easy integration with the laptop circuits. Also, by adding a simple external circuit, charging can be easily controlled by the host computer, allowing for more sophisticated charging schemes. Step-Down Conversion, continued from page 30 cuitry works in the same manner as in Figure 1. Efficiency and performance are virtually the same as the LTC1649 solution, but parts count and system cost are lower. In a 3.3V to 2.5V application, the steady-state, no-load duty cycle is 76%. If the input supply drops to 3.135V (3.3V – 5%), the duty cycle requirement rises to 80% at no load, and even higher under heavy or transient load conditions. Both the LTC1649 and the LTC1430A guarantee a maximum duty cycle of greater than 90% to provide acceptable load regulation and transient response. The standard LTC1430 (not the LTC1430A) can max out as low as 83%—not high enough for 3.3V to 2.5V circuits. Applications with larger step-down ratios, such as 3.3V to 2.0V, can use the circuit in Figure 3 successfully with a standar d LTC1430. Other Applications lower cost LTC1430A replacing the LTC1649. The LTC1430A does not include the 3.3V to 5V charge pump and requires a 5V supply to drive the external MOSFET gates. The current drawn from the 5V supply depends on the gate charge of the external MOSFETs but is typically below 50mA, regardless of the load current on the 2.5V output. The drains of the Q1/Q2 pair draw the main load current from the 3.3V supply. The remaining cirLinear Technology Magazine • February 1999 35