advertisement Monolithic Synchronous Step-Down Regulator Delivers up to 12A from a Wide Input Voltage Range – Design Note 457 Charlie Zhao and Henry Zhang Typical Application Example Figure 1 shows a typical application schematic of the LTC3608. This 7mm × 8mm regulator supplies 2.5V at 8A maximum load current from a 4.5V to 20V input source. This application switches at a nominal 650kHz, which allows the use of low profile inductor and capacitors while maintaining high efficiency. The switching frequency can be easily adjusted by a resistor (RON in Figure 1, connected to the ION pin). Figure 2 shows the operating efficiency. Introduction The LTC ®3610 is a high power monolithic synchronous step-down DC/DC regulator that can deliver up to 12A of continuous output current from a 4V to 24V (28V maximum) input supply. It is a member of a high current monolithic regulator family (see Table 1) that features integrated low RDS(ON) N-channel top and bottom MOSFETs. This results in a high efficiency and high power density solution with few external components. This regulator family uses a constant on-time valley current mode architecture that is capable of operating at very low duty cycles at high frequency and with very fast transient response. All are available in low profile (0.9mm max) QFN packages. The FCB pin is connected to ground to force continuous mode operation at light load for both low noise and small output ripple. The FCB pin can also be tied to INTVCC to enable discontinuous mode for higher efficiency at light load. Soft-start is programmable with a capacitor from the RUN/SS pin to ground. Forcing the RUN/SS pin below 0.8V shuts down the device. Table 1. Comparison of LTC36XX Family Members LTC3608 LTC3609 LTC3610 LTC3611 INPUT VOLTAGE 4 to 20 4.5 to 32 4 to 24 4.5 to 32 RANGE (V) MAX LOAD 8 6 12 10 CURRENT (A) QFN PACKAGE 7 × 8 × 0.9 7 × 8 × 0.9 9 × 9 × 0.9 9 × 9 × 0.9 SIZE (mm) VIN 4.5V TO 20V 10μF 25V 10μF 25V 10μF 25V GND L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 1μH IHLP2525CZ-01 SW PVIN 0.22μF CMDHS-3 25V INTVCC The LTC36XX family are valley current mode regulators so they inherently limit the cycle-by-cycle inductor current. The inductor current is sensed using the RDS(ON) 100μF 6.3V LTC3608 GND SW 4.7μF INTVCC 10V BOOST INTVCC RUN/SS 0.1μF VOUT EXTVCC 2200pF 60.4k 1% PGND ION SGND 0.47μF 25V VOUT VFB ITH FCB SVIN SVIN VRNG 100pF VIN 1Ω VON PGOOD 19.1k 1% 12k fSW = 650kHz RON 162k 1% DN457 F01 SVIN Figure 1. 4.5V to 20V Input to 2.5V/8A High Density Step-Down Converter 01/09/457 100μF 6.3V VOUT 2.5V 8A limiting feature to further limit current in the event of a short circuit. If the output drops more than 25%, the maximum sense voltage is lowered to about one sixth of its original value. 100 VIN = 5V 95 EFFICIENCY (%) 90 85 VIN = 20V Paralleling Regulators for >12A These parts can be easily paralleled for high output current applications. Figure 3 shows a 1.2V/24A application using two parallel LTC3610s. Because of the valley current mode control architecture, the paralleled regulators can operate at very low duty cycles with fast transient response and excellent load balance. 80 75 70 65 60 VOUT = 2.5V FREQ = 650kHz 55 50 0 1 2 7 6 4 3 5 LOAD CURRENT (A) 8 AN457 F02 Figure 2. Efficiency vs Load Current of Figure 1 of the bottom MOSFET —no additional current sense resistor is required. The current limit is also adjustable with the voltage at the VRNG pin. When the VRNG pin is tied to ground, in the Figure 1 example, the current limit is set to about 16A. An open-drain logic power good output voltage monitor (PGOOD) is pulled low when the output voltage is outside ±10% of the regulation point. In the case of overvoltage, the internal top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition clears. The LTC36XX also includes a foldback current The current sharing is simple. Connect the ITH pins together, since the ITH pin voltage determines the cycleby-cycle valley inductor current. The feedback pins of paralleled LTC3610s share a single voltage divider. The RUN/SS pins are connected so that the LTC3610s start up with same slew rate. The paralleled LTC3610s have excellent thermal balance due to good current sharing. Conclusion With broad input and output ranges, high current capability and high efficiency, these monolithic regulators provide small size, low external component count power solutions for many applications from communications infrastructure to industrial distributed power systems. 10μF PVIN RUN/SS 3.83k 150μF 0.33μH SW LTC3610 ITH 10μF VIN 11V TO 13V GND + ITH PGND 22pF VFB SGND VFB 60.4k 100pF 1800pF 10μF PVIN RUN/SS 0.1μF 100pF ITH PGND 47μF s8 + 150μF GND 0.33μH SW 22pF LTC3610 ITH 10μF VOUT 1.2V 24A VFB SGND VFB DN457 F03 fSW = 750kHz Figure 3. Two LTC3610s in Parallel Can Provide 24A Output Current Data Sheet Download For applications help, call (408) 432-1900, Ext. 3258 www.linear.com Linear Technology Corporation dn457 LT/TP 0109 116K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●